Nitazoxanide

In pharmaceutical terms, stability refers to the storage time that can elapse before any degradation product(s) reach a level sufficient to present a risk to the patient. One approach to evaluate the stability of a drug is the use of stress tests; stress tests comprise a set of assays aimed at facilitating the development of analytical methodology. The results of these assays provide a better understanding of the active pharmaceutical ingredient and of the drug’s stability, as well as information about the degradation pathways and degradation products. There have been reports published on the stability of many drugs, including antihistamines such as fexofenadine, antihypertensive agentsmsuch as enalapril maleate, anti-inflammatory agents such as piroxicam, antiparasitic agents such as nitazoxanide, anti-emetic agents such as alizapride and antidepressant drugs such as citalopram, in addition to other classes of drugs. A review of the literature revealed that a large number of methods have been reported for the analysis of the stability of antifungal compounds belonging to the chemical class of azoles, a large group of synthetic compounds that includes imidazoles and triazoles. All azoles operate via a common mode of action: they prevent the synthesis of ergosterol, the major sterol component of the fungal plasma membrane, through inhibition of the fungal cytochrome P450-dependent enzyme lanosterol 14-a-demethylase.

The previously reported organoruthenium anticancer compounds did not exhibit significant in vitro activity against E. multilocularis, nor did the first set of newly synthesised compounds (20a–c, Fig. 15). However, the three variants of compound 21 did show significant activity, with 21a (EC50 1.4 μM) proving three times more active than 21b and 21c (EC50 4.2–4.7 μM) and with comparable activity to the reference drug, nitazoxanide (EC50 1.2 μM). Unfortunately, the compounds were found to be toxic to human fibroblasts, though the group is hopeful that this may not translate in further in vivo studies. Though the mechanism of action for the complexes is still undetermined, it was found that the activity of the compounds against E. multilocularis increased with hydrolytic stability, or the ability to resist hydrolysis. This likely leaves the compounds more lipophilic and therefore more readily taken up into parasitic cell membranes. Assertions were made about possible mechanisms of action, including the ability of the phosphite and ^tBu₂acac ligands to form a ‘lipophilic sphere’ around the ruthenium centre, protecting the charged Ru²⁺ from forming aqua complexes. Bearing this in mind, it was proposed that the variants of complex 21 likely crossed the cell membrane and exerted their action on an intracellular target. Given that the ruthenium centre is able to donate π-electrons into both ligands, it was also asserted that the complex could lose its p-cymene ligand as a result, opening up sites at the ruthenium centre to possibly coordinate to the DNA of the parasite.

The prodrug nitazoxanide is deacetylated to tizoxanide (TIZ) as the active metabolite which transforms in vivo to the inactive 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. NTZ has poor aqueous solubility (7.55 mgmL^-1in water) and even lower bioavailability (258 ng mL^-1, 7.5 mg kg^-1 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. We report two isomorphous cocrystals (drug–drug pharmaceuticals) of NTZ with PASAandPABA(1:1stoichiometry) 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 multi-component pharmaceutical solids as improved medicines. The supramolecular approach (Scheme 2) shows that binary isomorphous cocrystals may be ground in a multivariate stoichiometricratiotogivecocrystalalloys.