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Cocrystals and alloys of nitazoxanide: enhanced pharmacokinetics Kuthuru Suresh,a M. K. Chaitanya Mannava,b and Ashwini Nangia*a,b Received 00th January 20xx, Accepted 00th January 20xx
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Two isomorphous cocrystals of nitazoxanide (NTZ) with paminosalicylic acid (PASA) and p-aminobenzoic acid (PABA) as well as their alloys were prepared by slurry and grinding techniques. The cocrystals exhibit faster dissolution rate and higher pharmacokinetic properties compared to the reference drug, and surprisingly the cocrystal alloy NTZ-PABA : NTZ-PASA (0.75:0.25) exhibited 4 fold higher bioavailability of NTZ in Sprague Dawley rats. This study opens the opportunity for cocrystal alloys as improved medicines. The principal advantages of cocrystals in improving the physical and chemical stability, hygroscopicity, tabletability, increasing the dissolution rate and solubility, and enhanced bioavailability of Active Pharmaceutical Ingredients (APIs) were highlighted recently.1-3 Apart from fixed stoichiometry cocrystals, there are relatively few examples of multi-component solids forming solid 4-7 solutions and alloys in organic systems involving drug molecules. Furthermore, there is no report on the pharmaceutical utility of 8 such cocrystal alloys, though eutectics are reported. The difference between cocrystal, eutectic and cocrystal alloy for the discussion in this paper is that while they all typically have binary components, the stoichiometry is fixed in cocrystal and eutectic but can vary over a range (multiple stoichiometries) for cocrystal alloy (and solid solution). The molecular arrangement is distinct from the individual components and characterized by long range order in cocrystal, whilst in eutectic and cocrystal alloy the differences lie in the shortrange order only and at the limit of X-ray diffraction to detect such changes. The latter microstructures are indistinguishable at long range by Bragg reflections. A cocrystal alloy may be defined as a single crystalline phase comprising a mixture of two or more isomorphous/ isostructural cocrystals, also referred to as multivariate stoichiometry cocrystals. Whereas cocrystals and
eutectics are known to make drug-drug combinations for improvement in pharmacokinetic and pharmacodynamic properties,9,10 we report in this paper the first example of a cocrystal alloy (combination of two cocrystals of a drug in variable stoichiometry) which exhibits enhanced bioavailability in SD rats compared to the individual cocrystals NTZ-PASA and NTZ-PABA, for the anti-protozoal drug nitazoxanide. The prodrug Nitazoxanide is deacetylated to Tizoxanide (TIZ) as the active metabolite which transforms in vivo to the inactive 11 metabolite glucuronide ether (Scheme 1). In a screening program to evaluate the activity of known chemical entities against Mycobacterium tuberculosis, Nathan et al. discovered that nitazoxanide is a promising drug candidate for multi-drug resistant bacteria.12 NTZ has poor aqueous solubility (7.55 µg/mL in water)13 14 and even lower bioavailability(258 ng/mL, 7.5 mg/kg dose in rats). A Cambridge Structural Database search of NTZ gave four hits, of which one is a guest free form and three are cocrystals.15,16 We report two isomorphous cocrystals (drug-drug pharmaceuticals) of NTZ with PASA and PABA (1:1 stoichiometry) as well as solid solutions of cocrystal alloys NTZ-PABA : NTZ-PASA of 0.75:0.25 and 0.67:0.33 composition. Our solubility and pharmacokinetic data validate the need to explore cocrystal alloys as a new class of multicomponent pharmaceutical solids as improved medicines. The supramolecular approach (Scheme 2) shows that binary isomorphous cocrystals may be ground in multivariate stoichiometric ratio to give cocrystal alloys. OH HO
CH3 O
O
NO2 O
S N H
NTZ
OH
NO2 OH
O
N
N H
Deacetylation
NO2
HO
S
O
O
O
S
O
N
N H
+
Glucuronidation
TIZ
TIZ Glucuronide
Scheme 1 Biochemical transformation of NTZ prodrug to the active metabolite TIZ and TIZ glucuronide.
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Journal Name Figure 1 Two adjacent non-equivalent NTZ molecules are connected via CH∙∙∙N interaction in a dimeric R22(6) ring motif.
Scheme 2 Schematic representation of isomorphous cocrystals. Both cocrystals 1 and 2 can result in cocrystal alloys.
