Assessment of active transport of HIV protease inhibitors in various cell lines and the in vitro blood--brain barrier

Role of P-glycoprotein in tissue uptake of indinavir in rat

The effect of p-glycoprotein inhibition on tissue distribution of indinavir, an anti-HIV (human immunodeficiency virus) protease inhibitor drug, has been evaluated. Indinavir was co-administered intravenously in rats along with a p-glycoprotein inhibitor, PSC833, and the drug concentrations in plasma and various tissues were determined using a HPLC method. Additionally, initial uptake clearance of indinavir was evaluated in the brain and testes. The highest increasing effect of p-glycoprotein inhibition on the tissue uptake ratios of indinavir was found in central nervous system (CNS). The estimated tissue extraction the drug was indicative of (i) limited drug entry to brain parenchyma, which was increased significantly by p-glycoprotein inhibition, (ii) non-restricted drug entry to testes, heart and spleen, which was increased significantly in the case of heart and decreased in the case of testes and spleen as a result of p-glycoprotein inhibition, and (iii) drug accumulation in liver and small intestine and, to a lesser extent, kidney, which was not affected by p-glycoprotein inhibition. The uptake clearances of indinavir by brain parenchyma in PSC833-treated and control rats were 68.80+/-8.65 and 21.63+/-4.28 micro/min/g and the corresponding values for the testes were 39.84+/-4.90 and 36.65+/-2.54 microl/min/g. The difference was significant only in the case of brain parenchyma (P<0.001). These data showed that p-glycoprotein inhibition increases the CNS uptake of indinavir markedly and has some transient minor effects on drug uptake by some other tissues.

HIV-1 persistence, viral reservoir, and the central nervous system in the HAART era

HAART therapy has led to a significant reduction of general and neurological morbidity, and mortality among HIV-1 infected patients. It can also decrease HIV-1 RNA titres in plasma and CSF towards undetectable level. However, the initial hope of achieving total eradication of the virus from the body has vanished. Even in patients who do not develop viral resistance or treatment intolerance, two kinds of viral persistence have been demonstrated both in lymphoid and central nervous system. The first one is a smoldering infection that persists, despite prolonged and apparently efficient HAART, in monocytes, tissue macrophages and most probably microglia. The second one is an integration of proviral DNA in the genome of subpopulations of CD4 lymphocytes of patients receiving efficient HAART. A similar viral integration in astrocytes and less likely in resting microglia is suggested by several studies, although it has yet to be demonstrated conclusively.

Cost-effectiveness of enfuvirtide in HIV therapy for treatment-experienced patients in the United States

Enfuvirtide (ENF) is the first of a new class of antiretrovirals (ARVs) known as the HIV fusion inhibitors. Two phase III studies of ENF, TORO 1 and TORO 2, demonstrated that ENF given in combination with optimized background (OB) therapy significantly improved virological response, increased the time to virological failure, and increased CD4-cell count compared with OB alone among highly treatment-experienced patients. The present study investigated the long-term clinical outcomes, costs, and cost-effectiveness of ENF. Outcomes, costs, and cost-effectiveness were estimated using a Markov model. Viral suppression and immune reconstitution were determined from the outcomes of the clinical trials. Time to immunological failure, time to AIDS-defining event (ADE), and time to death were estimated based on published mathematical models of disease progression. Costs were based on published estimates of the use and costs of ARVs, cost of managing ADEs, and cost of laboratory and other outpatient services. Cost-effectiveness was calculated as the incremental cost per year of life gained, adjusted for quality of life. The combined effects of an increase in CD4 count and delayed time to virological and immunological failure with ENF + OB were predicted to produce a mean life expectancy of 7.4 years from initiation of therapy, which was 1.8 years (1.5 quality-adjusted lifeyears [QALYs]) greater than the life expectancy associated with OB alone. The incremental cost-effectiveness of ENF + OB was estimated to be Dollars 24,604 per QALY. ENF is projected to increase time to immunological failure, delay onset of new AIDS-defining events, and increase life expectancy by more than 1.5 years among treatment-experienced HIV-infected patients. The cost-effectiveness of ENF is comparable to many existing treatment and prevention management strategies for HIV.

