Synthesis and in vitro characterization of drug conjugates of l-carnitine as potential prodrugs that target human Octn2

The objective was to evaluate the potential of drug conjugates with l-carnitine as prodrugs that target organic cation/carnitine transporter (OCTN2). Twenty-two l-carnitine analogues were evaluated for human organic cation/carnitine transporter (hOCTN2) inhibition; the 3′-hydroxyl group was found to be the only functional group not contributing to l-carnitine interaction with hOCTN2 among the three functional groups on l-carnitine (i.e., 3′-hydroxyl, amine, and carboxylate). The 3′-hydroxyl group on l-carnitine was therefore chosen as the conjugate site. Three drug–l-carnitine conjugates (i.e., valproyl–l-carnitine, naproxen–l-carnitine, and ketoprofen–l-carnitine) were synthesized along with two ketoprofen analogues that incorporated a linker group (glycolic acid or glycine) between ketoprofen and l-carnitine (i.e., ketoprofen–glycolic acid–l-carnitine and ketoprofen–glycine–l-carnitine). These potential prodrugs were evaluated for their in vitro inhibition, transport, and metabolism properties. All three drug–l-carnitine conjugates and ketoprofen–glycine–l-carnitine were OCTN2 inhibitors, as well as substrates. For valproyl–l-carnitine, Ki = 155 ± 19 μM, Km = 132 ± 23 μM, and normalized Jmax = 0.467 ± 0.028; for naproxen–l-carnitine, Ki = 5.97 ± 0.81 μM, Km = 257 ± 57 μM, and normalized Jmax = 0.141 ± 0.012; for ketoprofen–l-carnitine, Ki = 82.2 ± 5.3 μM, Km = 77.0 ± 4.0 μM, and normalized Jmax = 0.412 ± 0.015; for ketoprofen–glycine–l-carnitine, Ki = 14.4 ± 1.4 μM, Km = 58.5 ± 8.7 μM, and normalized Jmax = 0.0789 ± 0.0037. Ketoprofen–glycolic acid–l-carnitine was unstable in metabolic buffers and chemical buffers. On the contrary, naproxen–l-carnitine, ketoprofen–l-carnitine, and ketoprofen–glycine–l-carnitine were stable in chemical and metabolic buffers. The results demonstrate the potential of drug–l-carnitine conjugates to serve as prodrugs that target OCTN2. © 2011 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 100:3802–3816, 2011