NTZ is a functionally rich molecule (five acceptors: amide carbonyl O, thiazolidine N, ester C=O, nitro O atoms; one donor: amide NH) capable to form cocrystals with COOH, NH2, and OH functional 17 The cocrystal group-containing coformers of GRAS status. structures of NTZ-PASA and NTZ-PABA (1:1) were established by single crystal X-ray diffraction (see crystallographic data in Table S1 and hydrogen bonds in Table S2, ESI). The similarity in unit cell parameters, isostructurality index and XPac analysis of the two cocrystal structures are explained in Figure S1 (see ESI for details). Based on the near identity of the two structures, which differ by an OH/H group only in coformer of PASA/ PABA, the possibility of pharmaceutical cocrystal alloys for NTZ-PASA and NTZ-PABA was explored in different stoichiometric ratios. The integrity of the cocrystal alloy was established by a low R factor achieved upon assigning the O atom of PASA variable occupancy in the least squares refinement cycles. The product cocrystal alloys (CA) have the stoichiometry 0.67 : 0.33 for CA1 and 0.75 : 0.25 for CA2 of NTZPABA : NTZ-PASA by X-ray diffraction (Table S1). The preparation and characterizaIon of cocrystals and alloys is described in ESI†.
Figure 2 Sheet structure of NTZ-PABA is sustained by R22(8) dimer synthon and N−H∙∙∙O hydrogen bonds.
The crystal structure of NTZ-PASA (1:1 in space group P-1 with similar unit cell parameters to those of NTZ-PABA) contains the heterodimer of N−H∙∙∙O and O−H∙∙∙N hydrogen bonds (2.00 Å, 162°; 2 1.88 Å, 169°) in R2 (8) ring motif, which are connected via aminonitro H bond (2.19 Å, 162°) (Figure 3). A reason for the isostructurality between the two cocrystal structures by exchange 18 of H/ OH groups is the intramolecular H bond in PASA, which in effect makes the OH donor engaged in a S(6) motif (1.90 Å), and buried inside the hydrogen bonded network.
The reported crystal structure15 of NTZ (in space group Pna21) shows two symmetry-independent molecules constrained in a rigid conformation by intramolecular N-H∙∙∙O hydrogen bond (2.03 Å, 132° and 2.01 Å, 131°; S(6) graph set) and chalcogen S∙∙∙O interactions (2.61 Å, 160°and 2.64 Å, 159°; S(5) graph set). Two molecules are connected by C−H∙∙∙N dimer R22(6) motif (2.74 Å, 142°; Figure 1). The overall structure of this nearly planar (except the acetyl group) molecule exists as a layered stacking.
Figure 3 Sheet structure of NTZ-PASA is sustained by R22(8) dimer synthon and N−H∙∙∙O hydrogen bonds.
Isostructurality between the binary pharmaceutical cocrystals encouraged us to make combinations in variable stoichiometry with the idea that (1) this will lead to novel multi-component systems with a different stoichiometry of NTZ and coformers, and (2) that
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The crystal structure of NTZ-PABA (1:1 in space group P-1) is sustained by the robust heterodimer of amidothiazole-acid groups via N−H∙∙∙O and O−H∙∙∙N hydrogen bonds (2.00 Å, 168°; 1.93 Å, 2 174°) in R2 (8) ring (Figure 2). Such dimeric ring motifs are arranged in a 1D tape via amino N−H∙∙∙O to nitro acceptor hydrogen bond (2.22 Å, 169°).
