**4.1 Synergy with P-gp modulators**

*Advances in Precision Medicine Oncology*

increased drug levels in liver and kidney [110].

**3.2 P-gp effects on doxorubicin's Pharmacodynamic aspects**

inhibition of activity or down-regulation of protein expression.

chemotherapeutic drugs in the therapeutic regimen.

toxicity and patients' intolerability.

**doxorubicin**

are as follows:

through P-gp inhibition, reduced drug excretion into bile and urine, leading to

Critically, overexpression of P-gp on the plasma membrane of cancer cells is a major determinant in preventing intracellular doxorubicin accumulation up to its cytotoxic level. Doxorubicin resistant cancer cells clearly display significant lower intracellular doxorubicin retention with more tolerable to doxorubicin exposure than their parental sensitive cells [65, 66]. Thus, P-gp can be a potential therapeutic target for either MDR reversal or bio-enhancing effect in cancer therapy. The presence of P-gp modulators clearly demonstrates their abilities to restore doxorubicinmediated killing effect in various cancer cells by increasing intracellular level of doxorubicin [66, 111]. Several plant-based compounds such as limonin, quercetin, resveratrol, curcumin and rhinacanthin-C at their non-cytotoxic concentration have been reported to significantly enhance doxorubicin-mediated cytotoxicity in various cancer resistance cells through modulation of P-gp function [66, 112]. These phytochemical P-gp modulators may suppress P-gp function either by direct

Moreover, the influence of P-gp on clinical resistance to doxorubicin-based treatment has been reported in cancer patients [113–116]. In order to improve drug efficacy and patient tolerability, several approaches targeting at the P-gp function and expression have been introduced to increase cellular doxorubicin drug level and restore drug sensitivity without the need of higher concentration or additional

**4. Strategic approaches to overcome P-gp mediated resistance to** 

Taken that doxorubicin is a known substrate of P-gp, the drug efflux transporters in the ATP binding cassette (ABC) family. Hence, any approaches target at the function of these transporters can be presumed to increase therapeutic success for doxorubicin-based chemotherapeutic regimens. Regarding this, the strategies

• Increases in dose of doxorubicin or number of cytotoxic drugs to achieve therapeutic success. This has not been a satisfactory approach due to drug

• Utilization of P-gp modulators to inhibit either function or expression.

to increase intracellular retention of doxorubicin within target cells.

• Development of better drug delivery platforms to bypass P-gp activity, leading

The current MDR reversal strategy has been exploited P-gp modulators that either directly inhibit P-gp activity or down-regulate P-gp expression in order to restore cell chemo-sensitivity to doxorubicin [107]. With the encapsulation technology, P-gp modulators can be co-administered with doxorubicin in the same drug delivery platform, and enhance intracellular doxorubicin accumulation. This approach can be accomplished if the potent, non-cytotoxic P-gp modulators that specifically target at cancer cells are implemented. In addition, the P-gp modulators that also target at non-transporter based resistance such as activation of cellular

**198**

This approach aims to suppress P-gp activity at plasma membrane of target cancer cells. Several P-gp modulators in combination with anti-cancer drugs have been evaluated for safety and efficacy in clinical trials. The clinical outcomes from the first three generations of ABC inhibitors such as quinine, verapamil, cyclosporine-A, tariquitor, PSC 833, LY335979, and GF120918 were quite disappointed, partly because of their dose-limiting adverse events. Most of the P-gp inhibitors required high doses for their clinical MDR reversal effects. In addition, their interference on the P-gp or other ABC transporters at non-target tissues such as brain and kidney could adversely increase accumulation of cytotoxic drugs in these tissues.

The fourth generation of P-gp modulators which are mostly natural products have gained a great interest as potential chemosensitizers in MDR cancer treatment. The advantages of being natural products with long history of use are inclined to the known safety profiles in human and potential hit multiple targets that can restore cell sensitivity to doxorubicin. In addition to direct inhibition of P-gp activity, a number of the natural compounds at non-cytotoxic concentration elicit their chemo sensitizing effects through down-regulation of MDR1 and signaling proteins in cell adaptive survival mechanisms. The higher degree of synergism between doxorubicin and a P-gp modulator can be anticipated with potential therapeutic success. Synergistic outcomes between doxorubicin and natural compounds such as resveratrol, quercetin, silymarin, gallic acid, curcumin, epigallocatechin-3-gallate have been demonstrated in various cancer cell models [82, 83, 103, 111, 117–120]. In addition to P-gp modulatory activity (inhibiting both P-gp function and expression), these natural compounds have a broad spectrum of pharmacological activities such as antioxidant, anticancer, anti-inflammation, possible through multiple signaling pathways. For example, the biological effects of curcumin have been related to multiple signaling pathways including NF-kB, Akt, MAPK, Nrf2, AMPK, JAK/STAT that involve in MDR1 expression, cell inflammation, and apoptosis [121]. Co-administration of doxorubicin and curcumin significantly improved doxorubicin-mediated cytotoxicity in vitro cell models and in vivo hepatic xenograft mice model, compared with doxorubicin alone [121–125].

In addition to chemical-based modulators, the uses of specific antibody against P-gp or RNA interference (RNAi) technology to silence P-gp expression may be effective approach to suppress P-gp activity and restore chemo-sensitivity to doxorubicin treatment. Clinical studies on these MDR reversing methods should be extensively conducted to support their uses and benefits in cancer patients.
