SIROLIMUS (RAPAMYCIN, RAPAMUNE(R)) AND COMBINATION THERAPY WITH CYCLOSPORINE-A IN THE RAT DEVELOPING ADJUVANT ARTHRITIS MODEL - CORRELATIONWITH BLOOD-LEVELS AND THE EFFECTS OF DIFFERENT ORAL FORMULATIONS

Objective and Design: To determine whole blood levels of sirolimus, a macrolide antibiotic in the rat developing adjuvant arthritis (AA) mod el after dosing orally with two different vehicles, and whether combin ational doses of sirolimus and cyclosporin A (CsA) produced additive o r synergistic inhibitory effects in this model. Material: Male Lewis r ats (150-180g). Treatment: Arthritis was induced by the injection (0.5 mg/rat) of heat-killed Mycobacterium butyricum suspended in light mine ral oil. Drugs were administered orally either in fine suspension (0.5 % Tween 80) or in emulsion (phosal 50 PG in 1% Tween 80) at doses of 0 .1 to 5 mg/kg in a 7 day, MWF or daily regimen. Method: Paw volumes (m l) were measured by automated mercury plethysmograph and sirolimus con centrations in whole blood were quantitated by liquid chromatography/m ass spectroscopy. Results: At 72 h (7 days after adjuvant) after recei ving the third oral dose (4.5 mg/kg p.o.), the phosal vehicle resulted in higher sirolimus blood levels (2.5 ng/ml) than in Tween 80 (1.6 ng /ml). After the rats received the last oral dose on day 14, (7 total d oses of sirolmus at 4.5 mg/kg) the sirolimus blood levels (2 h after t he last dose) were about 2 times higher for the phosal dosed rats (9.8 ng/ml) compared to Tween 80 dosed rats (4.6 ng/ml). Even 24h after th e last dose, sirolimus blood levels were still elevated in the phosal dosed rats (0.8 ng/ml) relative to 0.5% Tween 80 dosed rats (0.5 ng/ml ). At day 16 in the rat developing model, sirolimus, when given in pho sal vehicle, produced an ED50 of 0.28 mg/kg (i.e. inhibition of uninje cted paw edema) that was about 5.5 times lower than using 0.5% Tween 8 0 as the suspending agent (ED50 = 1.6 mg/kg). When combining sirolimus and CsA using precalculated doses for producing an additive effect in this adjuvant model, an additive inhibitory effect on uninjected paw edema was observed at equal combinational doses of 0.5 and 2 mg/kg, re spectively. Conclusions: The phosal vehicle used in administering siro limus increases the absorption and whole blood levels in the rat and t he elevated blood levels correlated positively with the therapeutic ef fect in the rat developing AA model. In addition, combination therapy using sirolimus and CsA produced an additive effect in rat developing AA.