In patients with colon cancer, OATP1B3 confers resistance to anticancer drugs like paclitaxel (see Figure 3) . In prostate cancer patients on androgen ablation therapy, variants of OATP1B3 with impaired function are associated with a longer progression-free and a longer overall survival, which is likely to be due to a reduced testosterone uptake into tumor cells [8, 9]. These findings recommend that therapeutic inhibition of OATP1B3 could be suitable for endocrine anticancer therapy. However, inhibiting this OATP therapeutically may interfere with normal physiological processes in the liver and impair the excretion of bilirubin, bile acids, drugs, and toxins. It may also
cause drug interactions because of the Inhibitors,research,lifescience,medical inhibition of the hepatic uptake of OATP1B3 Inhibitors,research,lifescience,medical Paclitaxel substrates and subsequently, with their biotransformation and excretion . Figure 3 Selected anticancer drugs as substrates for organic anion-transporting polypeptides [2, 5, 6, 24]. This paper focuses on the expression of OATP as a transporter for anticancer drugs and hormones in cancer. We provide an overview on the expression of specific OATPs and discuss their potential role as novel targets for anticancer therapy. 2. The OATP Family of Transporters The best characterized family is the OATP1 family with three transporters OATP1A2, OATP1B1, and OATP1B3 that transport a number of typical OATP substrates including steroid
Inhibitors,research,lifescience,medical hormone conjugates, thyroid hormones, prostaglandins, bile acids, and various drugs, for example, statins, antibiotics, and a number of anticancer drugs (for a review see ). The fourth Inhibitors,research,lifescience,medical member, OATP1C1, is regarded as thyroid hormone transporter, because of its high affinity for the thyroid hormones T3 and T4 . However, it also transports steroid hormone conjugates . The OATP2 family comprises two members, OATP2A1 and OATP2B1. OATP2A1 was originally identified as the prostaglandin transporter (PGT). It is thought to regulate prostaglandin (PG) levels in target tissues, for example, kidney, colon . OATP2B1 has broader substrate Inhibitors,research,lifescience,medical specificity at an acidic pH (pH 6.8) for various endogenous products and drugs, while at pH
7.4, it transports mainly steroid hormone conjugates . Typical OATP substrates (prostaglandins, thyroid Histamine H2 receptor hormones) are also transported by OATP3A1 and OATP4A1, but with varying affinity. For OATP3A1, transporting prostaglandins, thyroxin, vasopressin, deltorphin, and benzylpenicillin, two splice variants OATP3A1v1 and OATP3A1v2 were identified . Additional substrates for the second member of the family 4, the “kidney-specific transporter” OATP4C1, which is important for the removal of uremic toxins, are cyclic nucleotides, the anticancer drug methotrexate, and other common OATP substrates, including thyroid hormones . Transporters of the OATP family OATP5A1 and OATP6A1 are not characterized for their transport function yet.