Faculdade de Farmácia
n
From synthesis to formulation of gliclazide and glibenclamide
cocrystal pharmaceutical solid dosage forms
Bárbara Violinha Baptista
Dissertation to obtain the Master of Science Degree in
Pharmaceutical Engineering
Work supervised by:
Professor João Almeida Lopes (Supervisor)
Doctor Mafalda Cruz Sarraguça (Co-Supervisor)
Faculdade de Farmácia
From synthesis to formulation of gliclazide and glibenclamide
cocrystal pharmaceutical solid dosage forms
Bárbara Violinha Baptista
Dissertation to obtain the Master of Science Degree in
Pharmaceutical Engineering
Work supervised by:
Professor João Almeida Lopes (Supervisor)
Doctor Mafalda Cruz Sarraguça (Co-Supervisor)
i
Abstract
Low solubility of pharmaceutical drugs is very often a limiting factor in the development of drug products. Cocrystals are known for a long time but still little explored as an alternative for drug solubility increase. Cocrystals are crystalline homogenous structures composed by two or more molecules in the same crystal lattice, all solid at room temperature. They are different from the precursors held together by noncovalent interactions (normally hydrogen bonds) with a well-defined stoichiometry.
In this work, two sulfonylureas used in the treatment of non-insulin-dependent diabetes mellitus – glibenclamide (GBL) and gliclazide (GCZ) – were chosen as cocrystallization targets due to their categorization as class II drugs (low solubility, high permeability) by the Biopharmaceutical Classification System. The development of GBL and GCZ cocrystals was developed in two stages: screening and production experiments. Screening experiments performed by solvent evaporation, were designed to evaluate the most promising cocrystal forms produced with three different coformers – tromethamine (TRIS), nicotinamide (NICO) and p-aminobenzoic acid (PABA). Differential scanning calorimetry, near- and mid-infrared spectroscopy and powder x-ray diffraction comprehended the selected crystal characterization methods. Products of the cocrystallization process were characterized and compared with the respective drug, coformer and binary physical mixture. Screening experiments showed that cocrystals of GLB and GCZ were preferentially formed with TRIS.
Production experiments (still lab scale) were performed for the GBL:TRIS 1:1 and GCZ:TRIS 3:2 systems, aiming at obtaining a sufficient amount of product for incorporation in pharmaceutical solid oral dosage forms. For this, two different cocrystallization methods were tested – solvent evaporation and slurry. Contrarily to the slurry method, solvent evaporation operated at a 20-fold scale increase resulted in the production of impure cocrystals. To better understand this result, a D-optimal experimental design was proposed to understand the impact of temperature, volume of solvent and molar ratio on products purity. The slurry cocrystallization method was selected as preferential method for the production of higher cocrystals’ quantities ensuring a suitable level of purity.
The solubility increase was assessed for the cocrystals produced with the slurry method (GBL:TRIS and GCZ:TRIS) using three different dissolution media. Results revealed a maximum solubility enhancement in water, with a 54.7-fold and 82.5-fold increase for GBL and GCZ, respectively.
Key-words: pharmaceutical cocrystals, glibenclamide, gliclazide, solvent evaporation cocrystallization; slurry cocrystallization; cocrystals characterization
ii
Resumo
A solubilidade de fármacos é um fator importante a considerar no desenvolvimento de novos medicamentos. A produção de co-cristais, ou co-cristalização, constitui numa técnica de alteração de estado sólido há muito conhecida pela engenharia de cristais, que tem vindo a ganhar relevância junto da indústria farmacêutica desde as décadas de 1980-1990, principalmente devido à sua vasta aplicabilidade na melhoria da solubilidade de fármacos. Os co-cristais consistem em formas sólidas homogéneas, compostas por duas ou mais moléculas sólidas à temperatura ambiente e contidas numa única estrutura cristalina. Estes compostos têm uma estequiometria bem definida (por exemplo, A:B 1:1, A:B 1:2) e apresentam caraterísticas diferentes dos respetivos precursores ligados entre si por ligações não-covalentes (normalmente, pontes de hidrogénio).
A diabetes mellitus (DM) é uma doença do foro metabólico, caracterizada por níveis elevados de açúcar no sangue (hiperglicemia), e que exibe duas categorias etiopatogénicas: DM tipo 1 ou auto-imune e DM tipo 2 ou DM não-dependente de insulina. Esta última afeta mais de 8.5% da população mundial (422 milhões de pessoas), sendo considerada uma das maiores epidemias do século XXI. As atuais opções terapêuticas de primeira-linha incluem, numa fase inicial, a administração de metformina (biguanida), à qual se equaciona a concomitância com sulfonilureias ou inibidores de α-glucosidase com o agravamento da condição. No presente trabalho, duas das sulfonilureias mais prescritas em Portugal para o tratamento da NIDDM – glibenclamida (GBL) e gliclazida (GCZ) – foram selecionadas como alvo para aplicação de técnicas de co-cristalização, devido à sua categorização como fármacos de classe II pelo Sistema de Classificação Biofarmacêutica (BCS). Esta classificação revela que ambos os fármacos têm baixa solubilidade e elevada permeabilidade.
A síntese de co-cristais de GBL e GCZ foi desenvolvida em duas etapas: experiências de seleção ou triagem e produção à escala laboratorial. As experiências de seleção ou triagem foram elaboradas através da aplicação do método de co-cristalização por evaporação de solvente, com o objetivo de investigar os co-cristais mais promissores produzidos por combinação dos referidos fármacos com três diferentes co-formadores – trometamina (TRIS), nicotinamida (NICO) e ácido p-aminobenzoico (PABA). Nestas experiências, quantidades muito reduzidas (0.5-1.5 mmol) de cada fármaco foram misturadas fisicamente com cada um dos co-formadores, sendo a mistura física resultante dissolvida num determinado volume de metanol (20-40 mL). Após completa dissolução, as soluções foram deixadas em repouso, decorrendo cristalização até completa evaporação do solvente. Os produtos obtidos foram caraterizados quanto à sua estabilidade térmica por calorimetria diferencial de varrimento e quanto ao seu perfil químico por espetroscopia de infravermelho e infravermelho-próximo. Por constituir um método mais complexo e demorado, a avaliação da estrutura cristalina por difração de raios-X sobre pós foi apenas realizada para os produtos determinados como promissores pelas técnicas anteriormente referidas. Assumindo que um processo de cristalização bem sucedido resulta na produção de co-cristais com características físicas e químicas diferentes dos precursores, todos os produtos de co-cristalização obtidos foram caracterizados em comparação com o respetivo fármaco, co-formador e mistura física de ambos.
Os produtos das experiências de seleção sintetizados com NICO e PABA (à exceção do sistema GCZ:NICO) demonstraram caraterísticas térmicas e espectroscópicas sobreponíveis à respetivas misturas físicas, concluindo-se a ineficácia de co-cristalização de GBL e GCZ com estes formadores. Assim, dos três formadores em teste, os co-cristais de GBL e GCZ com TRIS foram considerados os mais promissores, havendo sido
iii identificadas caraterísticas discrepantes face aos precursores em todos os métodos de caracterização aplicados.
