OCT1 and OCT2 are membrane transporters belonging to the SLC22 family. The OCTs transport positively charged endogenous substances such as carnitine and epinephrine, as well as exogenous substrates like 1-Methyl- 4- phenylperidinium iodide (MPP). Cytostatic transport by human OCTs was demonstrated by uptake of cisplatin and oxaliplatin (Zhang et al, 2006). Our previous studies had determined that OCT1 and OCT2 mediated MPP uptake is significantly inhibited by irinotecan. OCT1 mediated MPP uptake was also significantly inhibited by two additional drugs, mitoxantrone and paclitaxel. The current experiments focused on transporter mediated cytostatic effects of these three drugs on stable transfected cells. Irinotecan mediated cytotoxicity on hOCT1 and hOCT2 expressing cells was determined using [³H]thymidine incorporation as a measure of cell proliferation. OCT1 and OCT2 cells showed a significant decrease in cell proliferation on treatment with irinotecan. [³H]Thymidine incorporation tests on hOCT1 expressing cells treated with mitoxantrone revealed decreased cell proliferation in control cells as well, indicating that mitoxantrone uptake may not be OCT1-specific. OCT1-mediated cytostatic effect of paclitaxel was examined using the MTT assay. Significant reduction in cell viability was observed in paclitaxel treated OCT1 cells. The expression of p53 in hOCT2 cells treated with irinotecan was analyzed. An increase in p53 expression was observed on incubation with irinotecan but no change was observed on simultaneous incubation with irinotecan and quinine, which is a blocker of OCTs. These effects were not observed in the control cells. The expression of OCT1 and OCT2 in six lymphoma cell lines was analyzed using real time PCR. OCT1 was expressed at much higher levels than normal in the six cell lines tested, particularly in SUDHL-4, a B-lymphoma cell line and Jurkat, a T-lymphoma cell line. OCT2 expression was upregulated in four lymphoma cell lines.

In these studies, we demonstrated hOCT1 and hOCT2 mediated cytostatic effect of irinotecan. We also showed hOCT1 mediated reduction in cell viability on treatment with paclitaxel.