An investigation of the destruction of the β-lactam ring of penems by the albumin drug-binding site

Abstract

The plasma proteins have been reported to exert irreversible effects on some β-lactams. The penems represent a class of antibiotics of great interest due to their wide spectrum of activity. However, many compounds of this class were found to be unstable in rat serum. A prototype compound, (±)-2-methylpenem-3-carboxylic acid (BCL-98), was selected for detailed investigations. The capacity of penems to inhibit the destruction of a chromogenic β-lactam (BCL-604) by serum was the technique selected to study the interaction of BCL-98 with serum proteins at the specific destructive site. The serum of various species was investigated and the magnitude of BCL-604 destruction was in the following order: rat = human > dog > mice. The human plasma proteins were fractionated on a Sephadex column and the fractions tested individually. The albumin fraction was found to be entirely responsible for the destruction, the globulin fractions being completely inactive. The reaction of Ellman's reagent with serum was not diminished in the presence of the BCL-98, thus demonstrating that the sulfhydryl group of albumin is not involved in this binding. Displacement of a fluorescent probe (5-dimethylaminonaphthalene-1-sulfonamide) by BCL-98 showed that the binding site for the latter is the same as the one occupied by L-tryptophan and thus involves the ε-lysine amino group of the albumin binding site. The exact nature of this binding is not yet established, but it can be inferred that it may involve acylation of the ε-lysine group at the binding site. Some analogs of BCL-98 were also investigated. The basic functionality seems to protect the molecule from the destructive site of the albumin, probably because this substituent induces binding at another nondestructive site resulting in a prolonged half-life in the blood.