A produção à escala laboratorial teve por objetivo a obtenção de quantidades suficientes de co-cristais para posterior utilização nos estudos de formulação farmacêutica. Esta produção foi testada através de dois diferentes métodos de co-cristalização: evaporação de solvente e o método com lamas. Primeiramente, procedeu-se à co-cristalização por evaporação de solvente de ambos os fármacos com TRIS, aplicando o mesmo protocolo experimental utilizado nas experiências de seleção, mas numa escala 20-vezes superior. Para além da comparação com os respetivos precursores, a caraterização dos produtos obtidos visou também a avaliação da sua homogeneidade. Para isto, foram retiradas amostras do centro de cada um dos quatro quadrantes e do centro do fundo dos cristalizadores, sendo cada amostra avaliada de forma individual (um cristalizador, cinco amostras). Os resultados de caracterização térmica e espectroscópica foram coerentes entre si, verificando-se a obtenção de produtos impuros. Para os dois sistemas, as impurezas foram associadas à presença de excesso de co-formador, a qual, por sua vez, determina a ocorrência de um processo de co-cristalização incompleto. Quanto à caraterização espectroscópica, os resultados foram interpretados por observação direta dos espetros obtidos (tal como procedido anteriormente) e também por avaliação de modelos de análise de componentes principais. Os modelos de análise de componentes principais auxiliaram a interpretação da correlação entre as várias amostras dos produtos de co-cristalização e entre estas e os respetivos precursores e mistura física. Adicionalmente, verificou-se que as amostras retiradas do centro dos cristalizadores foram aquelas que demonstraram maior correlação para com as respetivas misturas físicas, propondo-se que o defeito de interação entre a totalidade dos precursores poderá dever-se à utilização de agitação magnética central durante o passo de dissolução. No caso do sistema GCZ:TRIS, para além das considerações de co-cristalização incompleta, o elevado grau de impureza foi também associado à utilização de uma razão molar desadequada (3:2).
Procedeu-se ao desenvolvimento de um delineamento experimental (do tipo D-ótimo) com o objetivo de avaliar o impacto de parâmetros de processo críticos para a co-cristalização por evaporação de solvente em termos do grau de pureza dos produtos. Três valores diferentes de temperatura (10ºC, 15ºC e 20ºC) foram sistematicamente combinados com três diferentes volumes de solvente (20 mL, 25 mL e 30 mL), num total de onze experiências para cada um dos dois sistemas em teste. Para o sistema GCZ:TRIS, a razão molar foi readaptada para 1:1. Foi possível compreender que variações de temperatura durante a cristalização estão associadas a co-cristalização incompleta e que a temperatura intermédia testada (15ºC) é preferível para otimizar a pureza dos co-cristais de GBL:TRIS. Finalmente, e apesar da readaptação da razão molar, o produto GCZ:TRIS continuou a ser caraterizado por um excesso de precursor. De forma geral, a ausência de efeitos sistemáticos no grau de pureza dos co-cristais a partir das experiências delineadas, indicia a falta de controlo sobre a co-cristalização por evaporação de solvente e considera-se este método como desadequado para obtenção de quantidades maiores destes co-cristais.
Finalmente, os sistemas GBL:TRIS e GCZ:TRIS foram co-cristalizados pelo método das lamas. Este método requer que os precursores fiquem em suspensão no solvente selecionado, ocorrendo co-cristalização por indução de choque físico entre as moléculas. Quanto ao sistema GBL:TRIS, foi possível produzir maiores quantidades de co-cristais mantendo um elevado grau de pureza, considerando-se o método de lamas como adequado e preferencial para aumento de escala. A razão molar de GCZ:TRIS 1:1 confirmou-se a mais adequada. No entanto o produto obtido continuou a ser considerado como impuro, e portanto desadequado para posteriores incorporação na formulação a desenvolver.
iv A solubilidade dos co-cristais produzidos obtidos pelo método de lamas foi avaliada em três meios de dissolução diferentes: água, tampão fosfato pH 7.5 e tampão fosfato pH 8.5 foram os meios selecionados para GBL:TRIS; água, tampão fosfato pH 7.4 e 0.1N ácido clorídrico foram os meios selecionados para GCZ:TRIS. Para ambos os sistemas, o aumento máximo de solubilidade foi observado em água. Decorrente da aplicação da técnica de co-cristalização, a solubilidade aquosa de GBL aumentou 54.7-vezes e a de GCZ 82.5-vezes.
Os co-cristais de GBL:TRIS foram utilizados para formulação de formas orais sólidas, doseadas a 5mg, por compressão direta, em comparação com um medicamento atualmente comercializado (GBL 5 mg Generis Comprimidos).
Palavras-chave: cocristais farmacêuticos, glibenclamida, gliclazida, cocristalização por evaporação de solvente; cocristalização por lamas; caracterização de cocristais
v
vi
Acknowledgements
There are no words to express the feeling of thankfulness towards a true attitude of kindness. However, it is imperative for me to describe my deepest and sincere acknowledgments:
Firstly, and most importantly, to my Supervisors João Almeida Lopes and Mafalda Sarraguça, who have always been available to guide me through the whole process, both scientifically and personally;
To IST Professors João Figueirinhas and Carlos Cruz, who generously provided PXRD equipment and concerning knowledge support;
To Ana Salgado (FFUL), who openhandedly offered her availability and time to help me develop the solubility tests;
To Generis Farmacêutica S.A., for kindly providing the APIs;
To my family and friends, who showed me that there is always a light at the end of the tunnel.
vii “...the number of forms known for a given compound is proportional to
the time and energy spent in research on that compound.”
