A new porphyrin as selective substrate-based inhibitor of breast cancer resistance protein (BCRP/ABCG2).

The ABCG2 transporter plays a pivotal role in multidrug resistance, however, no clinical trial using specific ABCG2 inhibitors have been successful. Although ABC transporters actively extrude a wide variety of substrates, photodynamic therapeutic agents with porphyrinic scaffolds are exclusively transported by ABCG2. In this work, we describe for the first time a porphyrin derivative (4B) inhibitor of ABCG2 and capable to overcome multidrug resistance in vitro. The inhibition was time-dependent and 4B was not itself transported by ABCG2. Independently of the substrate, the porphyrin 4B showed an IC50 value of 1.6 µM and a mixed type of inhibition. This compound inhibited the ATPase activity and increased the binding of the conformational-sensitive antibody 5D3. A thermostability assay confirmed allosteric protein changes triggered by the porphyrin. Long-timescale molecular dynamics simulations revealed a different behavior between the ABCG2 porphyrinic substrate pheophorbide a and the porphyrin 4B. Pheophorbide a was able to bind in three different protein sites but 4B showed one binding conformation with a strong ionic interaction with GLU446. The inhibition was selective toward ABCG2, since no inhibition was observed for P-glycoprotein and MRP1. Finally, this compound successfully chemosensitized cells that overexpress ABCG2. These findings reinforce that substrates may be a privileged source of chemical scaffolds for identification of new inhibitors of multidrug resistance-linked ABC transporters.

The influence of rhein on the absorption of rehmaionoside D: In vivo, in situ, in vitro, and in silico studies.

ETHNOPHARMACOLOGICAL RELEVANCE: In traditional Chinese Medicine, Rehmannia glutinosa (Gaertn.) DC., as the principle herb of ShengDiHuang Decotion (SDHD), has the effect of cooling blood and hemostasis, and tonifying the yin and kidney. Rheum L., as adjuvant herbs, assist Rehmannia glutinosa (Gaertn.) DC. to promote blood circulation to remove blood stasis. AIM OF STUDY: To study the mechanism of Rhein (RH) involved in the promotion of Rehmannioside D (RD) absorption by pharmacokinetic studies, single-pass intestinal perfusion, Caco-2 cell models, molecular docking technique and western blotting. MATERIALS AND METHODS: Initially, the intestinal absorption of RD in the presence or absence of RH was conducted through pharmacokinetic studies. Thereafter, the intestinal absorption of RD and RH was studied using the single-pass intestinal perfusion and Caco-2 cell models. Finally, using molecular docking technique and western blotting. RESULTS: We found that the promotion of RD absorption by RH was mediated by breast cancer resistance and multidrug resistance-associated protein 2, thereby affecting the permeability of the intestinal epithelium. Additionally, RH and RD can competitively bind to breast cancer resistance and multidrug resistance-associated protein 2, and that RH inhibits the expression of breast cancer resistance and multidrug resistance-associated protein 2 in the ileum to promote the intestinal absorption of RD. CONCLUSION: This study reveals the mechanisms associated with the RH-mediated promotion of RD absorption and provides a basis for further exploring the synergistic effect of Rehmannia glutinosa (Gaertn.) DC and rhubarb.

Cellular Models and In Vitro Assays for the Screening of modulators of P-gp, MRP1 and BCRP.

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are highly expressed in tumor cells, as well as in organs involved in absorption and secretion processes, mediating the ATP-dependent efflux of compounds, both endogenous substances and xenobiotics, including drugs. Their expression and activity levels are modulated by the presence of inhibitors, inducers and/or activators. In vitro, ex vivo and in vivo studies with both known and newly synthesized P-glycoprotein (P-gp) inducers and/or activators have shown the usefulness of these transport mechanisms in reducing the systemic exposure and specific tissue access of potentially harmful compounds. This article focuses on the main ABC transporters involved in multidrug resistance [P-gp, multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP)] expressed in tissues of toxicological relevance, such as the blood-brain barrier, cardiovascular system, liver, kidney and intestine. Moreover, it provides a review of the available cellular models, in vitro and ex vivo assays for the screening and selection of safe and specific inducers and activators of these membrane transporters. The available cellular models and in vitro assays have been proposed as high throughput and low-cost alternatives to excessive animal testing, allowing the evaluation of a large number of compounds.