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both coformers being present in the same crystalline solid could lead to unusual properties for NTZ. We were guided by recent reports wherein organic alloys exhibit advanced functional behavior 19,20 in solar cells and semiconductors. Because the H atom is smaller than an OH group, a solid solution or alloy of two cocrystals NTZPABA and NTZ-PASA will have a higher molar ratio of PABA cocrystal than that of PASA cocrystal. The stoichiometry of CA1 and CA2 obtained by refining the occupancy of electron density at the H/ OH site in the crystal structure gave the lowest R-factor for NTZ-PABA : NTZ-PASA composition as 0.67 : 0.33 (CA1, Rf = 0.0593) and 0.75 : 0.25 (CA2, R = 0.0781); the cocrystal structures R-factor is 0.0478, 0.0487. The crystal structures of CA1 and CA2 are similar to those of the individual cocrystals (Figure S2, Table S1 and S2). Additionally, in both cocrystals and cocrystal alloy structures, slight conformational changes were observed due to reorganization of the homomeric interactions of NTZ (Figure S3, see ESI for details). PXRD and DSC of the bulk materials are displayed in Figure 4 and 5. Their melting point and FT-IR peaks are listed in ESI† (Table S3, and FT-IR spectra Figure S4, Table S4). The lower Tonset for CA1 and CA2 in DSC compared to the cocrystals means that they are different in terms of chemical composition and the single peak endotherm is indicative of a unique phase. Given the purpose of kinetic enhancement in drug solubility (or apparent solubility) for metastable phases such as polymorphs, cocrystals and eutectics,21,8 intrinsic dissolution rate (IDR) measurements were performed in 3% CTAB (cetyltrimethyl ammonium bromide) phosphate buffer (pH 7) medium for 240 minutes by the rotating disk intrinsic dissolution rate (DIDR) method at 37 °C. Interestingly, the cocrystals exhibit higher IDR than pure NTZ (which is common) but surprisingly the cocrystal alloys are even superior to the cocrystals (this is unexpected). The dissolution rate order is CA2 > CA1 > NTZ-PABA > NTZ-PASA (Figure 6, Table S5). The general observation that the higher solubility coformer (PABA, 6.1 mg/mL) gives higher dissolution rate for the cocrystal is followed in this system. Secondly, the melting point of the alloys is lower than those for the cocrystals, and this is another reason for higher solubility. Generally, for crystal forms of the same composition, or nearly the same composition, the lower melting solid has higher solubility. Even though the cocrystals and alloys are isomorphous/ isostructural, their dissolution rate is remarkably different. There is a similar report for isostructural cocrystals exhibiting different thermodynamic stability as explosives.22
Figure 5 DSC thermograms of NTZ cocrystals and alloys exhibit single endotherm melting behavior. The exotherm above 200 °C is due to decomposition of coformer
Figure 4 Rietveld refinement of experimental PXRD pattern of NTZ cocrystals and cocrystal alloys (black) showed good match with their calculated line profile from the X-ray crystal structure (red) indicating bulk purity and phase homogeneity.
Figure 6 IDR of NTZ cocrystals and alloys in 3% CTAB (pH 7) buffer.
Next the pharmacokinetics was measured by oral administration of NTZ cocrystals/ alloys (45 mg/kg active drug, equivalent 500 mg human dosage) to Sprague Dwaley rats. The Cmax and AUC of CA2 are far superior to those for CA1 and cocrystals (Figure 7, Table 1). CA2 exhibits short Tmax (30 min) and highest Cmax 7.6 µg/mL compared to the other 3 crystal forms. The AUC(0-12) (total drug delivered in 12 h) is 24.2 µg.hr/mL for CA2 compared to 7.5 µg.hr/mL for NTZ (it transforms to TIZ in vivo).
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10. 11. 12.
Figure 7 Peak and mean plasma concentration of TIZ vs. Time profile for NTZ cocrystals and alloys. NTZ transforms to TIZ in vivo.
13. 14. 15.
Table 1 Pharmacokinetic parameters of NTZ cocrystals and alloys. SD for 6 readings is given in parenthesis.
16.
Solid form
NTZ
Parameter Tmax (min) Cmax (µg/mL) T1/2 (h) AUC(0-12) (µg.hr/mL) AUC(0-∞) (µg.hr/mL)
NTZPASA n=6
CA1 (0.67-0.33) n=6
CA2 (0.75-0.25) n=6
17.
n=6
NTZPABA n=6
90 (0.42) 1.88 (0.91) 3.85 (0.29) 7.47 (1.07) 42.50 (6.89)
90 (0.55) 4.32 (0.30) 4.20 (0.90) 17.06 (3.80) 107.2 (22.15)
30 (0.20) 3.45 (0.74) 3.68 (0.95) 7.89 (1.13) 43.17 (9.66)
30 (0.26) 3.45 (0.44) 4.57 (0.60) 10.01 (2.58) 67.62 (21.95)
30 (0.00) 7.58 (1.21) 2.96 (0.67) 24.18 (2.14) 105.27 (36.80)
18. 19. 20.
21. 22.