– W .C. McCrone off-quoted statement, in Physics and Chemistry of the Organic Solid State, Vol. 2, Wiley Inerscience,1965)
viii
Contents
Abstract ... i Resumo ... ii Acknowledgements ... vi Contents ... viii List of figures ... xList of tables ... xiii
List of abbreviations ... xiv
Chapter 1 – Objectives ... 1 Chapter 2 – Introduction ... 2 2.1. Drug development ... 2 2.2. Solid forms ... 4 2.2.1. Amorphous ... 5 2.2.2. Polymorphs ... 6 2.2.3. Solvates ... 6 2.2.4. Salts ... 6 2.2.5. Cocrystals ... 8 2.3. Cocrystallization techniques ... 16 2.3.1. Solvent-based cocrystallization ... 18 2.3.2. Scale-up ... 20
2.4. Dissolution, solubility and bioavailability ... 20
2.4.1. Dissolution ... 21 2.4.2. Solubility... 21 2.4.3. Bioavailability ... 24 2.4.4. Bioequivalence ... 25 2.5. Formulation ... 25 2.6. Diabetes mellitus ... 27 2.6.1. Sulfonylureas ... 28
Chapter 3 – Materials and methods ... 33
3.1. APIs and coformers ... 33
3.2. Cocrystals production ... 35
3.2.1. Screening experiments ... 36
3.2.2. Production experiments (still lab scale) ... 37
3.3. Characterization methods ... 41
3.3.1. Powder X-ray diffraction ... 41
3.3.2. Differential scanning calorimetry ... 44
ix
3.3.4. Mid infrared spectroscopy ... 45
3.3.5. Solubility assays ... 46
3.4. Data analysis... 47
3.4.1. Principal components analysis ... 47
3.4.2. Standard normal variate pre-processing ... 48
3.5. Cocrystals formulation on solid oral dosage forms ... 48
3.5.1. Formulation design ... 49
3.5.2. Formulation process ... 51
3.5.3. Solid oral dosage forms characterization ... 53
Chapter 4 – Results and discussion ... 54
4.1. Cocrystals production ... 54
4.1.1. Screening experiments ... 54
4.1.2. Production experiments (still lab scale) ... 68
4.2. Solubility studies ... 103
4.3. Cocrystals formulation on solid oral dosage forms ... 104
4.3.1. Compression force selection ... 104
4.3.2. Final hardness studies ... 105
Chapter 5 – Conclusions and future perspectives ... 107
5.1. Conclusions ... 107
5.2. Future perspectives ... 110
x
List of figures
Figure 1 – Drug development timeline: stages of drug development and respective events .. 3 Figure 2 – Schematic representation of API solid forms classification ... 5 Figure 3 – Evolution of the number of publications on “cocrystals” and “pharmaceutical cocrystals” over the decades 1960-2017(present) posted on four different search engines: (a) <google.com>, (b) <b-on.pt>, (c) <ncbi.nlm.nih.gov/pubmed> and (d) <sciencedirect.com> . 9 Figure 4 – Graphic illustration of industry indecision concerning manufacturing allocation of pharmaceutical cocrystals (regulatory uncertainty) ... 11 Figure 5 – Scheme for rational design of cocrystal ... 11 Figure 6 – Examples of supramolecular synthons: (a) homosynthon formed between two carboxylic acid groups, (b) heterosynthon formed between one carboxylic acid group and one amide group ... 12 Figure 7 – Selection of a cocrystals candidate typical decision tree. Adapted from (Schulteiss N., Newman A., 2009)(254) ... 15 Figure 8 – Cocrystallization as a transformation process of raw material characteristics (API and coformer) into outputs (cocrystal), respecting to the adjacent CQAs and CPPs ... 18 Figure 9 – Typical pH-solubility profile of a basic compound. Image retrieved from (Sugano, K., et al., 2007)(158) ... 23 Figure 10 – Schematic representation of the absorption, distribution and metabolism process of a drug in the human body after intravenous (black arrows) and oral administration (grey arrows) ... 24 Figure 11 – Formulation process stages ... 26 Figure 12 – Simplified representation of the NIDDM therapeutic management algorithm according to IDF and AACE/ACE guidelines ... 28 Figure 13 – Illustrative scheme of sulfonylurea mechanism of action: pancreatic cell highlighting K+ and Ca2+ polarized membrane cell flux, type 2 diabetes pancreatic cell
without treatment during glucose intake, and pancreatic cell with sulfonylurea treatment during glucose cell intake ... 30 Figure 14 – GBL and GCZ chemical structures and molecular formulas ... 30 Figure 15 – TRIS molecular structure, formula and weight. Image retrieved from
https://pubchem.ncbi.nlm.nih.gov/compound/6503 (220) ... 33 Figure 16 – NICO molecular structure, formula and weight. Image retrieved from
https://pubchem.ncbi.nlm.nih.gov/compound/936 (225) ... 34 Figure 17 – PABA molecular structure, formula and weight. Image retrieved from
https://pubchem.ncbi.nlm.nih.gov/compound/978 (228) ... 35 Figure 18 – Schematic representation of a cocrystallization experiment by solvent evaporation ... 36 Figure 19 – SE scale-up cocrystallization products categorical sampling: centre of the 4 crystallization-dish quadrants (1Q, 2Q, 3Q and 4Q) and centre of the crystallization-dish itself ... 38 Figure 20 – Full-factorial DoE combinations scheme ... 38 Figure 21 – D-optimal DoE combinations scheme with summary table of the number of experiences to be used in the candidate experimental set including the combination of the related variables ... 39 Figure 22 – Controlled temperature water-bath installation and crystallization-dishes placement... 40 Figure 23 – Phase diagram involving API, conformer and solvent, highlighting the “cocrystal+coformer” window, where later SE acquired products are integrated ... 41 Figure 24 – PXRD equipment: a) X-ray tubeshield transmitter, b) distance ring sample container, c) detector, d) temperature variation chamber ... 42
xi Figure 25 – PXRD command module: a) detector monitoring and temperature chamber
control module, b) high voltage generator module ... 42
Figure 26 – Sample assembly process: [1] sample container movable and fixed parts coupled, [2] removal of the sample container movable part, [3] sample container movable part secure placement, [4] insertion of the mark-tube into the sample container movable part orifice, [5] capillary alignment apparatus: a) sample container movable part locking support equipment and b) calibration monocle, [6] mark-tube alignment observation through the calibration monocle, [7] replacement of the sample container movable part into the sample container fixed part. ... 43
Figure 27 – DSC crucibles: container and lid ... 44
Figure 28 – NIR correlation chart ... 45
Figure 29 – MIR correlation chart. Retrieved from (Andy Brunning, 2015)(255) ... 45
Figure 30 – Cocrystals production steps scheme ... 54
Figure 31 – DSC screening thermograms for (a) GBL:TRIS, (b) GBL:PABA, (c).GBL:NICO) and GCZ ((d) GCZ:TRIS, (e) GCZ:PABA, (f) GCZ:NICO) systems. ... 58
Figure 32 – GBL systems spectroscopic characterization: (a) NIR spectra for the cocrystallization with TRIS, (b) MIR spectra for the cocrystallization with TRIS, (c) NIR spectra for the cocrystallization with PABA, (d) MIR spectra for the cocrystallization with PABA, (e) NIR spectra for the cocrystallization with NICO, (f) MIR spectra for the cocrystallization with NICO. ... 62
Figure 33 – GCZ systems spectroscopic characterization: (a) NIR spectra for the cocrystallization with TRIS (b) MIR spectra for the cocrystallization with TRIS , (c) NIR spectra for the cocrystallization with PABA (d) MIR spectra for the cocrystallization with PABA, (e) NIR spectra for the cocrystallization with NICO, (f) MIR spectra for the cocrystallization with NICO. ... 64
Figure 34 – PXR diffractograms of GBL:TRIS (a) and GCZ:TRIS (b) systems: pure APIs are displayed in black-line, pure TRIS in dark-blue-line, MIXs in blue-line and cocrystallization products in light-blue-line. Black-dashed-vertical-lines highlight new cocrystal peaks and grey-highlighted-zones correspond to shift zones ... 66
Figure 35 – DSC thermograms for SE cocrystallization: (a) GBL:TRIS system; (b) GCZ:TRIS system ... 69
Figure 36 – (a) PCA score plots of the NIR spectra from the products obtained from the screening and production experiments, from the MIX, and from pure GLB and TRIS; (b) Second component loadings (down) and NIR spectra form the production experiment, MIX, and pure GLB and TRIS (up). ... 71
Figure 37 – (a) PCA score plots of the MIR spectra from the products obtained from the screening and production experiments, from the MIX, and from pure GLB and TRIS; (b) Third component loadings (down) and MIR spectra form the production experiment, MIX, and pure GLB and TRIS (up). ... 72
