Improved delivery through biological membranes. 39. Brain-specific chemical delivery systems for .beta.-lactam antibiotics. In vitro and in vivo studies of some dihydropyridine and dihydroisoquinoline derivatives of benzylpenicillin in rats

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Abstract
Four chemical delivery systems (CDS''s) based on a dihydropyridine .dblarw. quaternary pyridinium salt redox system were used for the brain delivery of benzylpenicillin (BP). CDS''s 5 and 9 are diesters of C1 and C2 diols in which one hydroxyl group is esterified by the benzylpenicillin-3-carboxylic group and the other by dihydrotrigonelline. CDS''s 13a and 17 are benzylpenicillin esters of amino alcohols in which the amine group is acylated by dihydrotrigonelline (13a) or by 1,2-dihydro-2-methyl-4-isoquinolinecarboxylic acid (17). In vitro relative stability studies showed that both CDS''s and quaternary pyridinium salts were quite unstable in rat and rabbit blood or brain but much more stable in dog or human blood. Kinetic studies performed in rat brain homogenate demonstrated the facile enzymatic oxidation of the CDS''s to the corresponding quaternary salts. Hydrolysis of the CDS''s and the quaternary salts resulted in the release of benzylpenicillin. In biological media CDS 13a also yielded a water addition product, the 6-hydroxy-1,4,5,6-tetrahydropyridine derivative. In vivo distribution studies were carried out in rats. After iv administration of equimolar doses of BP and CDS''s brain benzylpenicillin levels were found to be substantially higher and more prolonged in case of 5 and 9 than of BP itself. However, administration of 13a and 17 resulted in lower brain benzylpenicillin levels due to the water addition reaction and a nonspecific brain delivery, respectively. The remarkable increase of BP levels as well as the prolonged effect after the administration of 5 and 9 is a result of an improved penetration through the blood-brain barrier of the lipophilic CDS''s and a "lock-in" effect of the corresponding quaternary salts generated in situ.