Dependence of nelfinavir brain uptake on dose and tissue concentrations of the selective P-glycoprotein inhibitor zosuquidar in rats

Most reverse transcriptase and protease inhibitors used in highly active antiretroviral therapy for treating human immunodeficiency virus (HIV) infections exhibit poor penetration into the brain, raising the concern that the brain may be a sanctuary site for the development of resistant HIV variants. This study explores the relationship between the dose and plasma and brain concentrations of zosuquidar and the effect of this selective P-glycoprotein inhibitor on central nervous system penetration of the HIV protease inhibitor nelfinavir maintained at steady state by intravenous infusions in rats. Nelfinavir was infused (10 mg/kg/h) for up to 10 h with or without concurrent administration of an intravenous bolus dose of 2, 6, or 20 mg/kg zosuquidar given at 4 h. Brain tissue and plasma were analyzed for both drug concentrations. Brain tissue/plasma nelfinavir concentration ratios (uncorrected for the vascular contribution) increased nonlinearly with zosuquidar dose from 0.06 +/- 0.03 in the absence of zosuquidar and 0.09 +/- 0.02 between 2 and 6 h after 2 mg/kg zosuquidar to 0.85 +/- 0.19 after 6 mg/kg and 1.58 +/- 0.67 after 20 mg/kg zosuquidar. Zosuquidar brain tissue/plasma concentration ratios exhibited a similar abrupt increase from 2.8 +/- 0.3 after a 2 mg/kg dose to approximately 15 after the 6 and 20 mg/kg doses. The apparent threshold in the plasma concentration of zosuquidar necessary to produce significant enhancement in brain uptake of nelfinavir appears to be close to the plasma concentrations associated with the maximum tolerated dose reported in the literature after repeated dosing of zosuquidar in patients.

Fosamprenavir

Fosamprenavir is one of the most recently approved HIV-1 protease inhibitors (PIs) and offers reductions in pill number and pill size, and omits the need for food and fluid requirements associated with the earlier-approved HIV-1 PIs. Three fosamprenavir dosage regimens are approved by the US FDA for the treatment of HIV-1 PI-naive patients, including fosamprenavir 1,400 mg twice daily, fosamprenavir 1,400 mg once daily plus ritonavir 200mg once daily, and fosamprenavir 700 mg twice daily plus ritonavir 100mg twice daily. Coadministration of fosamprenavir with ritonavir significantly increases plasma amprenavir exposure. The fosamprenavir 700 mg twice daily plus ritonavir 100mg twice daily regimen maintains the highest plasma amprenavir concentrations throughout the dosing interval; this is the only approved regimen for the treatment of HIV-1 PI-experienced patients and is the only regimen approved in the European Union. Fosamprenavir is the phosphate ester prodrug of the HIV-1 PI amprenavir, and is rapidly and extensively converted to amprenavir after oral administration. Plasma amprenavir concentrations are quantifiable within 15 minutes of dosing and peak at 1.5-2 hours after fosamprenavir dosing. Food does not affect the absorption of amprenavir following administration of the fosamprenavir tablet formulation; therefore, fosamprenavir tablets may be administered without regard to food intake. Amprenavir has a large volume of distribution, is 90% bound to plasma proteins and is a substrate of P-glycoprotein. With <1% of a dose excreted in urine, the renal route is not an important elimination pathway, while the principal route of amprenavir elimination is hepatic metabolism by cytochrome P450 (CYP) 3A4. Amprenavir is also an inhibitor and inducer of CYP3A4. Furthermore, fosamprenavir is commonly administered in combination with low-dose ritonavir, which is also extensively metabolised by CYP3A4, and is a more potent CYP3A4 inhibitor than amprenavir. This potent CYP3A4 inhibition contraindicates the coadministration of certain CYP3A4 substrates and requires others to be co-administered with caution. However, fosamprenavir can be co-administered with many other antiretroviral agents, including drugs of the nucleoside/nucleotide reverse transcriptase inhibitor, non-nucleoside reverse transcriptase inhibitor and HIV entry inhibitor classes. Coadministration with other HIV-1 PIs continues to be studied.The extensive fosamprenavir and amprenavir clinical drug interaction information provides guidance on how to co-administer fosamprenavir and fosamprenavir plus ritonavir with many other commonly co-prescribed medications, such as gastric acid suppressants, HMG-CoA reductase inhibitors, antibacterials and antifungal agents.

