Disrupting P-glycoprotein function in clinical settings: what can we learn from the fundamental aspects of this transporter?

P-glycoprotein is one of the most well-studied drug transporters, significant for its role in cancer multiple drug resistance. However, using P-gp inhibitors with the aim of enhancing the therapeutic efficacy of anti-cancer drugs has led to disappointing outcomes. Furthermore, several lead compounds suggested by in vitro and pre-clinical studies have shown variable pharmacokinetics and therapeutic efficacies when applied in the clinical setting. This review will highlight the need to revisit a sound approach to better design and apply P-gp inhibitors in light of safety and efficacy. Challenges confronting the issue hinge upon myriad studies that do not necessarily represent the heterogeneous target population of this therapeutic approach. The application of P-gp modulators has also been complicated by the promiscuous substrate-binding behaviour of P-gp, as well as toxicities related to its intrinsic presence in healthy tissue. This review capitalizes on information spanning genetics, energetics, and pharmacology, bringing to light some fundamental aspects that ought to be reconsidered in order to improve upon and design the next generation of P-gp inhibitors.

Autologous Adipose-Derived Mesenchymal Stromal Cells for the Treatment of Psoriasis Vulgaris and Psoriatic Arthritis: A Case Report.

Psoriasis is a dermatologic disease of immune origins with no definitive cure. We report the Makati Medical Center experience of utilizing autologous mesenchymal stromal cells (MSCs) for one patient with psoriasis vulgaris (PV) and another with psoriatic arthritis (PA). Patients were educated and gave informed consent, according to the principles of the Declaration of Helsinki. The protocol was approved by the Cellular Transplantation Ethics Committee of the Makati Medical Center. Autologous MSCs were cultured from lipoaspirate and expanded in a clean room class 100 facility (Cellular Therapeutics Center, Makati Medical Center). MSCs were infused intravenously at a dose of 0.5-3.1 million cells/kg after complying with quality control parameters. Psoriasis area and severity index (PASI) evaluations were conducted by third-party dermatologists. The PA patient, who was previously unresponsive to standard treatment modalities, demonstrated a decrease in PASI (from 21.6 to 9.0, mild state after two infusions). No improvements were noted in joint pain until further treatment with etanercept and infliximab. The PV patient, who was previously dependent on methotrexate, showed a decrease in PASI from 24.0 to 8.3 after three infusions; this clinical improvement was sustained for 292 days (9.7 months) without methotrexate. The PV patient illustrated a marginal reduction in serum tumor necrosis factor-α (TNF-α), while significant (3.5- to 5-fold) decreases in reactive oxygen species (ROS) activity were noted. The ROS levels correlated with the clinical improvement of the PV patient. No serious adverse events were noted for either patient as a result of MSC infusions. This report demonstrates safe and tolerable transplantation of autologous MSCs for the treatment of psoriasis and warrants large clinical studies to investigate the long-term safety and efficacy of this approach.

Modeling cyclosporine A inhibition of the distribution of a P-glycoprotein PET ligand, 11C-verapamil, into the maternal brain and fetal liver of the pregnant nonhuman primate: impact of tissue blood flow and site of inhibition.

UNLABELLED: Through PET imaging, our laboratory has studied the dynamic biodistribution of (11)C-verapamil, a P-gp substrate, in the nonhuman primate Macaca nemestrina. To gain detailed insight into the kinetics of verapamil transport across the blood-brain barrier (BBB) and the blood-placental barrier (BPB), we analyzed these dynamic biodistribution data by compartmental modeling. METHODS: Thirteen pregnant macaques (gestational age, 71-159 d; term, ∼172 d) underwent PET imaging with (11)C-verapamil before and during infusion (6, 12, or 24 mg/kg/h) of cyclosporine A (CsA, a P-glycoprotein [P-gp] inhibitor). Dynamic (11)C-verapamil brain or fetal liver (reporter of placental P-gp function) activity was assessed by a 1- or 2-tissue-compartment model. RESULTS: The 1-tissue-compartment model best explained the observed brain and fetal liver distribution of (11)C-radioactivity. When P-gp was completely inhibited, the brain and fetal liver distribution clearance (K1) approximated tissue blood flow (Q); that is, extraction ratio (K1/Q) was approximately 1, indicating that in the absence of P-gp function, the distribution of (11)C-verapamil radioactivity into these compartments is limited by blood flow. The potency of CsA to inhibit P-gp was tissue-independent (maternal BBB half-maximal inhibitory concentration [IC50], 5.67 ± 1.07 µM, vs. BPB IC50, 7.63 ± 3.16 µM). CONCLUSION: We propose that on deliberate or inadvertent P-gp inhibition, the upper boundary of increase in human brain (or fetal) distribution of lipophilic drugs such as verapamil will be limited by tissue blood flow. This finding provides a means to predict the magnitude of P-gp-based drug interactions at the BBB and BPB when only the baseline distribution of the drug (i.e., in the absence of P-gp inhibition) across these barriers is available through PET. Our data suggest that P-gp-based drug interactions at the human BBB and BPB can be clinically significant, particularly for those P-gp substrate drugs for which P-gp plays a significant role in excluding the drug from these privileged compartments.

