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The potency, efficacy, and pharmacokinetic properties of -amino]-propionylamino}-4-oxo-5-(2,3,5,6-tetrafluoro-phenoxy)-pentanoic acid), a first-in-class caspase inhibitor in clinical trials for the treatment of liver diseases, were characterized in vivo in rodent models. In the mouse ␣-Fas model of liver injury, i.p. administration of IDN-6556 resulted in marked reduction of alanine aminotransferase (ALT), apoptosis, and caspase activities at a dose of 3 mg/kg. At this dose, IDN-6556 was also effective when given up to 2 h before ␣-Fas and as late as 4 h after ␣- Pharmacokinetic analysis in the rat demonstrated rapid clearance after i.v., i.p., and s.c. administration with terminal t 1/2 ranging from 46 to 51 min. Low absolute bioavailability after p.o. administration was seen (2.7-4%), but portal drug concentrations after oral administration were 3-fold higher than systemic concentrations with a 3.7-fold increase in the terminal t 1/2 , indicating a significant first-pass effect. Liver concentrations remained constant after oral administration for at least a 4-h period, reaching a C max of 2558 ng/g liver at 120 min. Last, 51 Ϯ 20 and 4.9 Ϯ 3.4% of IDN-6556 was excreted intact in bile after i.v. and p.o. administration, respectively. This evaluation indicates that IDN-6556 has marked efficacy in models of liver disease after oral administration and thus, is an excellent candidate for the treatment of liver diseases characterized by excessive apoptosis.

A series of oxamyl dipeptides were optimized for pan caspase inhibition, anti-apoptotic cellular activity and in vivo efficacy. This structure-activity relationship study focused on the P4 oxamides and warhead moieties. Primarily on the basis of in vitro data, inhibitors were selected for study in a murine model of alpha-Fas-induced liver injury. IDN-6556 (1) was further profiled in additional in vivo models and pharmacokinetic studies. This first-in-class caspase inhibitor is now the subject of two Phase II clinical trials, evaluating its safety and efficacy for use in liver disease.

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