MDR1 G1199A polymorphism alters permeability of HIV protease inhibitors across P-glycoprotein-expressing epithelial cells

OBJECTIVE

To evaluate the impact of the human multidrug resistance gene (MDR1) G1199A polymorphism (amino acid change Ser400Asn) on P-glycoprotein (P-gp)-dependent transepithelial permeability and uptake kinetics of HIV protease inhibitors (PI), by using recombinant epithelial cells expressing wild-type MDR1 (MDR1wt) or the G1199A variant (MDR1(1199A)).

METHODS

Using a recombinant expression system developed previously, the transepithelial permeability and uptake kinetic parameters of five PI, amprenavir, indinavir, lopinavir, ritonavir, and saquinavir were estimated across polarized epithelial cells.

RESULTS

For all PI, the transepithelial permeability ratio (basolateral-to-apical transport divided by apical-to-basolateral transport) was significantly greater in MDR1(1199A) cells than MDR1wt cells: amprenavir (1.7-fold), indinavir (1.8-fold), lopinavir (1.5-fold), ritonavir (2.8-fold), and saquinavir (2.1-fold). However, the impact of G1199A on P-gp activity appeared to primarily influence drug permeability in the apical-to-basolateral direction. Kinetic analysis of ritonavir and saquinavir uptake by MDR1wt- and MDR1(1199A)-expressing cells showed that Vmax was similar, while uptake Km was significantly higher in cells expressing the G1199A variant suggesting that alterations in P-gp-dependent efflux mediated by G1199A were due to changes in transporter affinity.

CONCLUSIONS

Alterations in transepithelial permeability of HIV PI due to the G1199A polymorphism may impact oral bioavailability of PI and penetration into cells and tissues of the lymphoid and central nervous systems.

Increased expression of MDR1 mRNAs and P-glycoprotein in placentas from HIV-1 infected women

P-glycoprotein transports several compounds including protease inhibitors, actually used in the clinical management of HIV-1 infection. Since P-glycoprotein is expressed in placental trophoblasts, its efflux activity could interfere with placental transfer of antiretrovirals. The purpose of this study was to investigate the expression of P-gp-encoding MDR1 gene and P-gp itself in full-term placentas from uninfected (n=35) and HIV-1 infected women (n=24). MDR1 transcripts were quantified by real-time PCR using relative (MDR1 normalized upon 28S levels) and absolute (copy number) determinations. P-glycoprotein localization and expression were evaluated by immunohistochemistry and western blot analysis, respectively. Relative or absolute PCR quantification showed a significant 3.3-fold (p<0.0009) or 3.7-fold (p<0.0002) mean increase in MDR1 placental transcription in HIV-infected compared to non-infected women, respectively. Ratios of individual HIV-positive values to HIV-negative mean ranged from 0.1 to 21.8. Moreover a significant 2.5-fold increased expression of immunoreactive P-glycoprotein was evidenced in placentas from HIV-infected women (p<0.0001). This MDR1 overexpression was observed in a similar extent in placentas from pregnant women treated with Zidovudine alone or in combination with Nelfinavir and/or Lamivudine. Our findings suggest that P-glycoprotein in placentas from HIV-infected women would contribute to modulate the materno-fetal transport of antiretrovirals across the placental barrier and consequently diminish fetal exposure to these compounds.

Evolution of human immunodeficiency virus type 1 protease genotypes and phenotypes in vivo under selective pressure of the protease inhibitor ritonavir