Figure 38 – (a) PCA score plots of the NIR spectra from the products obtained from the screening and production experiments, from the MIX, and from pure GCZ and TRIS; (b) Third component loadings (down) and NIR spectra form the production experiment, MIX, and pure GCZ and TRIS (up). ... 73
Figure 39 – (a) PCA score plots of the MIR spectra from the products obtained from the screening and production experiments, from the MIX, and from pure GCZ and TRIS; (b) Third component loadings (down) and MIR spectra form the production experiment, MIX, and pure GCZ and TRIS (up). ... 74
Figure 40 – DSC thermograms of (a) GBL:TRIS (1:1) and (b) GCZ:TRIS (1:1) systems for the DoE experiments, screening experiments, MIXs, and pure API’s and TRIS... 77 Figure 41 – (a) PCA score plots of the NIR spectra from the products obtained from the screening and DoE experiments, from the MIX, and from pure GLB and TRIS; (b) Second
xii component loadings (up) and NIR spectra form the DoE experiments, MIX, and pure GLB and TRIS (down). ... 79 Figure 42 – (a) PCA score plots of the MIR spectra from the products obtained from the screening and DoE experiments, from the MIX, and from pure GLB and TRIS; (b) First component loadings (up) and MIR spectra form the DoE experiments, MIX, and pure GLB and TRIS (down). ... 80 Figure 43 – Suggestion of most probable synthons involved in GBL:TRIS cocrystal bonding ... 81 Figure 44 – (a) PCA score plots of the NIR spectra from the products obtained from the screening and DoE experiments, from the MIX, and from pure GCZ and TRIS; (b) First component loadings (up) and NIR spectra form the DoE experiments, MIX, and pure GCZ and TRIS (down). ... 83 Figure 45 – (a) PCA score plots of the MIR spectra from the products obtained from the screening and DoE experiments, from the MIX, and from pure GCZ and TRIS; (b) First component loadings (up) and MIR spectra form the DoE experiments, MIX, and pure GCZ and TRIS (down). ... 84 Figure 46 – GBL:TRIS system thermograms: (a) 1st and 2nd DSC heating cycles and
in-between cooling cycle results for slurry GBL:TRIS(1:1). (b) Comparison of the thermally characterized profile of GBL:TRIS (1:1) to the respective MIX, to the pure GBL and TRIS, and to the other obtained GBL:TRIS (1:1) products. ... 87 Figure 47 – GCZ:TRIS system thermograms: (a) comparison of GCZ:TRIS 1:1 cocrystals replicates acquired from slurry; (b) comparison of GCZ:TRIS cocrystals in two different molar ratios (3:2 and 1:1) acquired in three different experimental moments (screening, DoE and slurry) with pure precursors and respective MIXs ... 89 Figure 48 – GBL:TRIS 1:1 screening and slurry spectroscopic analysis: comparison of both cocrystals, pure precursors and MIX through (a) NIR spectra and NIR PCA model, and (b) MIR spectra and MIR PCA model ... 91 Figure 49 – GCZ:TRIS 3:2 screening, DoE (or 1:1 SE) and. 1:1 slurry spectroscopic analysis: comparison of both cocrystals, pure precursors and MIX through (a) NIR spectra and NIR-PCA model, and (b) MIR spectra and MIR- NIR-PCA model ... 94 Figure 50 – GBL:TRIS PXRD results: (a) stacked PXR-diffractograms comparing GBL:TRIS 1:1 cocrystals acquired from slurry and SE (screening) cocrystallization experiments with respective pure precursors and MIX; (b) summary table of GBL:TRIS 1:1_slurry diffractogram peaks in correspondence to the same system cocrystals acquired in screening experiments and respective discrimination as new peaks or related to pure precursors ... 97 Figure 51 – GCZ:TRIS system PXR-diffractograms: comparison of GCZ:TRIS cocrystals in two different molar ratios (3:2 and 1:1) acquired in three different experimental moments (screening, DoE and slurry) with pure precursors and respective MIXs ... 99 Figure 52 – GCZ:TRIS 1:1_(slurry) and MIX GCZ:TRIS 1:1 VT-PRX diffractograms stacked accordingly to temperature levels tested ... 101 Figure 53 – Solubility results comparison in different dissolution media: (a) GBL vs. GBL:TRIS solubility in 7.5 pH and 8.5 pH phosphate buffers and distilled water;(1:1), (b) GCZ vs. GCZ:TRIS solubility in 7.4 pH phosphate buffer, HCl 0,1N and distilled water ... 103 Figure 54 – Relation between compression force (N) and GBL 5 mg tablets hardness (kN) ... 105
xiii
List of tables
Table 1 – Stage of drug development and respective development activities. ... 2 Table 2 – Most common functional groups used for formation of supramolecular synthons by H-bonding... 13 Table 3 – Cocrystals characterization techniques and corresponding evaluated characteristics ... 14 Table 4 – Solubility classification according to European Pharmacopoeia ... 22 Table 5 – Evaluation of BCS classification of marketed drugs and drugs in R&D pipeline. Image retrieved from (Babu, N.J. and Nangia, A., 2011)(25) ... 25 Table 6 – Predicted physicochemical properties of glibenclamide and gliclazide ... 31 Table 7 – Cocrystals systems ratio and respective quantities, volume of methanol and stirring time used for cocrystallization exploratory experiments ... 37 Table 8 – D-optimal design specifications ... 39 Table 9 – Melting points of the pure compounds used in the cocrystallization experiments . 44 Table 10 – Formulation examples from Glyburide Composition patents WO 2001051463 A1 and US 6830760 B2 ... 50 Table 11 – Formulation example from patent “Solid dosage form of an antidiabetic drug WO 2006109175 A2” and respective optimization towards 5 mg GBL and GBL:TRIS tablets to be prepared ... 51 Table 12 – Tablet specifications for compression force determination: diameter, thickness and tablet hardness ... 52 Table 13 – Cocrystallization products visual inspection pictures and descriptions ... 56 Table 14 – DSC results for the screening experiments. ... 57 Table 15 – GBL:TRIS PXRD results regarding the total number of peaks for each system component, peak inter-system components correspondence and respective 2-angles ... 67 Table 16 – Summary of cocrystals screening results interpretation for all four characterization methods performed (YES = cocrystallization success; NO = cocrystallization failure; NA = “Not Applicable”)... 67 Table 17 – Comparison of SE cocrystallization products with respective MIXs ... 69 Table 18 – PCA captured variance for models constructed on NIR and MIR spectra taken for GBL:TRIS and GCZ:TRIS acquired from SE cocrystallization scale increase. ... 70 Table 19 – DSC results for the DoE samples. ... 76 Table 20 – PCA captured variance for models constructed on NIR and MIR spectra taken for GBL:TRIS and GCZ:TRIS acquired from DoE ... 78 Table 21 – PCA captured variance for models constructed on NIR and MIR spectra taken for GBL:TRIS and GCZ:TRIS acquired from slurry cocrystallization ... 90 Table 22 – Summary of cocrystals production characterization. NA is an abbreviation for “Not Applicable” ... 102 Table 23 – GBL produced tablets evaluation: mass of powder used per tablet, final tablet mass and tablet hardness discriminative and mean values ... 106 Table 24 – GBL:TRIS produced tablets evaluation: mass of powder used per tablet, final tablet mass and tablet hardness discriminative and mean values ... 106
xiv
List of abbreviations
ANDA – Abbreviated New Drug Application API – Active Pharmaceutical Ingredient ATR – Attenuated Total Reflectance
BCS – Biopharmaceutical Classification System BSM – Basic Structural Motifs
CED – Cohesive Energy Density CPP – Critical Process Parameter CQA – Critical Quality Attribute
CSD – Cambridge Structural Database DE – Dissolution Efficiency
DLS – Dynamic Light Scattering DM – Diabetes mellitus
DoE – Design of Experiments DR – Dissolution Rate
DSC – Differential Scanning Calorimetry EMA – European Medicines Agency
FDA – Food and Drug Administration (United Sates of America) FT – Fourrier Transform technology
GBL – Glibenclamide GCZ – Gliclazide
GRAS – Generally Regarded As Safe HCl – Hydrochloric Acid
HPLC – High Performance Liquid Chromatography HSP – Hansen Solubility Parameters
ICH – International Conference of Harmonization IDF – International Diabetes Federation
