Brain and CSF specific chemical delivery systems for beta-lactam antibiotics. Study of two dihydropyridine derivatives of benzylpenicillin in rabbits and dogs.

Following previous studies in rats, the ability of two chemical delivery systems (CDSs) to deliver benzyl penicillin (1) to the central nervous system of rabbits and dogs was investigated. One of the systems (3) was a diester of methylene diol, and the other (5) a diester of ethylene 1,2-diol; in both, one hydroxyl group of the diol was esterified by the 3-carboxylic acid group of benzylpenicillin, and the other by the carboxy group of an N-methyldihydropyridine (dihydrotrigonelline). The basis of the system is the ability of the dihydropyridine components to undergo oxidation to quaternary pyridinium salts (2 from 3, and 4 from 5). In vitro relative stability studies were first performed in 10% rabbit brain homogenate, rabbit CSF and dog CSF. The results showed that the CDSs (3 and 5) were more stable than the corresponding quaternary salts (2 and 4). Hydrolysis of 2 and 3 resulted in the release of 1, whereas hydrolysis of 4 and 5 released both 1 and the hydroxyethyl ester (6) of 1. In vivo distribution studies were performed in rabbits and dogs. After i.v. administration of equimolar doses of 1 or the CDSs, levels of 1 in brain and CSF were substantially higher and more prolonged in the cases of the CDSs than in the case of 1 itself. Brain levels of 1 were lower following administration of 5, as compared with 3, due to the release of the intermediate compound, the hydroxyethyl ester (6) of 1, which was not hydrolyzed efficiently to 1 in rabbit or dog brain. The substantially increased and prolonged penicillin levels following administration of the CDSs arise as the result of improved penetration of the lipophilic CDSs across the blood-brain barrier, and a "lock-in" effect of the corresponding quaternary salts generated in situ.