MONITORING OF PHARMACODYNAMICS (PD), BUT NOT PHARMACOKINETICS (PK), DIFFERENTIATES THE RELATIVE IMMUNOSUPPRESSIVE POTENCIES OF LEFLUNOMIDE AND ITS MALONONITRILAMIDE (MNA) ANALOGS

Abstract

160 Using novel, mitogen-driven whole blood lymphocyte proliferation assays to quantitate drug effects in vivo, we show for the first time that PD, but not PK, predicts the relative in vivo immunosuppressive potencies of LFM and its analogs. Although that monitoring drug concentrations (PK) fails to predict immunosuppressive efficacy reliably, the utility of monitoring drug effects(PD) has not been previously shown. Our goal was to determine the relationships among dose, PK, PD and severity of histologic rejection in rat heart allograft recipients treated with LFM or MNAs. METHODS: Lewis rat recipients of BN heart grafts (n = 5/dose group) were treated daily for 21 d with doses of 1-15 mg/kg of oral LFM or its analogs 279, 715. Beginning on d 21, multiple blood samples were analyzed by HPLC to produce full PK profiles of A77 1726 ([A77], a MNA and the active metabolite of LFM), 279 and 715. The same blood samples were used for PD profiles by assessing levels of inhibition of ConA-driven whole blood lymphocyte proliferation ([3H] TdR). Drug effects were monitored by flow cytometry for inhibition of lymphocyte proliferation (% PCNA+ [proliferating cell nuclear antigen] / DNA+ cells) and inhibition of activation (%CD25+ [IL-2 R] and CD134+ [TNF R] cells). Following blood sampling, all grafts were removed and blindly scored for severity of histologic rejection. RESULTS: Plasma concentrations for each MNA correlated directly and significantly with PD measurements of lymphocyte function (A77: r2=0.77; 279: r2=0.77; 715: r2=0.81). These correlations followed an Emax model from which in vivo IC50's (mg/L) for inhibitions of lymphocyte proliferation were calculated: A77=0.8; 279=8.5; 715=36 demonstrating the wide range of in vivo potencies among the 3 MNAs which also predicted their relative potencies for inhibition of rejection (A77>279>715). Measurement of drug exposure (AUC) alone would not have predicted these differences in drug effects, since when the same dose (10 mg/kg) of each drug was administered to groups of rats, the exposure was much greater for 715 (895 mg.h/L) than 279 (412 mg.h/L) and A77 (130 mg.h/L). For each drug, severity of histologic rejection correlated inversely with its drug exposure and its inhibition of lymphocyte proliferation and activation. Administration of 2 mg/kg CsA or 5 mg/kg 279 showed no PD effects and failed to suppress rejection; coadministration did not alter PK but produced substantial PD effects and completely suppressed rejection. CONCLUSIONS: 1) PD, but not PK, predicted the relative in vivo immunosuppressive potencies of MNAs, 2) For each MNA, there was a significant correlation among dose, PK, PD and severity of rejection. 3) PD, but not PK, predicted the benefits of combined vs monotherapy with CsA and 279 demonstrating the value of PD for monitoring net immunosuppression. Funded by Hoechst-Marion-Roussel.