IND – Investigational New Drug
INFARMED – Autoridade Nacional do Medicamento e Produtos de Saúde, I.P. (Portugal) LDL – Low-Density Lipoprotein (cholesterol)
lnGaAs – Gallium-Arsenide and Indium-arsenide alloy (NIR detector) MeOH – Methanol
xv MIR – Mid-Infrared
MIX – Physical Mixture
NAS – New Active Substance NDA – New Drug Application NICO – Nicotinamide
NIDDM – Non-Insulin-Dependent Diabetes mellitus NIR – Near-Infrared
NSAID – Non-steroidal Anti-Inflammatory Drugs PABA – p-Aminobenzoic Acid
PC – Principal Component
PCA – Principle Components Analysis PCoC – Pharmaceutical Cocrystal PSD – Particle Size Distribution PXRD – Powder X-Ray Diffraction QbD – Quality by Design
R&D – Research and Development RED – Relative Energy Difference
SCFP – Sample Container Fixed Part (PXRD equipment) SCMP – Sample Container Movable Part (PXRD equipment) SDF – Solid Dispersion Formulation
SE – Solvent Evaporation
SEM – Scanning Electron Microscopy
SNV – Standard Normal Variate pre-processing SODF – Solid Oral Dosage Form
SPC – Summary of Product Characteristics SU – Sulfonylurea
TRIS – Tromethamine UV – Ultraviolet
VT-PXRD – Variable-Temperature Powder X-Ray Diffraction WHO – World Health Organization
1
Chapter 1 – Objectives
The main objectives were:
1. to synthesize several cocrystal forms of glibenclamide and gliclazide;
2. to characterize produced products through several state of the art techniques, probing for the most promising pharmaceutical cocrystals;
3. to produce quantities of the above selected cocrystals sufficient for further formulation studies;
4. to develop formulation of solid oral dosage forms according to Generis, S.A. commercial drugs and embedding the produced pharmaceutical cocrystals;
5. to perform pharmacothecnical studies over the obtained pharmaceutical cocrystals solid oral dosage forms.
2
Chapter 2 – Introduction
2.1. Drug development
The development of a new medicinal product from a novel compound (either synthesized, retrieved from a natural source or produced by biotechnological pathways) is a complex procedure that can be subdivided into five distinct stages: strategic research, exploratory research, candidate drug selection, exploratory development and full development. Table 1 summarizes the main activities that run at each stage. It is also important to note that some of those activities run in parallel aiming market distribution as briefly and efficiently as possible. (1)
Table 1 – Stage of drug development and respective development activities.
Stage of drug development Development activities
Strategic research
Feasibility and viability studies:
Research competence and expertise;
Definition of therapeutic areas of unmet medical need; Market potential/commercial viability evaluations.
Exploratory research
Investigation and identification of chemical or biological lead:
Manipulation of discovered natural compound with medicinal interest; Synthesis of new chemical entities:
Combinatorial chemistry; High-throughput screening;
Creation of representative libraries of compounds.
Candidate drug selection
Nomination of one or more drug candidates for development:
Generation and characterization of specific chemical compounds (stem from chemical or biological leads) aiming the fulfilment of optimal desired characteristics:
Potency; Safety;
Duration of effect; Specificity;
Pharmacotechnical aspects; Phase 0 clinical studies:
In vitro; Ex vivo;
Animal experiments (in vivo);
Biopharmaceutics and preformulation studies.
Exploratory development
Ascertain suitability of the drug candidate to the human body: Phase I clinical trials:
Healthy humans;
Pharmacodynamic and pharmacokinetic studies; Pharmacovigilance and tolerability tests;
Single ascending dose and multiple ascending dose studies; Establishment of the effects of interaction with food and other
drugs.
Full development
Long-term safety and post-market trials: Phase II and III clinical trials:
Patients with target disease; Dose-ranging studies;
Several hundreds to thousands population extension;
Market authorization application submission and regulatory approval; Market launch;
Phase IV clinical trials: Pharmacovigilance.
3 As so, the drug pipeline resumes an intricate set of events that can take close to $1000 million dollars and 10-20 years to follow a lead or drug candidate to final market approval. (2) It is typically divided by the regulatory entities in four stages (Figure 1).
Phase I – Discovery and development; Phase II – Preclinical research;
Phase III – Clinical research; Phase IV – Approval and review.
Also, a fifth phase concerning post-market safety monitoring can be included.
Preformulation (phase I) is paramount for successful candidate drug selection and profiling, providing information about four main aspects:
Analytical analysis
Molecular structure determination; Chemical analysis (quantification, purity). Physical analysis
Solid state and crystal structure; Melting point;
Hygroscopicity;
Particle size and shape; Powder compaction and flow. Physicochemical analysis
Chemical stability (degradants);
Physicochemical properties (pKa, partition and distribution coefficients, solubility).
Pharmaceutical analysis
Biopharmaceutical properties (pharmacokinetics and pharmacodynamics); Figure 1 – Drug development timeline: stages of drug development and respective events
4 Chemical compatibility (processability, impact of additives/excipients);
Regulatory stability (shelf-life).
For instance, solid state and crystalline structure assessment can be understood as the main basis of preformulation studies, as every other study must be constructed on the base knowledge of the target chemical entity and relative physicochemical structure.
2.2. Solid forms
Presently, “trial-and-error” approaches are generally practiced in the pharmaceutical industry, where a molecule is passed through development stages until it meets a limited set of criteria with little contribution from other functional areas and developability knowledge. As so, drug candidates often advance to human trials without sufficient information on potential crystal forms, physical properties and manufacturing capabilities. Consequently, the lack of full characterization of the drug candidate through multiple perspectives traditionally leads to empirically, complex and costly handled problems on late stages of development, which could be early avoided.(3) In the present context of paradigm shift imposed by the regulatory entities where risk knowledge and management approaches are paramount so that quality should be built into the drug product from its inception to its full and late development stages, solid-state and crystal engineering are merging in the pharmaceutical sciences as strategies to:
alter solubility and dissolution rate of drugs;(4–6)
controlled-release drug forms;(5)
improve drug efficacy and mitigation of adverse reactions;(5) improve drug permeability;(5,7)
alter melting points, resulting in improved milling and new formulation opportunities;(5,8)
extend patent protection;(5,9) improve thermal stability;(7,10) improve hydrolytic stability;(10)
create targeted drug delivery systems;(11,12) reduce hygroscopicity;(13,14)
improve organoleptic properties (e.g., taste acceptability);(14) facilitate purification, handling and processability;(14)
improve compactability an tableting performance and (15,16)
racemic resolution and absolute configuration of chiral drugs through single-crystal X-ray diffraction.(17)
Given the background, screening and selection of existent solid state of an active pharmaceutical ingredient (API) is one of the most important procedures to be established during early stages of drug development, since it can impact API’s physicochemical and mechanical properties and chemical stability, as well as it can affect its biopharmaceutical properties and manufacturability. Divergences between different solid states of the same API merge as a result of divergences in molecular interactions, structure, composition and molecular arrangement.(18–22)
As solids, APIs can exist mainly in two morphological structures: crystals or amorphous (see Figure 2).