T. Friščić, A.V. Trask, W.Jones and W. D. S. Motherwell, Angew. Chem. Int. Ed., 2006, 45, 7546–7550. A. A. Bredikhin, Z. A. Bredikhina, D. V. Zakharychev, A. T. Gubaidullin and R. R. Fayzullin, CrystEngComm, 2012, 14, 648–655. K. D. Prasad, S. Cherukuvada, L. D. Stephen and T.N. Guru Row, CrystEngComm, 2014, 16, 9930–9938. S. Cherukuvada and A. Nangia Chem. Commun., 2014, 58, 906–923. V. Ferretti, A. Dalpiaz, V. Bertolasi, L. Ferraro S. Beggiato, F. Spizzo, E. Spisni, and B. Pavan, Mol. Pharm., 2015, 12, 1501−1511. M. M. Buckley and P. Benfield, Drugs, 1993, 46, 126-151. G. Gargala, A. Delaunay, X. Li, P. Brasseur, L. Favennec and J. J. Ballet, J. Antimicrob. Chemother., 2000, 46, 57–60. L. Pedro S. D. Carvalho,G. Lin, X. Jiang and C. Nathan, J. Med. Chem., 2009, 52, 5789–5792. http://www.drugbank.ca/drugs/DB00507 M. I. Ruiz-Olmedo, J. L. Gallegos-Perez, K. G. Calderon-Gonzalez, J. Franco-Perez, H. Jung-Cook, Pharmazie, 2009, 64, 419–422. F. P. Bruno, M. R. Caira, G. A. Monti, D. E. Kassuha, N. R. Sperandeo, J. Mol. Str., 2010, 984, 51–57. B. C. Félix-Sonda,J. Rivera-Islas,D. Herrera-Ruiz,H. Morales-Rojas, andH. Höpfl, Cryst. Growth Des., 2014, 14, 1086–1102. Generally Regarded as Safe chemicals by the US-FDA. http://www.fda.gov/Food/IngredientsPackagingLabeling/FoodAdditive sIngredients/ucm091048.htm A, Anthony, M. Jaskólski, A. Nangia and G. R. Desiraju, Chem. Commun., 1998, 2537–2538. J. Zhang,Y. Zhang,J. Fang,K. Lu,Z. Wang,W. Ma, andZ. Wei. J. Am. Chem. Soc., 2015, 137, 8176–8183. J. B. Sherman, K. Moncino, T. Baruah, G. Wu, S. R. Parkin, B. Purushothaman, R. Zope, J. Anthony and M. L. Chabinyc. J. Phys. Chem. C, 2015, 119, 20823–20832. N. J. Babu and A. Nangia, Cryst. Growth Des., 2011, 11, 2662–2679. K. B. Landenberger,O.Bolton and A. J. Matzger, Angew. Chem. Int. Ed., 2013, 52, 6468–6471.
The present study shows that non-stoichiometric multi-component solid forms present yet another opportunity for oral drug bioavailability alongside the stoichiometric cocrystals and salts. These results open opportunities for the repositioning of nitazoxanide as a multi-strain resistant anti-TB drug with improved bioavailability. This research was funded by JC Bose Fellowship (SR/S2/JCB06/2009), CSIR project on Pharmaceutical polymorphs and cocrystals (02(0223)/15/EMR-II), and SERB scheme on Multicomponent cocrystals (EMR/2015/002075). DST-IRPHA, DST-FIST, UGC-PURSE and UPE-II are thanked for providing instrumentation and infrastructure facilities. KS thanks UGC for research fellowship. MKCM thanks Crystalin Research, Hyderabad, for their support, and Dr. Durga Bhavani, Head of Animal Facility at Virchow Biotech, Hyderabad, for bioavailability experiments.
References 1. 2. 3. 4.
N. K. Duggirala, M. L. Perry, Ö. Almarsson and M. J. Zaworotko, Chem. Commun., 2016, 52, 640–655. S. Aitipamula, P. S. Chow and R. B. H. Tan, CrystEngComm,2014, 16, 3451–3465. J.W. Steed, Trends Pharmacol Sci., 2013, 34, 185–193. S. P. Thomas, R. Sathishkumar and T. N. G. Row Chem. Commun., 2015, 51, 14255–14258.
