THE SYNERGISTIC INTERACTIONS INVITRO AND INVIVO OF BREQUINAR SODIUM WITH CYCLOSPORINE OR RAPAMYCIN ALONE AND IN TRIPLE COMBINATION

The rigorous median-effect analysis was used to assess the interaction s between cyclosporine and drugs that inhibit nucleotide synthesis pat hways. Using in vitro proliferation assays wherein human lymphocytes w ere triggered by phytohemagglutin, anti-CD3 monoclonal antibody, or mi xed lymphocyte reactions, CsA was shown to display additive interactio ns with 6-mercaptopurine (6-MP), mizorbine (MZB), and mycophenolic aci d (MPA), and a synergistic interaction with brequinar (BQR). In the in vitro assays, BQR contributed a further synergistic effect to the dou ble-drug combination CsA/rapamycin (RAPA). Of the four inhibitors of n ucleotide synthesis pathways, only BQR noncompetitively inhibited IL-2 -stimulated proliferation of the CTLL-2 cell line. Using the in vivo a ssay of heterotopic Buffalo (BUF, RT-1b) cardiac allografts in Wistar- Furth (WFu, RT-1u) hosts, oral administration of BQR displayed about 1 00% bioavailability-which, like the bolus intravenous (i.v.) mode, was eight-fold more effective than continuous i.v. infusions. Furthermore median-effect analysis of serial amounts of orally administered BQR d emonstrated that it contributes synergistically to the immunosuppressi ve effects of intravenously delivered CsA/RAPA (0.5/0.01 mg/kg/day). T he degree of synergism was proportionate to the extent of the immunosu ppression. These findings document the potency of the CsA/RAPA/BQR tri ple-drug combination and suggest that the synergistic effects may perm it dose reductions of each component, thereby mitigating toxicities re sulting from the large amounts of individual agents necessary to achie ve allo-unresponsiveness.