Coumermycin is from the aminocoumarin class of antibiotic compounds which acts by inhibiting DNA gyrase. Coumermycin is effective against gram-positive bacteria, but not gram-negative bacteria. Coumermycin its derivatives have been studied since the 1950's as potential antibiotic. However, it has seen little to no clinical development because of its low water solubility, toxicity profile, and ineffectiveness against gram-negative bacteria.

Valspodar (PSC-833) is a derivative of cyclosporin but devoid of the immunosuppressive and nephrotoxic properties seen in cyclosporin A. It exhibited high-affinity binding to Mdr1 P-glycoprotein (P-gp) and demonstrated multidrug resistance-reversing activity superior to cyclosporin A and verapamil both in vitro and in vivo. Preclinical and phase I/II clinical data have indicated that plasma levels of PSC-833 with multidrug resistance-reversing activities are achievable. Potent inhibition of intestinal, hepatobiliary and blood-brain barrier P-gp function has been demonstrated. The toxicity profiles of valspodar are acceptable and dose-limited by transient and reversible cerebellar ataxia. It has shown multidrug resistance-modulating activities towards acute myeloid leukemia, multiple myeloma and ovarian cancer in phase I/II clinical trials. However, the company discontinued development of valspodar in April 2001 following disappointing results reported from several multicentre phase III studies.