We examined the population dynamics of human immunodeficiency virus type 1 pro variants during the evolution of resistance to the protease inhibitor ritonavir (RTV) in vivo. pro variants were followed in subjects who had added RTV to their previously failed reverse transcriptase inhibitor therapy using a heteroduplex tracking assay designed to detect common resistance-associated mutations. In most cases the initial variant appeared rapidly within 2 to 3 months followed by one or more subsequent population turnovers. Some of the subsequent transitions between variants were rapid, and some were prolonged with the coexistence of multiple variants. In several cases variants without resistance mutations persisted despite the emergence of new variants with an increasing number of resistance-associated mutations. Based on the rate of turnover of pro variants in the RTV-treated subjects we estimated that the mean fitness of newly emerging variants was increased 1.2-fold (range, 1.02 to 1.8) relative to their predecessors. A subset of pro genes was introduced into infectious molecular clones. The corresponding viruses displayed impaired replication capacity and reduced susceptibility to RTV. A subset of these clones also showed increased susceptibility to two nonnucleoside reverse transcriptase inhibitors and the protease inhibitor saquinavir. Finally, a significant correlation between the reduced replication capacity and reduced processing at the gag NC-p1 processing site was noted. Our results reveal a complexity of patterns in the evolution of resistance to a protease inhibitor. In addition, these results suggest that selection for resistance to one protease inhibitor can have pleiotropic effects that can affect fitness and susceptibility to other drugs.

Transport of HIV protease inhibitors through the blood-brain barrier and interactions with the efflux proteins, P-glycoprotein and multidrug resistance proteins

HIV protease inhibitors (HPIs) have limited penetration into the brain. This poor transport through the blood-brain barrier is mainly due to active efflux by proteins such as P-glycoprotein (P-gp) preventing drugs from clearing the brain of the virus. The present paper focuses on cerebral uptake of HPIs and interactions between HPIs and efflux proteins, either as substrates or modulators. Most of the studies described HPIs as P-gp substrates. Studies are more controversial when investigating HPIs as inhibitors of P-gp. HPIs seem to be able to inhibit efflux proteins of in vitro cell models but with limited consequences in vivo. Moreover, after repeated administrations of HPIs, most of them are also able to induce the expression and functionality of P-gp. For these reasons, certain combinations of HPIs may not efficiently increase brain uptake of HPIs as would combinations of more potent efflux inhibitors.

Quantitative assessment of HIV-1 protease inhibitor interactions with drug efflux transporters in the blood-brain barrier

PURPOSE

To quantitatively characterize the drug efflux interactions of various HIV-1 protease inhibitors in an in vitro model of the blood-brain barrier (BBB) and to compare that with HIV-1 protease inhibitor stimulated P-glycoprotein (P-gp)-ATPase activity.

METHODS

Cellular accumulation of the P-gp sensitive probe, rhodamine 123 (R123), and the mixed P-gp/multidrug resistance-associated protein (MRP) probe, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), were evaluated in primary cultured bovine brain microvessel endothelial cells (BBMEC) in the presence of various concentrations of HIV-1 protease inhibitors. The potency (IC50) and efficacy (Imax) of the drugs in the cell accumulation assays for P-gp and/or MRP was determined and compared to activity in a P-gp ATPase assay.

RESULTS

For R123 (P-gp probe), the rank order potency for inhibiting R123 accumulation in the BBMEC was saquinavir=nelfinavir>ritonavir=amprenavir>indinavir. This correlated well with the rank order affinity in the P-gp ATPase assay. The rank order potency for MRP-related drug efflux transporters, was nelfinavir>ritonavir>saquinavir>amprenavir>indinavir.

CONCLUSIONS

HIV-1 protease inhibitors potently interact with both P-gp and MRP-related transporters in BBMEC. Characterization of the interactions between the HIV-1 protease inhibitors and drug efflux transporters in brain microvessel endothelial cells will provide insight into potential drug-drug interactions and permeability issues in the BBB.

Role of drug efflux transporters in the brain for drug disposition and treatment of brain diseases

