BCRP Inhibitors: Mechanisms, Types, and Clinical Implications

Breast Cancer Resistance Protein (BCRP) is a key efflux transporter that influences drug absorption, distribution, and elimination, affecting therapeutic efficacy and potential toxicity. Inhibiting BCRP can enhance drug bioavailability but may also lead to unintended drug interactions, making it a crucial consideration in pharmacology and clinical practice.

BCRP Structure And Function

Breast Cancer Resistance Protein (BCRP), also known as ATP-binding cassette subfamily G member 2 (ABCG2), is a transmembrane efflux transporter that limits drug accumulation in cells. Structurally, BCRP is a half-transporter, consisting of a single nucleotide-binding domain (NBD) and a single transmembrane domain (TMD), unlike full transporters that require dimerization for function. This structure enables it to efficiently extrude a range of substrates, including chemotherapeutic agents, antibiotics, and endogenous compounds. It is highly expressed in the liver, intestine, kidney, and blood-brain barrier, regulating drug absorption, metabolism, and excretion.

BCRP operates through ATP hydrolysis, which provides the energy to transport substrates against concentration gradients. Upon substrate binding, conformational changes in the NBD facilitate ATP binding and hydrolysis, triggering a shift in the TMD that expels the substrate. This process is highly selective, recognizing specific molecular features such as planar ring structures and hydrophobicity, which are common among xenobiotics and endogenous metabolites. Its broad substrate specificity makes it a major determinant of drug bioavailability and resistance, particularly in cancer cells, where overexpression reduces intracellular drug concentrations and leads to therapeutic failure.

Beyond drug transport, BCRP protects physiological barriers. In the placenta, it limits fetal exposure to harmful xenobiotics by pumping them back into maternal circulation. In the blood-brain barrier, it restricts entry of neurotoxic compounds and certain pharmaceuticals, influencing central nervous system drug delivery. Its presence in the intestine and liver modulates oral drug absorption and biliary excretion, affecting systemic drug levels. Genetic polymorphisms in the ABCG2 gene can significantly alter BCRP function, leading to variability in drug response. For instance, the c.421C>A polymorphism reduces transporter activity, increasing plasma concentrations of BCRP substrates such as rosuvastatin and sulfasalazine, which can enhance efficacy but also raise the risk of adverse effects.

Mechanisms Of Inhibition

BCRP inhibition occurs through direct and indirect mechanisms that interfere with substrate transport. Direct inhibition includes competitive and non-competitive interactions. Competitive inhibitors, such as tyrosine kinase inhibitors (TKIs) like gefitinib and erlotinib, bind to the substrate recognition site, preventing efflux and increasing intracellular drug levels. Non-competitive inhibitors, like cyclosporine A, bind to allosteric sites, altering conformational dynamics and reducing substrate expulsion.

Indirect inhibition affects BCRP expression or function through transcriptional, post-transcriptional, or post-translational modifications. Nuclear receptors such as pregnane X receptor (PXR) and aryl hydrocarbon receptor (AhR) regulate ABCG2 gene expression, and their inhibition reduces BCRP levels. MicroRNAs like miR-328 suppress BCRP translation, affecting drug transport capacity. Post-translational modifications, including phosphorylation and ubiquitination, influence BCRP stability and degradation. Some kinase inhibitors induce proteasomal degradation, reducing BCRP presence on the plasma membrane and altering drug pharmacokinetics.

Common Types Of BCRP Inhibitors

A wide range of compounds inhibit BCRP, with small-molecule inhibitors being the most studied. Many were initially developed for other purposes but later found to interfere with BCRP-mediated transport. TKIs such as imatinib, gefitinib, and lapatinib not only target aberrant kinase signaling in cancer cells but also reduce BCRP activity, increasing intracellular retention of chemotherapeutic agents like topotecan and mitoxantrone. This can enhance treatment efficacy but also heighten toxicity risks, requiring careful dose adjustments.

