BACKGROUND AND PURPOSE Intracellular pharmacokinetics of anticancer drugs in multi‐drug resistance (MDR) cancer cells is hugely important in the evaluation and improvement of drug efficacy. By using adriamycin as a probe drug in MDR cancer cells, we developed a cellular pharmacokinetic‐pharmacodynamic (PK‐PD) model to reveal the correlation between cellular pharmacokinetic properties and drug resistance. In addition, the ability of 20(S)‐ginsenoside Rh2 (20(S)‐Rh2) to reverse MDR was further investigated. EXPERIMENTAL APPROACH The cellular pharmacokinetics of adriamycin were analysed visually and quantitatively in human breast cancer cells MCF‐7 and in adriamycin‐resistant MCF‐7 (MCF‐7/Adr) cells. Mitochondria membrane potential was assayed to evaluate the apoptotic effect of adriamycin. Subsequently, a PK‐PD model was developed via MATLAB. KEY RESULTS Visual and quantitative data of the dynamic subcellular distribution of adriamycin revealed that it accumulated in cells, especially nuclei, to a lesser and slower extent in MCF‐7/Adr than in MCF‐7 cells. 20(S)‐Rh2 increased the rate and amount of adriamycin entering cellular/subcellular compartments in MCF‐7/Adr cells through inhibition of P‐glycoprotein (P‐gp) activity, in turn augmenting adriamycin‐induced apoptosis. The integrated PK‐PD model mathematically revealed the pharmacokinetic mechanisms of adriamycin resistance in MCF‐7/Adr cells and its reversal by 20(S)‐Rh2. CONCLUSIONS AND IMPLICATIONS P‐gp, which is overexpressed and functionally active at cellular/subcellular membranes, influences the cellular pharmacokinetic and pharmacological properties of adriamycin in MCF‐7/Adr cells. Inhibition of P‐gp activity represents a key mechanism by which 20(S)‐Rh2 attenuates adriamycin resistance. Even more importantly, our findings provide a new strategy to explore the in‐depth mechanisms of MDR and evaluate the efficacy of MDR modulators.

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Pharmacokinetics, pharmacology, and toxicology are the major determinants of the success or failure of candidates during drug development. Because inappropriate pharmacokinetics often leads to inefficacy, even toxicity, pharmacokinetics studies have been regarded as crucial components in drug preclinical and clinical research. However, new data increasingly reveal that drug concentrations in plasma or tissues cannot totally explain the efficacy of drug on the target organ. For most drugs that interact with targets localized in cells, intracellular penetration, accumulation, distribution, and elimination are important parameters governing the efficacy in the target cells. So, there is a pressing need to clarify the cellular pharmacokinetics and thus evaluate the efficacy of drugs in the target cells. This review provides a general overview regarding current knowledge about cellular pharmacokinetics in some specific cells and also summarizes the factors that can influence cellular pharmacokinetics. It concludes by discussing potential strategies for optimizing cellular pharmacokinetics and advocating that global cellular pharmacokinetics studies be conducted in future research toward improving drug efficacy.

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Background and Purpose The clinical use of doxorubicin, an effective anticancer drug, is severely hampered by its cardiotoxicity. 23‐Hydroxybetulinic acid (23‐HBA), isolated from Pulsatilla chinensis, enhances the anticancer effect of doxorubicin while simultaneously reducing its cardiac toxicity, but does not affect the concentration of doxorubicin in the plasma and heart. As the metabolite doxorubicinol is more potent than doxorubicin at inducing cardiac toxicity, in the present study we aimed to clarify the role of doxorubicinol in the protective effect of 23‐HBA. Experimental Approach Doxorubicin was administered to mice for two weeks in the presence or absence of 23‐HBA. The heart pathology, function, myocardial enzymes and accumulation of doxorubicin and doxorubicinol were then analysed. A cellular pharmacokinetic study of doxorubicin and doxorubicinol, carbonyl reductase 1 (CBR1) interference and molecular docking was performed in vitro. Key Results 23‐HBA alleviated the doxorubicin‐induced cardiotoxicity in mice, and this was accompanied by inhibition of the metabolism of doxorubicin and reduced accumulation of doxorubicinol selectively in hearts. In H9c2 cells, the protective effect of 23‐HBA was shown to be closely associated with a decreased rate and extent of accumulation of doxorubicinol in mitochondria and nuclei. siRNA and docking analysis demonstrated that CBR1 has a crucial role in doxorubicin‐mediated cardiotoxicity and 23‐HBA inhibits this metabolic pathway. Conclusions and Implications Inhibition of CBR‐mediated doxorubicin metabolism might be one of the protective mechanisms of 23‐HBA against doxorubicin‐induced cardiotoxicity. The present study provides a new research strategy guided by pharmacokinetic theory to elucidate the mechanism of drugs with unknown targets. Linked Articles This article is part of a themed section on Chinese Innovation in Cardiovascular Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-23