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Cocrystals and alloys of nitazoxanide: enhanced pharmacokinetics Kuthuru Suresh,a M. K. Chaitanya Mannava,b and Ashwini Nangia*a,b Received 00th January 20xx, Accepted 00th January 20xx
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Two isomorphous cocrystals of nitazoxanide (NTZ) with paminosalicylic acid (PASA) and p-aminobenzoic acid (PABA) as well as their alloys were prepared by slurry and grinding techniques. The cocrystals exhibit faster dissolution rate and higher pharmacokinetic properties compared to the reference drug, and surprisingly the cocrystal alloy NTZ-PABA : NTZ-PASA (0.75:0.25) exhibited 4 fold higher bioavailability of NTZ in Sprague Dawley rats. This study opens the opportunity for cocrystal alloys as improved medicines. The principal advantages of cocrystals in improving the physical and chemical stability, hygroscopicity, tabletability, increasing the dissolution rate and solubility, and enhanced bioavailability of Active Pharmaceutical Ingredients (APIs) were highlighted recently.1-3 Apart from fixed stoichiometry cocrystals, there are relatively few examples of multi-component solids forming solid 4-7 solutions and alloys in organic systems involving drug molecules. Furthermore, there is no report on the pharmaceutical utility of 8 such cocrystal alloys, though eutectics are reported. The difference between cocrystal, eutectic and cocrystal alloy for the discussion in this paper is that while they all typically have binary components, the stoichiometry is fixed in cocrystal and eutectic but can vary over a range (multiple stoichiometries) for cocrystal alloy (and solid solution). The molecular arrangement is distinct from the individual components and characterized by long range order in cocrystal, whilst in eutectic and cocrystal alloy the differences lie in the shortrange order only and at the limit of X-ray diffraction to detect such changes. The latter microstructures are indistinguishable at long range by Bragg reflections. A cocrystal alloy may be defined as a single crystalline phase comprising a mixture of two or more isomorphous/ isostructural cocrystals, also referred to as multivariate stoichiometry cocrystals. Whereas cocrystals and
eutectics are known to make drug-drug combinations for improvement in pharmacokinetic and pharmacodynamic properties,9,10 we report in this paper the first example of a cocrystal alloy (combination of two cocrystals of a drug in variable stoichiometry) which exhibits enhanced bioavailability in SD rats compared to the individual cocrystals NTZ-PASA and NTZ-PABA, for the anti-protozoal drug nitazoxanide. The prodrug Nitazoxanide is deacetylated to Tizoxanide (TIZ) as the active metabolite which transforms in vivo to the inactive 11 metabolite glucuronide ether (Scheme 1). In a screening program to evaluate the activity of known chemical entities against Mycobacterium tuberculosis, Nathan et al. discovered that nitazoxanide is a promising drug candidate for multi-drug resistant bacteria.12 NTZ has poor aqueous solubility (7.55 µg/mL in water)13 14 and even lower bioavailability(258 ng/mL, 7.5 mg/kg dose in rats). A Cambridge Structural Database search of NTZ gave four hits, of which one is a guest free form and three are cocrystals.15,16 We report two isomorphous cocrystals (drug-drug pharmaceuticals) of NTZ with PASA and PABA (1:1 stoichiometry) as well as solid solutions of cocrystal alloys NTZ-PABA : NTZ-PASA of 0.75:0.25 and 0.67:0.33 composition. Our solubility and pharmacokinetic data validate the need to explore cocrystal alloys as a new class of multicomponent pharmaceutical solids as improved medicines. The supramolecular approach (Scheme 2) shows that binary isomorphous cocrystals may be ground in multivariate stoichiometric ratio to give cocrystal alloys. OH HO
CH3 O
O
NO2 O
S N H
NTZ
OH
NO2 OH
O
N
N H
Deacetylation
NO2
HO
S
O
O
O
S
O
N
N H
+
Glucuronidation
TIZ
TIZ Glucuronide
Scheme 1 Biochemical transformation of NTZ prodrug to the active metabolite TIZ and TIZ glucuronide.
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Journal Name Figure 1 Two adjacent non-equivalent NTZ molecules are connected via CH∙∙∙N interaction in a dimeric R22(6) ring motif.
Scheme 2 Schematic representation of isomorphous cocrystals. Both cocrystals 1 and 2 can result in cocrystal alloys.
