Rifamycin L

The absorption and tissue distribution of a new rifamycin, L/105, which chemically is 4-deoxy-4'-methylpyrido [1',2'-1,2]imidazo [5,4 C] rifamycin SV, were studied in rats after oral administration of 100 mg/kg. This study showed that L/105, contrary to what was observed for rifampicin, was practically not absorbed: neither serum nor significant tissue levels were evidenced. Also in dogs, after oral administration of L/105 in a single dose (25 mg/kg) and a multiple dosage (10 mg/kg/day, for 8 days), no traces of the compound were detected in the serum. The elimination was investigated in the rat: the highest recovery of L/105 (more than 50% of the administered dose) was in feces after 72 h, hardly any traces were found in urine.

Rifamycin-derived drugs, including rifampin, rifabutin, rifapentine, and rifaximin, have long been used as first-line therapies for the treatment of tuberculosis and other deadly infections. However, the late steps leading to the biosynthesis of the industrially important rifamycin SV and B remain largely unknown. Here, we characterize a network of reactions underlying the biosynthesis of rifamycin SV, S, L, O, and B. The two-subunit transketolase Rif15 and the cytochrome P450 enzyme Rif16 are found to mediate, respectively, a unique C-O bond formation in rifamycin L and an atypical P450 ester-to-ether transformation from rifamycin L to B. Both reactions showcase interesting chemistries for these two widespread and well-studied enzyme families.

This paper reports the determination of the X-ray molecular structure of 4-deoxy-3'-bromopyrido[1',2'-1,2]imidazo[5,4-c]rifamycin S, carried out in order to unequivocally define the general structure of a new series of rifamycin SV derivatives, which are potent antibacterial agents, and are not absorbed at the gastroenteric level. They have been prepared by Alfa Farmaceutici, Bologna, by condensing 2-aminopyridine derivatives to 3-bromorifamycin S. The solid state X-ray study has confirmed the structure proposed on the basis of 1H NMR studies in solution. It has also shown that the newly formed pyridoimidazo system is in a mesomeric betaine form, the pyrido nitrogen being positively charged and the imidazo nitrogen being negatively charged. This feature is believed responsible for the pharmacokinetic behavior of these new drugs, one of which, denoted either as rifamycin L 105 or rifaximin, is actually under clinical trial as a topical intestinal disinfectant.