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ABSORPTION, DISTRIBUTION, METABOLISM, AND EXCRETION

Liver-Targeted Drug Delivery Using HepDirect¹ Prodrugs

Metabasis Therapeutics, Inc., San Diego, California

Targeting drugs to specific organs, tissues, or cells is an attractive strategy for enhancing drug efficacy and reducing side effects. Drug carriers such as antibodies, natural and manmade polymers, and labeled liposomes are capable of targeting drugs to blood vessels of individual tissues but often fail to deliver drugs to extravascular sites. An alternative strategy is to use low molecular weight prodrugs that distribute throughout the body but cleave intracellularly to the active drug by an organ-specific enzyme. Here we show that a series of phosphate and phosphonate prodrugs, called HepDirect prodrugs, results in liver-targeted drug delivery following a cytochrome P450-catalyzed oxidative cleavage reaction inside hepatocytes. Liver targeting was demonstrated in rodents for MB06866 [(2R,4S)-9-[2-[4-(3-chlorophenyl)-2-oxo-1,3,2-dioxaphosphorinan-2-yl]methoxyethyl]adenine (remofovir)], a Hep-Direct prodrug of the nucleotide analog adefovir (PMEA), and MB07133 [(2R,4S)-4-amino-1-[5-O-[2-oxo-4-(4-pyridyl)-1,3,2-dioxaphosphorinan-2-yl]--D-arabinofuranosyl]-2(1H)-pyrimidinone], a HepDirect prodrug of cytarabine (araC) 5'-monophosphate. Liver targeting led to higher levels of the biologically active form of PMEA and araC in the liver and to lower levels in the most toxicologically sensitive organs. Liver targeting also confined production of the prodrug byproduct, an aryl vinyl ketone, to hepatocytes. Glutathione within the hepatocytes rapidly reacted with the byproduct to form a glutathione conjugate. No byproduct-related toxicity was observed in hepatocytes or animals treated with HepDirect prodrugs. A 5-day safety study in mice demonstrated the toxicological benefits of liver targeting. These findings suggest that HepDirect prodrugs represent a potential strategy for targeting drugs to the liver and achieving more effective therapies against chronic liver diseases such as hepatitis B, hepatitis C, and hepatocellular carcinoma.

Address correspondence to: Dr. Mark D. Erion, Executive Vice President, Research and Development, Metabasis Therapeutics, Inc., 9390 Towne Centre Drive, Building 300, San Diego, CA 92121. E-mail: erion{at}mbasis.com

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