NTZ is a functionally rich molecule (five acceptors: amide carbonyl O, thiazolidine N, ester C=O, nitro O atoms; one donor: amide NH) capable to form cocrystals with COOH, NH2, and OH functional 17 The cocrystal group-containing coformers of GRAS status. structures of NTZ-PASA and NTZ-PABA (1:1) were established by single crystal X-ray diffraction (see crystallographic data in Table S1 and hydrogen bonds in Table S2, ESI). The similarity in unit cell parameters, isostructurality index and XPac analysis of the two cocrystal structures are explained in Figure S1 (see ESI for details). Based on the near identity of the two structures, which differ by an OH/H group only in coformer of PASA/ PABA, the possibility of pharmaceutical cocrystal alloys for NTZ-PASA and NTZ-PABA was explored in different stoichiometric ratios. The integrity of the cocrystal alloy was established by a low R factor achieved upon assigning the O atom of PASA variable occupancy in the least squares refinement cycles. The product cocrystal alloys (CA) have the stoichiometry 0.67 : 0.33 for CA1 and 0.75 : 0.25 for CA2 of NTZPABA : NTZ-PASA by X-ray diffraction (Table S1). The preparation and characterizaIon of cocrystals and alloys is described in ESI†.
Figure 2 Sheet structure of NTZ-PABA is sustained by R22(8) dimer synthon and N−H∙∙∙O hydrogen bonds.
The crystal structure of NTZ-PASA (1:1 in space group P-1 with similar unit cell parameters to those of NTZ-PABA) contains the heterodimer of N−H∙∙∙O and O−H∙∙∙N hydrogen bonds (2.00 Å, 162°; 2 1.88 Å, 169°) in R2 (8) ring motif, which are connected via aminonitro H bond (2.19 Å, 162°) (Figure 3). A reason for the isostructurality between the two cocrystal structures by exchange 18 of H/ OH groups is the intramolecular H bond in PASA, which in effect makes the OH donor engaged in a S(6) motif (1.90 Å), and buried inside the hydrogen bonded network.
The reported crystal structure15 of NTZ (in space group Pna21) shows two symmetry-independent molecules constrained in a rigid conformation by intramolecular N-H∙∙∙O hydrogen bond (2.03 Å, 132° and 2.01 Å, 131°; S(6) graph set) and chalcogen S∙∙∙O interactions (2.61 Å, 160°and 2.64 Å, 159°; S(5) graph set). Two molecules are connected by C−H∙∙∙N dimer R22(6) motif (2.74 Å, 142°; Figure 1). The overall structure of this nearly planar (except the acetyl group) molecule exists as a layered stacking.
Figure 3 Sheet structure of NTZ-PASA is sustained by R22(8) dimer synthon and N−H∙∙∙O hydrogen bonds.
Isostructurality between the binary pharmaceutical cocrystals encouraged us to make combinations in variable stoichiometry with the idea that (1) this will lead to novel multi-component systems with a different stoichiometry of NTZ and coformers, and (2) that
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The crystal structure of NTZ-PABA (1:1 in space group P-1) is sustained by the robust heterodimer of amidothiazole-acid groups via N−H∙∙∙O and O−H∙∙∙N hydrogen bonds (2.00 Å, 168°; 1.93 Å, 2 174°) in R2 (8) ring (Figure 2). Such dimeric ring motifs are arranged in a 1D tape via amino N−H∙∙∙O to nitro acceptor hydrogen bond (2.22 Å, 169°).
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both coformers being present in the same crystalline solid could lead to unusual properties for NTZ. We were guided by recent reports wherein organic alloys exhibit advanced functional behavior 19,20 in solar cells and semiconductors. Because the H atom is smaller than an OH group, a solid solution or alloy of two cocrystals NTZPABA and NTZ-PASA will have a higher molar ratio of PABA cocrystal than that of PASA cocrystal. The stoichiometry of CA1 and CA2 obtained by refining the occupancy of electron density at the H/ OH site in the crystal structure gave the lowest R-factor for NTZ-PABA : NTZ-PASA composition as 0.67 : 0.33 (CA1, Rf = 0.0593) and 0.75 : 0.25 (CA2, R = 0.0781); the cocrystal structures R-factor is 0.0478, 0.0487. The crystal structures of CA1 and CA2 are similar to those of the individual cocrystals (Figure S2, Table S1 and S2). Additionally, in both cocrystals and cocrystal alloy structures, slight conformational changes were observed due to reorganization of the homomeric interactions of NTZ (Figure S3, see ESI for details). PXRD and DSC of the bulk materials are displayed in Figure 4 and 5. Their