Tuning Diffusion Permeability of an Anti-Retroviral Drug, Emtricitabine, via Multicomponent Crystallizations

Emtricitabine (ECB) is an anti-retroviral drug that inhibits HIV reverse transcriptase and prevents transcription of RNA to DNA. ECB exhibits high solubility and low permeability (log P < 0). To modify the diffusion behavior of ECB, a high throughput cocrystal screening has been carried out with coformers that contain carboxylic acid/amide functionalities via solvent assisted grinding. The screening study resulted in the formation of cocrystals with benzoic acid (BA), caprolactam (CPR), and salts with 2,6-dihydroxybenzoic acid (DHBA), malonic acid (MLN), maleic acid (MLE), and saccharin (SAC), which were confirmed with single crystal X-ray diffraction. In addition, ¹⁵N solid state NMR spectroscopy was exploited to define the ionization state of the multicomponent systems. The 2-aminopyrimidine homodimer of the cytosine analogue in the ECB is replaced by aminopyrimidine···carboxylic acid/amide in the cocrystals and aminopyrimidinium···carboxylate/saccharinate heterosynthons in the salts. The terminal hydroxyl group of the ECB forms a hydrogen bond with its carbonyl group, which is consistent in the ECB–BA cocrystal, ECB–DHBA and ECB–MLN salts. In addition, the hydroxyl group of ECB is hydrogen bonded with the relatively stronger acceptors like the carbonyl/sulfonyl group of caprolactam, maleate, and saccharinate in their corresponding multicomponent crystals. The diffusion studies of ECB multicomponent crystals using a Franz diffusion cell suggest that the ECB–BA cocrystal exhibited an enhanced diffusion and flux compared to that of native drug and other multicomponent crystals. An inverse correlation was observed partially between the flux values with crystal densities and binding energies of the ECB multicomponent systems.