5
2.2.1. Amorphous
Amorphous are isotropic shapeless solid forms that display two characteristic properties: when cleaved or broken they produce irregular fragments with often curved surfaces and exhibit irregular external structures with poorly defined patterns due to their irregularly arranged components and variable bonds strength. Also, rather than having a well-defined melting point, amorphous compounds tend to soften slowly over a wide temperature range, being considered less stable and more prone to degradation.(18,23) Concerning pharmaceutical applications, amorphous solids demonstrate as main advantage a higher solubility profile when compared to their relative crystalline form. This fact is related to the high free energy and low density of the amorphous phase, which favors the formation of intermolecular interactions and hydrogen bonds between the solvent and the solute, convenient for solubility and bioavailability considerations. However, the metastable behavior and tendency to degradation or transformation to the crystalline form during storage determine an higher risk and a serious concern for drug developers.(24–27)
Divergently, crystals are anisotropic solid forms in which the molecular structure is periodically organized in a repeating three-dimensional pattern.(7,28) The regularity of the crystalline lattices conditions the formation of uniform intermolecular forces with high density and low free energy. Thus, crystalline solid forms are considered more stable than their amorphous relatives, therefore determining lower risk and better manufacturability and acceptance criteria for drug developers. Nonetheless, lower solubility is a major disadvantage.(24–27)
6
2.2.2. Polymorphs
Polymorphs (from the Greek ‘polus’=many and ‘morph’=shape) are single-component solid compounds that retain the same chemical composition yet appearing in different crystalline geometric and spatial organizations, which can determine different physicochemical properties and biological activities.2,3 Polymorphism phenomenon is quite
common for organic molecules (more than 59% of substances are known to exist in more than one crystal form)(29,30), and, concerning pharmaceutical applications, screening of polymorphic forms is of utmost importance since not all the polymorphs may present the same therapeutic interest; even if they are considered pharmaceutically equivalent (in terms of efficacy and safety), their synthetic pathways may differ in crucial steps, allowing the decision for the morphological form that reveals the least expensive, convergent, safest and overall easiest method of production.(6,28,31–33) Some examples of polymorphism that gained a spot in the pharmaceutical industry history are: “the disappearing polymorphs of benzocaine:picric acid” (34), “the ritonavir story” (35), “Apotex Paroxetin vs GSK Paroxetin”(36).
2.2.3. Solvates
Solvates are defined as crystalline molecular compounds in which molecules of the crystallization solvent are entrapped in the host lattice. Regarding that definition, hydrates are unique types of solvates in which the crystallization solvent retained is water.12,14,15
Solvates/hydrates are often referred to as “pseudo-polymorphs” as they consist of a different solvated solid form of a certain organic compound, but cannot be considered true polymorphs due to the solvent molecules addition to the crystal lattice.(28) The presence of solvent molecules influences the intermolecular interactions and confers different physical (internal energy, enthalpy, entropy, Gibbs free energy and thermodynamic activity) and chemical (solubility, dissolution rate and, consequently, bioavailability) properties that those characteristic of the unsolvated solid form. Also, a solvate is thought to have lower solubility in the solvent from which it crystallizes than its respective unsolvated form. As each solvent convenes unique characteristics, different solvates will demonstrate different solid form properties.(32)
Usually, solvate formation during molecular synthesis is undesired for pharmaceutical industry applications, as solvate stability is inadvertently intertwined with the entrapped solvent thermodynamic activity and respective reaction to temperature and pressure variations, which makes storage stability prediction very complicated. In the specific case of hydrates, possible conversion into anhydrous crystal forms during storage can determine major physical properties modifications thereby compromising dosage form appearance and integrity.(39) Also, hydrates often display lower solubility and dissolution profiles than their relative anhydrous drug form.(24) Most hazardously, most solvents used in synthetic manufacturing are organic, which are biologically toxic and must not integrate APIs.(40)
2.2.4. Salts
According to the IUPAC “Gold Book”, a salt is a chemical compound comprising an assembly of cations and anions”, thus it is a crystalline solid form that results from a neutralization reaction.(38) To such a definition, a pharmaceutical salt comprises an ionized molecular API (cationic or anionic form) linked to a counterion (molecular or monoatomic) by ionic bonds, both compounds having a definite stoichiometry for charge
7 balance. For ionic interactions to occur, protons (H+) are transferred from an acid (A) to a
base (B), according to the following theoretical chemical equation:
𝐴 ⋯ 𝐻 + 𝐵 → (𝐴−)(𝐵+⋯ 𝐻) Eq. 1
The extent of proton transfer mainly depends on the difference of pKa values between the acid and the conjugate acid of the base in solution. Salt formation comprehends the formation of ionic bonds between the API and the conjugated compound, which is achieved by proton transfer in any ionization extent (full or incomplete). If two compounds become linked with no proton transfer, hydrogen bonds are mainly responsible for the linkage, thus resulting in the formation of products called cocrystals (which will be further discussed on section 2.2.5.).
Depending on the chosen counterion, a drug salt can be either basic or acidic, what will play major influence in its biological dissolution and absorption profile after oral absorption. While basic drug salts display greater solubility at lower pH values, acidic drug salts will have greater solubility at higher pH values, therefore basic drug salts tend to be preferably absorbed in the stomach or in upper portions of the small intestine and acidic drug salts will rather be absorbed over the small intestine or even in the large intestine.(19,41–44)
Because of the broad capacity to design and modulate an API according to desired drug properties, salt formation became most appealing to the pharmaceutical industry, being estimated that today more than 50% of all drug molecules are administered as salts.(42,45) However, salt formation is not a ‘one-size-fits-all’ approach as, besides the inadequacy to non-ionisable drugs, it may exhibit other disadvantages, such as:
additional step in the synthesis of a medicinal compound;(32)
decreased percentage of active content of drug candidate in the formulation: inactive counterions generally represent 20-50% of the weight of the drug
substance;
increased powder volume causes problems for tableting and capsule filling and patient compliance (smaller tablet/capsule size increases the ease of swallowing).(5)
corrosiveness of salts, resulting in tableting problems;(5)
increased formation of hydrates and polymorphs, resulting in greater variability;(5,29)
possible disproportionation (dissociation) of hydrochloride or hydrobromide, resulting in the release of hydrohalide gas or reaction with excipients or process-related chemicals;(7)
reduced dissolution rate or solubility for hydrochloride salts in gastric fluid resulting from precipitated free acid or base ate the surface of solid dosage form and (41) increased chance of poor solid-sate stability at the microenvironment pH of the
8
2.2.5. Cocrystals
Although cocrystals concrete definition is still debated in the scientific community(46– 48), the European (European Medicines Agency – EMA) and American (Food and Drug Administration – FDA) regulatory entities have recently come forward with a different definitions of cocrystals. In the draft document “Regulatory Classification of Pharmaceutical Cocrystals – Guidance for Industry (August, 2016)”, FDA states that “cocrystals are crystalline materials composed of two or more different molecules within the same crystal lattice that are associated by non-ionic and noncovalent bonds”.(49) In the document “Reflection paper on the use of cocrystals of active substances in medicinal products (May 2015)”, EMA reveals a further definition stating that “cocrystals are in general defined as homogenous (single phase) crystalline structures made up of two or more components in a definite stoichiometric ratio where the arrangement in the crystal lattice is not based on ionic bonds (as with salts). The components of a cocrystal may nevertheless be neutral as well as ionized.”(50) Although it has not been clearly stated in conclusive definitions by neither of the two regulatory entities, it is also important to consider that from a need to distinguish between other multiple-component crystalline materials (i.e., solvates and hydrates), cocrystals are comprised of two or more compounds that are solid under ambient conditions.(48)
To solve the salts-cocrystals continuum, both regulatory entities agree in referring the extent of proton transfer as differentiation motif. While cocrystals (whether neutral, acidic or basic) are held together by weaker interactions, such as hydrogen bonding (H-bonding), pi-pi-stacking or van der Waals interactions, salts are typically formed by acid-base reactions. Although the ionization state can generally be evaluated by the difference between the pKa values (∆pKa) of the acid and the conjugated base, EMA states that:
there is no strict borderline between incomplete (salt formation) and complete (cocrystal formation) proton transfer and
crystalline forms cannot be clearly differentiated from one another by the ionization state of the individual components.(50)
Differently, FDA states that, in general, if ∆pKa>1 salt formation is at hand, but if ∆pKa<1 there will be less than substantial proton transfer and therefore cocrystals are formed.(49) Besides regulatory acknowledgments, others have stated a “rule of thumb” that when the ∆pKa is greater than 2.7-3 units, salt formation is expected rather than cocrystals.(20,37)
Regarding the definitions from a pharmaceutical point of view, a pharmaceutical cocrystal (PCoC) convenes at least one API and one or more cocrystal former (or coformer) in the crystal lattice, being all components solid at room temperature.(50–52) Coformers chosen for PCoC development must be generally recognized as safe (GRAS) compounds, i.e., substances that are adequately recognized as safe for the intended use, as well as not affecting the API's pharmacological activity.(49,50,53)
A research in major search engines (“google.com”, “b-on.pt”,
“ncbi.nlm.nih.gov/pubmed” and “sciencedirect.com”) aiming at better understanding the evolution of the concepts “cocrystals” and “pharmaceutical cocrystals” showed that scientific interest in this field increased significantly over the last three decades (Figure 3, data assessed on 20/01/2017).
