Case studies in current drug development: 'glycylcyclines'

Facile conversion of tetracycline antibiotics to 4,11a-bridged derivatives via oxidative mannich cyclization

Treatment of a variety of tetracyclines (tigecycline, minocycline, tetracycline and doxycycline) with Ag(2)CO(3)/EDTA or Hg(OAc)(2) cleanly gave the 4,11a-bridged derivatives in high yields. The reactions proceeded through a novel, intramolecular Mannich cyclization of an iminium species generated by oxidation of the tertiary dimethylamino group at C(4) by Ag(I) or Hg(II). Tetracyclines without 5-OH-substitution (tigecycline, tetracycline and minocycline) gave the 4-OH-substituted, 4,11a-bridged compound, whereas doxycycline gave the 4-dimethylamino-substituted, 4,11a-bridged product. In the case of tetracycline, the 4,11a-bridged compound can equilibrate further to a 4,6-bridged hemiketal. Some of the bridged compounds underwent a novel decarboxylation--rearrangement sequence under acidic conditions to give tricyclic, open chain 1,4-quinoid compounds.

Molecular mechanics models for tetracycline analogs

Tetracyclines (Tcs) are an important family of antibiotics that bind to the ribosome and several proteins. To model Tc interactions with protein and RNA, we have developed a molecular mechanics force field for 12 tetracyclines, consistent with the CHARMM force field. We considered each Tc variant in its zwitterionic tautomer, with and without a bound Mg(2+). We used structures from the Cambridge Crystallographic Data Base to identify the conformations likely to be present in solution and in biomolecular complexes. A conformational search by simulated annealing was undertaken, using the MM3 force field, for tetracycline, anhydrotetracycline, doxycycline, and tigecycline. Resulting, low-energy structures were optimized with an ab initio method. We found that Tc and its analogs all adopt an extended conformation in the zwitterionic tautomer and a twisted one in the neutral tautomer, and the zwitterionic-extended state is the most stable in solution. Intermolecular force field parameters were derived from a standard supermolecule approach: we considered the ab initio energies and geometries of a water molecule interacting with each Tc analog at several different positions. The final, rms deviation between the ab initio and force field energies, averaged over all forms, was 0.35 kcal/mol. Intramolecular parameters were adopted from either the standard CHARMM force field, the ab initio structure, or the earlier, plain Tc force field. The model reproduces the ab initio geometry and flexibility of each Tc. As tests, we describe MD and free energy simulations of a solvated complex between three Tcs and the Tet repressor protein.

Microbial drug discovery: 80 years of progress

Microbes have made a phenomenal contribution to the health and well-being of people throughout the world. In addition to producing many primary metabolites, such as amino acids, vitamins and nucleotides, they are capable of making secondary metabolites, which constitute half of the pharmaceuticals on the market today and provide agriculture with many essential products. This review centers on these beneficial secondary metabolites, the discovery of which goes back 80 years to the time when penicillin was discovered by Alexander Fleming.

Mother Nature's gifts to diseases of man: the impact of natural products on anti-infective, anticholestemics and anticancer drug discovery

This chapter is designed to demonstrate that compounds derived from nature are still in the forefront of drug discovery in diseases such as microbial and parasitic infections, carcinomas of many types and control of cholesterol/lipids in man. In each disease area we have provided short discussions of past, present and future agents, in general only considering compounds currently in clinical Phase II or later, that were/are derived from nature's chemical skeletons. Finishing with a discussion of the current and evolving role(s) of microbes (bacteria and fungi) in the production of old and new agents ostensibly produced by higher organisms.