melting point and FT-IR peaks are listed in ESI† (Table S3, and FT-IR spectra Figure S4, Table S4). The lower Tonset for CA1 and CA2 in DSC compared to the cocrystals means that they are different in terms of chemical composition and the single peak endotherm is indicative of a unique phase. Given the purpose of kinetic enhancement in drug solubility (or apparent solubility) for metastable phases such as polymorphs, cocrystals and eutectics,21,8 intrinsic dissolution rate (IDR) measurements were performed in 3% CTAB (cetyltrimethyl ammonium bromide) phosphate buffer (pH 7) medium for 240 minutes by the rotating disk intrinsic dissolution rate (DIDR) method at 37 °C. Interestingly, the cocrystals exhibit higher IDR than pure NTZ (which is common) but surprisingly the cocrystal alloys are even superior to the cocrystals (this is unexpected). The dissolution rate order is CA2 > CA1 > NTZ-PABA > NTZ-PASA (Figure 6, Table S5). The general observation that the higher solubility coformer (PABA, 6.1 mg/mL) gives higher dissolution rate for the cocrystal is followed in this system. Secondly, the melting point of the alloys is lower than those for the cocrystals, and this is another reason for higher solubility. Generally, for crystal forms of the same composition, or nearly the same composition, the lower melting solid has higher solubility. Even though the cocrystals and alloys are isomorphous/ isostructural, their dissolution rate is remarkably different. There is a similar report for isostructural cocrystals exhibiting different thermodynamic stability as explosives.22
Figure 5 DSC thermograms of NTZ cocrystals and alloys exhibit single endotherm melting behavior. The exotherm above 200 °C is due to decomposition of coformer
Figure 4 Rietveld refinement of experimental PXRD pattern of NTZ cocrystals and cocrystal alloys (black) showed good match with their calculated line profile from the X-ray crystal structure (red) indicating bulk purity and phase homogeneity.
Figure 6 IDR of NTZ cocrystals and alloys in 3% CTAB (pH 7) buffer.
Next the pharmacokinetics was measured by oral administration of NTZ cocrystals/ alloys (45 mg/kg active drug, equivalent 500 mg human dosage) to Sprague Dwaley rats. The Cmax and AUC of CA2 are far superior to those for CA1 and cocrystals (Figure 7, Table 1). CA2 exhibits short Tmax (30 min) and highest Cmax 7.6 µg/mL compared to the other 3 crystal forms. The AUC(0-12) (total drug delivered in 12 h) is 24.2 µg.hr/mL for CA2 compared to 7.5 µg.hr/mL for NTZ (it transforms to TIZ in vivo).
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10. 11. 12.
Figure 7 Peak and mean plasma concentration of TIZ vs. Time profile for NTZ cocrystals and alloys. NTZ transforms to TIZ in vivo.
13. 14. 15.
Table 1 Pharmacokinetic parameters of NTZ cocrystals and alloys. SD for 6 readings is given in parenthesis.
16.
Solid form
NTZ
Parameter Tmax (min) Cmax (µg/mL) T1/2 (h) AUC(0-12) (µg.hr/mL) AUC(0-∞) (µg.hr/mL)
NTZPASA n=6
CA1 (0.67-0.33) n=6
CA2 (0.75-0.25) n=6
17.
n=6
NTZPABA n=6
90 (0.42) 1.88 (0.91) 3.85 (0.29) 7.47 (1.07) 42.50 (6.89)
90 (0.55) 4.32 (0.30) 4.20 (0.90) 17.06 (3.80) 107.2 (22.15)
30 (0.20) 3.45 (0.74) 3.68 (0.95) 7.89 (1.13) 43.17 (9.66)
30 (0.26) 3.45 (0.44) 4.57 (0.60) 10.01 (2.58) 67.62 (21.95)
30 (0.00) 7.58 (1.21) 2.96 (0.67) 24.18 (2.14) 105.27 (36.80)
18. 19. 20.
21. 22.
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The present study shows that non-stoichiometric multi-component solid forms present yet another opportunity for oral drug bioavailability alongside the stoichiometric cocrystals and salts. These results open opportunities for the repositioning of nitazoxanide as a multi-strain resistant anti-TB drug with improved bioavailability. This research was funded by JC Bose Fellowship (SR/S2/JCB06/2009), CSIR project on Pharmaceutical polymorphs and cocrystals (02(0223)/15/EMR-II), and SERB scheme on Multicomponent cocrystals (EMR/2015/002075). DST-IRPHA, DST-FIST, UGC-PURSE and UPE-II are thanked for providing instrumentation and infrastructure facilities. KS thanks UGC for research fellowship. MKCM thanks Crystalin Research, Hyderabad, for their support, and Dr. Durga Bhavani, Head of Animal Facility at Virchow Biotech, Hyderabad, for bioavailability experiments.
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