9 This increasing research activity on cocrystals is understandable as this unique crystal engineering approach(54–56) has already proven many advantages, mainly in the enhancement of APIs solubility profiles, which directly impacts drug dissolution and bioavailability.(6,20,57–71) Also, the fact that, theoretically, all types of APIs are capable of forming cocrystals (including non-ionisable ones) combined with the availability of a very large number of coformers (including other APIs), makes cocrystallization a flexible technique to fine-tune drugs material properties.(72) Cocrystallization can additionally be used for physical properties improvements. Manipulation of thermodynamics, admitting control of thermally labile APIs melting point(73), and of hydrodynamic stability, allowing enhancement of moisture-sensitive pharmaceutical materials(73,74), have been referred as concrete possibilities through cocrystallization. In this scope, ameliorated manufacturability of APIs is another advantage conceived to cocrystals, especially as these systems might be used APIs isolation and purification strategies during processing, being the coformer later discarded prior to formulation. This can be conveniently applied to chiral resolutions(75), using homochiral coformers to enantiomers separation.(55) As
Figure 3 – Evolution of the number of publications on “cocrystals” and “pharmaceutical cocrystals” over the decades 1960-2017(present) posted on four different search engines: (a) <google.com>, (b) <b-on.pt>, (c) <ncbi.nlm.nih.gov/pubmed> and (d) <sciencedirect.com>
10 taste-masking formulation alternative, selection of saccharin as coformer has also demonstrated advantageous characteristics through several studies.(51,76–79)
Another convenient leverage that cocrystallization has to offer to the pharmaceutical industry is the possibility of intellectual property extension, as new patents can be granted to the same API if the cocrystal product demonstrates the three patent awarding obligations: novelty, utility and non-obviousness. As a result, cocrystallization can be regarded as a commercial advantage(80), but the lack of consensus within the world regulatory agencies concerning cocrystals classification and potential applications is discouraging industry stakeholders.(81)
Other mentioned disadvantages are the fact that solubility and bioavailability studies involving PCoCs are still in their inception(82,83) and the inability to accurately predict the outcome of crystallization processes, requiring complex experimental screening during development.(72,84–87) These, however, can be considered erratic and changeable
disadvantages, as time and technological advancements are optimistic considerations for cocrystals.
Regulatory framework
Besides differences in terminology between “cocrystals” (EMA) and “co-crystals” (FDA), the two regulatory entities do not meet in all considerations (classification, identity, identity, industry position and manufacturing) attended for the referred crystalline solid form.
Classification wise, in its first released reflection paper (2013) FDA classifies PCoCs as drug product intermediates as those solid forms are nothing more than the association of an API with a pharmaceutical excipient, designed to achieve the best performance characteristics that ensure drug product (DP) quality; therefore, the identity of a PCoC when compared to its precursor API is unchanged, regardless of its effect on efficacy and/or safety.(88) Two years later (2015), EMA releases a reflection paper where PCoCs are classified as new active substances (NAS) unless they are demonstrated to be equal with respect to efficacy and/or safety to an already authorised product, i.e., PCoCs are classified as APIs; here, the identity of a PCoC is directly related to its effect on efficacy and/or safety. In this paper, EMA not only states cocrystals regulatory definitions and demands, but also criticizes cocrystals definition limited to the allegation that precursors should be solid under ambient conditions, referring that potential coformers but also APIs may be liquids at room temperature (e.g., valproic acid). In consequence, solvates and hydrates status is reintegrated as a subgroup of cocrystals where solvents, including water, serve as coformers (liquid coformers).(50)
Subsequently, as the first decision generated significant public discussion discouraging manufacturers to develop cocrystal-based products, FDA relaunched a new perspective paper (2016) relating PCoCs as analogous to a polymorph of the API (49), thus referring that the identity of each PCoC must be assessed case-by-case regarding drug product stability and bioequivalence assays demonstrated by the abbreviated new drug application (ANDA) applicant.(89)
Classification and identity divergences imposed by the aforementioned regulatory entities have decisive impact on the industry position in relation to cocrystals manufacturing. On the one hand, the EMA guidance leads PCoCs to be manufactured as APIs, maybe proposing the incorporation of cocrystallization processes at the end of API
11 synthesis (API manufacturing sites); on the other hand, the FDA reported point manufacture of PCoCs as drug product intermediates (or as APIs), dealing cocrystallization processes during drug product manufacture (drug product manufacturing site) but also possible to accommodate at the end of API synthesis (API manufacturing site) (see Figure 4). Manufacturers tend to be more amiable to EMA’s regulatory approach but it is not conceivable to adequate a different cocrystals manufacturing strategy to different regulatory demands.(90,91) This uncertainty hinders worldwide development of cocrystal-based products as drug developers prefer to take a “wait-and-see” approach facing a rather increasing risk associated with the actual application of this type of solid forms.
Design and coformer selection
Design of PCoCs can be defined as the combination of strategies theoretically and experimentally employed to plan and determine the best cocrystal product that can be achieved for a given system of precursors. It is a multifactorial rational approach that must be initiated by complete and exhaustive characterization of the API followed by coformer selection (with respective evaluation of coformer critical quality attributes (CQAs) and stoichiometric adequacy) and cocrystallization method selection (with concrete definition of the adequate method’s critical process parameters (CPPs) and rational solvent selection, if needed) (Figure 5).
