Impact of crystal polymorphism of rifaximin on dissolution behavior

Rifaximin is an intestinal antiseptic which has five (pseudo) polymorphs α, β, γ, δ and ε. These last (pseudo)polymorphs have different physicochemical properties. The objective of the study is to assess the impact of rifaximin polymorphism on its dissolution rate which could affect its bioavailability.

The analytical validation of dissolution assay method by UV-Visible spectrophotometry was carried out according to ICH Q2. The physicochemical characterization (solubility test, FTIR, DSC, XRD) was carried out on four active pharmaceutical ingredient (MP1, MP2, MP3, MP4). MP1 and MP2 were used by the manufacturer of generic brand 1 (G1) and MP3 and MP4 were used by the manufacturer of generic brand 2 (G2). The comparative in-vitro dissolution study was carried out on the leader brand (P), G1 and G2.

The four MPs were analyzed by XRD. The results of analysis showed that MP1 and MP4 were a mixture of α form and amorphous form. MP2 had an amorphous form and MP3 had a crystalline form β. The spectra of FTIR showed that the four MP had characteristics bands of rifaximin in the domain 4000-400 cm^-1. The differences between the spectra of the four MPs were observed among the amorphous form (MP2), around the region 1800 to 1820 cm^-1 which is attributed to the vibration of the CO group. An additional difference observed among the amorphous form (MP2) is around the region 1400 cm^-1 which is attributed to the banding OH. The thermograms of MP1, MP2 and MP4 showed endothermic peaks which are probably attributed to the departure of water which indicate that MP1, MP2 and MP4 are pseudopolymoph (hydrate). For the four MPs, probably the melting points are interrupted by the phenomenon of phase transformations (Crystallization) which are reflected by exothermic peaks around 200°C-250 °C.Our results showed that the crystalline polymorphism of rifaximin influences its solubility. According to the results of the solubility test, the β crystal form of rifaximin (MP3) had the lowest solubility (3.47 μg/ml). MP2 had the highest solubility (8.35 μg/ml) and MP1 and MP4 had intermediate solubilities (5.47 μg/ml and 6.74 μg/ml). Comparative in vitro dissolution results showed that the dissolution profile of P was not similar to that of G1 and G2 (% dissolution (P)_30min = 60%; % dissolution (G1) _30 min = 100% and % dissolution (G2) _30 min = 115%; f1(P versus G1) = 44; f1(P versus G2) = 61) in M1, while G1 and G2 had comparatively similar dissolution profiles (% dissolution (G1) _30 min = 100%; % dissolution (G1) _30 min = 110%; f1 (G1 versus G2) = 14) in M1.

This study highlighted the impact of rifaximin polymorphism on its physico-chemical properties (crystal structure, thermal behavior, solubility) and on its dissolution behavior which could affect the rifaximin bioavailability.