TY - JOUR T1 - <strong>Pharmacokinetic and pharmacodynamic factors contribute to synergism between let-7c-5p and 5-fluorouracil in inhibiting hepatocellular carcinoma cell viability</strong> JF - Drug Metabolism and Disposition JO - Drug Metab Dispos DO - 10.1124/dmd.120.000207 SP - DMD-AR-2020-000207 AU - Joseph L. Jilek AU - Mei-Juan Tu AU - Chao Zhang AU - Ai-Ming Yu Y1 - 2020/01/01 UR - http://dmd.aspetjournals.org/content/early/2020/10/13/dmd.120.000207.abstract N2 - Pharmacological interventions for hepatocellular carcinoma (HCC) are hindered by complex factors, and rational combination therapy may be developed to improve therapeutic outcomes. Very recently, we have identified a bioengineered microRNA let-7c-5p (or let-7c) agent as an effective inhibitor against HCC in vitro and in vivo. In this study, we sought to identify small-molecule drugs that may synergistically act with let-7c against HCC. Interestingly, we found that let-7c exhibited a strong synergism with 5-fluorouracil (5-FU) in the inhibition of HCC cell viability, as manifested by average combination indices of 0.3 and 0.5 in Hep3B and Huh7 cells, respectively. By contrast, co-administration of let-7c with doxorubicin or sorafenib inhibited HCC cell viability with rather surprisingly no or minimal synergy. Further studies showed that protein levels of multidrug resistance-associated protein 5 (MRP5/ABCC5), a 5-FU efflux transporter, were reduced around 50% by let-7c in HCC cells. This led to a greater degree of intracellular accumulation of 5-FU in Huh7 cells, as well as the second messenger cyclic adenosine monophosphate, an endogenous substrate of MRP5. Since 5-FU is an irreversible inhibitor of thymidylate synthetase (TS), we investigated the interactions of let-7c with 5-FU at pharmacodynamic level. Interestingly, our data revealed that let-7c significantly reduced TS protein levels in Huh7 cells, which was associated with the suppression of upstream transcriptional factors as well as other regulatory factors. Collectively, these results indicate that let-7c interacts with 5-FU at both pharmacokinetic and pharmacodynamic levels, and these findings shall offer insight into molecular mechanisms of synergistic drug combinations. Significance Statement Combination therapy is a common strategy that generally involves pharmacodynamic interactions. After identifying a strong synergism between let-7c-5p and 5-FU against HCC cell viability, we reveal the involvement of both pharmacokinetic and pharmacodynamic mechanisms. In particular, let-7c enhances 5-FU exposure (via suppressing ABCC5/MRP5 expression) and co-targets thymidylate synthase with 5-FU (let-7c reduces protein expression while 5-FU irreversibly inactivates enzyme). These findings provide insight into developing rational combination therapies based on pharmacological mechanisms. ER -