Autologous dendritic cell transplantation as an adjuvant treatment in pleomorphic lung carcinoma

We embarked on autologous dendric cells (DC) transplantation as an adjuvant therapy with chemotherapy in a pleomorphic lung carcinoma patient. DC were isolated from PBMC and primed with the autologous tumor lysate. No adverse event was noted in DC transplantation. DC administration also correlated with immunomodulation, as evidenced by an approximately 5-fold increase in serum interferon gamma after 2 months. The utility of autologous DC transplantation may offer a clinical benefit with virtually no adverse event.

Simultaneous PET imaging of P-glycoprotein inhibition in multiple tissues in the pregnant nonhuman primate.

UNLABELLED: Studies in rodents indicate that the disruption of P-glycoprotein (P-gp) function increases drug distribution into the developing fetus and organs such as the brain. To simultaneously and serially evaluate the effect of P-gp activity and inhibition on the tissue distribution of drugs in a more representative animal model, we tested the feasibility of conducting whole-body PET of the pregnant nonhuman primate (Macaca nemestrina). We used (11)C-verapamil as the prototypic P-gp substrate and cyclosporine A (CsA) as the prototypic inhibitor. METHODS: Four pregnant macaques (gestational age, 145-159 d; gestational term, 172 d) were imaged after the intravenous administration of (11)C-verapamil (30-72 MBq/kg) before and during intravenous infusion of CsA (12 or 24 mg/kg/h, n = 2 each). The content of verapamil and its metabolites in plasma samples was determined using a rapid solid-phase extraction method. The plasma and tissue time-radioactivity concentration curves of (11)C were integrated over 0-9 min after each verapamil injection. The tissue or arterial plasma area under the time-concentration curve (AUC(tissue)/AUC(plasma)) served as a measure of the tissue distribution of (11)C radioactivity. CsA effect on (11)C radioactivity distribution was interpreted as P-gp inhibition. The change in the fetal liver AUC ratio served as a reporter of placental P-gp inhibition. RESULTS: CsA effect on tissue distribution of (11)C radioactivity (AUC ratios) did not increase with the mean blood concentration of CsA, indicating a near-maximal P-gp inhibition. CsA increased maternal brain and fetal liver distribution of (11)C radioactivity by 276% +/- 88% (P < 0.05) and 122% +/- 75% (P < 0.05), respectively. Changes in other measured tissues were not statistically significant. CONCLUSION: These data demonstrate for the first time, to our knowledge, the feasibility of simultaneous, serial, noninvasive imaging of P-gp activity and inhibition in multiple maternal organs and the placenta in the nonhuman primate. Our findings, consistent with previous data in rodents, indicate that the activity of P-gp in the placenta and the blood-brain barrier is high and that the inhibition of P-gp facilitates drug distribution across these barriers.

The role of transporters in drug interactions.

Transport proteins play an important role in the adsorption, distribution and elimination of a wide variety of drugs. Therefore, it is not surprising that transporter-based drug interactions can occur in the clinic. These interactions can lead to changes in toxicity and/or efficacy of the affected drug. Here, we review such interactions and ask if these interactions could have been predicted from in vitro data. Conducting such in vitro-in vivo correlation is important for predicting future transporter-based drug interactions.