Methods and pharmaceutical compositions for inhibiting or decreasing transport of a drug by a transporter of multidrug resistance-associated protein comprising a compound of Formula (VI)wherein R1 and R2 are small peptides or modified peptides, are provided. The methods and compositions are useful in enhancing efficacy of drugs such as anti-inflammatory agents, neurological agents, thyroid agents, ocular agents, cancer chemotherapeutics, antibiotics, antimicrobials, antivirals and protease inhibitors to treat human immunodeficiency virus.