Figure 4 – Graphic illustration of industry indecision concerning manufacturing allocation of pharmaceutical cocrystals (regulatory uncertainty)
12 Rational coformer selection must be integrated through crystal engineering understanding. This concept was first introduced by Pepinsky in 1955(92), being later applied in the context of organic solid-sate photochemical reactions (by Schmidt)(93). In later 1980s and early 1990s, the concept gained further interest in the sciences field with Desiraju defining crystal engineering as a mean to understand the intermolecular interactions contextualized in crystal packing, which could be used in the design and modulation of new solid forms with fine-tuned properties.(94) This definition lent crystal engineering in the path of supramolecular synthesis of new compounds, which in turn constitutes the fundamental origin of rational design and development of cocrystals. Etter also proposed that rational synthesis of cocrystals can be achieved by combining the knowledge of geometric analysis (or graph-set analysis(95)) and the so called “hydrogen-bond rules”, which state that:
H-bonding is created in the presence of all good proton donors and acceptors; intramolecular H-bonds will preferably form instead of intermolecular H-bonds and in a given crystal structure, the best donor pairs with the best acceptor.(19,40,96) From this basis, a crystal of an organic compound is the ultimate supermolecule and its assembly is achieved connecting individual molecules through H-bonds, according to definite chemical and geometric factors. Supramolecular synthons are crystal’s “building-blocks” defined as the structural units that comprehend the pattern of interaction in a crystal lattice. They can further be classified as:
Supramolecular homosynthons, if identical self-complementary functional groups are combined (Figure 6 (a));
Supramolecular heterosynthons, if different but complementary functional groups are combined (Figure 6 (b)).(20,97)
Supramolecular synthons recognition is based on the determination of specific intermolecular interactions like hydrogen or halogen bonds, stacking interactions, etc. This approach can be very efficient for relatively strong interactions, but comparison of strengths between different types of specific intermolecular interactions (e.g., weak H-bonds, van der Waals forces) can be almost impossible due to the dependence of the energy of interactions on interatomic distances in different types of intermolecular interactions. Also, in the case of big molecules, the contribution of general dispersion and electrostatic interactions to the total energy of interaction may be comparable with the energy of specific interactions. This problem can only be solved by application of comparative quantum-chemical calculations, determining three-dimensionally the Basic Structural Motifs (BSM) of a crystal. Through this point of view, supramolecular synthons can be classified into four groups:
local or basic, leading to the formation of molecular complexes; Figure 6 – Examples of supramolecular synthons: (a) homosynthon formed between two carboxylic acid groups, (b) heterosynthon formed between one carboxylic acid group and one amide group
13 primary, responsible for the formation of a BSM of a crystal;
secondary, providing the packing of BSMs and
auxiliary, depending on energies of the interactions between molecules.(98)
From this crystal engineering considerations, theoretical models to cocrystals design are typically based on the hierarchy of supramolecular synthons, creating virtual-screening analysis on the propensities of precursors to be drawn together through H-bonds, derived from molecular electrostatic potential surface calculations. The most common functional groups probed as cocrystals synthons are depicted in Table 2.(33)
Table 2 – Most common functional groups used for formation of supramolecular synthons by H-bonding
Functional groups Typical supramolecular synthons used
in crystal engineering Carboxylic acid
(e.g., acetic acid, benzoic acid,
fumaric acid, maleic acid, malonic acid)
Amides
(e.g., nicotinamide and urea)
Amines
Alcohols
Another theoretical approach includes the evaluation of the Hansen Solubility Parameters (HSP).(20,99,100) HSP predicts the miscibility of the precursors based on the premise that materials with similar solubility parameters are miscible. In this concept, the total cohesive energy is divided into three individual components: dispersion, polar and H-bonding. For what concerns cocrystals design, HSP are used for comparing the distance between two molecules (Ra). Ra is a measure of likeliness, as the smaller the Ra the more likely two molecules are to be compatible. The solubility space or Relative Energy Difference (RED) is given by combining Ra with the interaction radius with the solvent, thus:
If RED<1, the molecules are alike and will dissolve; If RED=1, the system will partially dissolve and If RED>1, the system will not dissolve.(101)
On another perspective, statistical models can be built on Cambridge Structural Database (CSD) researches. CSD is a repository for organic or metal-organic small molecules crystal structures solved by single-crystal x-ray crystallography. This online platform permits the search for ordered, error-free organic crystal structures, filtering out duplicates and unreliable or incomplete structures, lending the remaining structures for
14 further molecular statistical analysis in external software. Here, distribution models are constructed on parametric and nonparametric correlation coefficients between pairs of descriptors (e.g., simple atoms, bonds, donors and acceptors, surface area descriptors, electrostatic descriptors), from which results the most probable final cocrystal structure.(20,99,100)
The aforementioned methods can be used individually or combined as strategies to cocrystals design and coformer selection. However, all of them depend on “trial-and-error” procedures which can be very complex and time-consuming. Besides, all the methods rely either on theoretical calculations or on previous deposited knowledge and may leave out of trial coformers that could actually work. Presently, coformer selection should be rationally approached in a short-listing of potentials manner but prediction of whether cocrystallization will or not occur is not possible and must be answered empirically.
Characterization methods
In cocrystallization studies, it is both necessary to confirm complete phase transformation and to characterize the cocrystal product. For those reasons, application of adequate characterization methods is of paramount importance for selection the most promising cocrystal candidates with respect to pharmaceutical application (Figure 7). Typical cocrystal characterization techniques include powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), vibrational spectroscopy, microscopy, morphology assessment methods and chromatographic techniques. Table 3 summarizes the mentioned characterization techniques and corresponding evaluated characteristics, which will be hereinafter explained in the present chapter.
Table 3 – Cocrystals characterization techniques and corresponding evaluated characteristics
Characterization technique Evaluated characteristic X-ray Powder Diffraction New phase formation, purity, crystallinity
Differential Scanning Calorimetry Melting-point, purity, crystallinity, polymorphism
Variable-Temperature X-ray Powder Diffraction Temperature of complete cocrystallization and polymorphic forms evaluation
Vibrational Spectroscopy Near-infrared Mid-infrared
RAMAN
Molecular bounds, polymorphism
Microscopy
Scanning Electron Microscopy Morphology of cocrystal particles Dynamic Light Scattering Particle size distribution
Chromatographic methods
High Performance Liquid Chromatography
15 Confirmation of complete phase transformation, i.e. complete transformation of the API:coformer physical mixture into cocrystals, is attained by comparison of precursors, physical mixture and cocrystallization product characteristics. PXRD (53,60,62) and DSC (53,61,102,103) are the most used characterization methods in the cocrystals field. PXRD is a fundamental and powerful technique for solid-state characterization, allowing unambiguous crystal structure determination in a relatively rapid manner; however, is a single-crystal cannot be obtained, complex indexing procedures are needed, taking time and specific qualifications to full use this technique, either from comparative diffractograms analysis.
DSC is a thermoanalytical technique which application to cocrystals characterization is mainly based on melting-point determination, but, if used in repeated cycles, it can also evaluate cocrystals thermodynamic stability. Combination of XRPD with thermal analysis, also known as variable-temperature x-ray powder diffraction (VT-XRPD), have as well been tested revealing interesting information on the temperature required for complete cocrystallization and determination of polymorphic forms of cocrystals.(104,105)
Figure 7 – Selection of a cocrystals candidate typical decision tree. Adapted from (Schulteiss N., Newman A., 2009)(254)