Immunosuppressants like cyclosporine A and tacrolimus also inhibit BCRP. Used primarily to prevent organ transplant rejection, these agents alter drug pharmacokinetics by increasing plasma concentrations of BCRP substrates such as rosuvastatin and methotrexate. Monitoring drug levels in patients receiving immunosuppressive therapy is essential to prevent unintended drug accumulation.

Certain antiviral drugs, including elacridar and zosuquidar, initially developed as P-glycoprotein inhibitors, also inhibit BCRP. These compounds have been explored in clinical trials to enhance drug bioavailability at the blood-brain barrier, offering potential benefits for central nervous system treatments.

Natural compounds, particularly flavonoids such as quercetin, genistein, and naringenin, found in fruits, vegetables, and soy products, also modulate BCRP. These dietary-derived inhibitors affect drug absorption in the intestine, potentially altering the bioavailability of orally administered medications. While their clinical significance remains under investigation, their interactions with certain drugs suggest dietary considerations in pharmacotherapy.

Laboratory Techniques For Assessing BCRP Inhibitors

Assessing BCRP inhibition involves in vitro, in situ, and in vivo methodologies. In vitro assays commonly use cell lines overexpressing BCRP, such as MDCKII-BCRP and HEK293-BCRP, to measure substrate accumulation and efflux. Fluorescent or radiolabeled substrates like Hoechst 33342 or [³H]-methotrexate provide quantifiable BCRP activity readouts, with increased intracellular retention indicating inhibition.

Membrane vesicle assays isolate BCRP-containing membranes to directly measure ATP-dependent substrate transport, eliminating confounding cellular processes. ATPase activity assays determine whether a compound stimulates or inhibits ATP hydrolysis, distinguishing between competitive and non-competitive inhibition.

Animal models, particularly ABCG2-knockout mice, provide pharmacokinetic data to validate in vitro findings. Comparing drug absorption and disposition in wild-type and knockout animals confirms whether a compound modulates BCRP activity in a physiological setting. Clinical studies further assess drug-drug interactions in human subjects by monitoring systemic exposure changes of known BCRP substrates when co-administered with potential inhibitors.

Interactions With Other Transporters

BCRP interacts with other efflux and uptake transporters that collectively influence drug disposition. These interactions can be synergistic, additive, or antagonistic, significantly impacting pharmacokinetics and therapeutic outcomes.

P-glycoprotein (P-gp), another ATP-binding cassette (ABC) transporter, frequently co-localizes with BCRP in tissues such as the intestine, liver, and blood-brain barrier. Both limit drug penetration into the brain and restrict intestinal absorption. When a compound is a substrate for both, inhibiting one transporter can lead to compensatory activity by the other. Dual inhibition, as seen with elacridar, can dramatically increase drug bioavailability and central nervous system exposure, a strategy explored in oncology to enhance chemotherapeutic efficacy. However, such inhibition also raises toxicity concerns, particularly for drugs with narrow therapeutic windows.

Multidrug resistance-associated proteins (MRPs), particularly MRP2 and MRP4, share substrate profiles with BCRP, influencing drug excretion into bile and urine. Inhibitors that target both BCRP and MRP2, such as cyclosporine A, can impair hepatic clearance, leading to elevated plasma drug concentrations. This interaction is particularly relevant in chemotherapy, where reduced elimination of antifolates can enhance efficacy but also increase adverse effects like nephrotoxicity.

Interactions between BCRP and organic anion-transporting polypeptides (OATPs) also affect hepatic drug uptake and biliary excretion. Statins, including rosuvastatin, are substrates for both transporters, and BCRP inhibition can lead to increased hepatic accumulation, altering lipid-lowering efficacy and myopathy risk. These transporter interactions highlight the complexity of drug disposition and the necessity for thorough pharmacokinetic evaluations when developing or prescribing BCRP inhibitors.