The blood-brain barrier (BBB) serves as a protective mechanism for the brain by preventing entry of potentially harmful substances from free access to the central nervous system (CNS). Tight junctions present between the brain microvessel endothelial cells form a diffusion barrier, which selectively excludes most blood-borne substances from entering the brain. Astrocytic end-feet tightly ensheath the vessel wall and appear to be critical for the induction and maintenance of the barrier properties of the brain capillary endothelial cells. Because of these properties, the BBB only allows entry of lipophilic compounds with low molecular weights by passive diffusion. However, many lipophilic drugs show negligible brain uptake. They are substrates for drug efflux transporters such as P-glycoprotein (Pgp), multidrug resistance proteins (MRPs) or organic anion transporting polypeptides (OATPs) that are expressed at brain capillary endothelial cells and/or astrocytic end-feet and are key elements of the molecular machinery that confers the special permeability properties to the BBB. The combined action of these carrier systems results in rapid efflux of xenobiotics from the CNS. The objective of this review is to summarize transporter characteristics (cellular localization, specificity, regulation, and potential inhibition) for drug efflux transport systems identified in the BBB and blood-cerebrospinal fluid (CSF) barrier. A variety of experimental approaches available to ascertain or predict the impact of efflux transport on brain access of therapeutic drugs also are described and critically discussed. The potential impact of efflux transport on the pharmacodynamics of agents acting in the CNS is illustrated. Furthermore, the current knowledge about drug efflux transporters as a major determinant of multidrug resistance of brain diseases such as epilepsy is reviewed. Finally, we summarize strategies for modulating or by-passing drug efflux transporters at the BBB as novel therapeutic approaches to drug-resistant brain diseases.

Atazanavir for the treatment of human immunodeficiency virus infection

Atazanavir is the first once-daily protease inhibitor for the treatment of human immunodeficiency virus type 1 infection and should be used only in combination therapy, as part of a highly active antiretroviral therapy (HAART) regimen. In addition to being the most potent protease inhibitor in vitro, atazanavir has a distinct cross-resistance profile that does not confer resistance to other protease inhibitors. However, resistance to other protease inhibitors often confers clinically relevant resistance to atazanavir. Currently, atazanavir is not a preferred protease inhibitor for initial HAART regimens. In treatment-naive patients, atazanavir can be given as 400 mg/day. However, atazanavir should be pharmacologically boosted with ritonavir in treatment-experienced patients or when coadministered with either tenofovir or efavirenz. Patients who receive atazanavir experience similar rates of adverse events compared with patients receiving comparator regimens. An exception is an increased risk of asymptomatic hyperbilirubinemia, which is due to competitive inhibition of uridine diphosphate-glucuronosyltransferase 1A1. Although hyperbilirubinemia is a common adverse drug reaction of atazanavir therapy (22-47%), fewer than 2% of patients discontinue atazanavir therapy because of this adverse effect. Common adverse effects reported with atazanavir include infection, nausea, vomiting, diarrhea, abdominal pain, headache, peripheral neuropathy, and rash. Of significance, fewer abnormalities have been observed in plasma lipid profiles in patients treated with atazanavir compared with other protease inhibitor-containing regimens. As with other protease inhibitors, atazanavir is also a substrate and moderate inhibitor of the cytochrome P450 (CYP) system, in particular CYP3A4 and CYP2C9. Clinically significant drug interactions include (but are not limited to) antacids, proton pump inhibitors, histamine type 2 receptor antagonists, tenofovir, diltiazem, irinotecan, simvastatin, lovastatin, St. John's wort, and warfarin. We conclude that atazanavir is a distinctively characteristic protease inhibitor owing to its in vitro potency, once-daily dosing, distinct initial resistance pattern, and infrequent association with metabolic abnormalities.

Blood-brain barrier active efflux transporters: ATP-binding cassette gene family

The blood-brain barrier (BBB) contributes to brain homeostasis by protecting the brain from potentially harmful endogenous and exogenous substances. BBB active drug efflux transporters of the ATP-binding cassette (ABC) gene family are increasingly recognized as important determinants of drug distribution to, and elimination from, the CNS. The ABC efflux transporter P-glycoprotein (Pgp) has been demonstrated as a key element of the BBB that can actively transport a huge variety of lipophilic drugs out of the brain capillary endothelial cells that form the BBB. In addition to Pgp, other ABC efflux transporters such as members of the multidrug resistance protein (MRP) family and breast cancer resistance protein (BCRP) seem to contribute to BBB function. Consequences of ABC efflux transporters in the BBB include minimizing or avoiding neurotoxic adverse effects of drugs that otherwise would penetrate into the brain. However, ABC efflux transporters may also limit the central distribution of drugs that are beneficial to treat CNS diseases. Furthermore, neurological disorders such as epilepsy may be associated with overexpression of ABC efflux transporters at the BBB, resulting in pharmacoresistance to therapeutic medication. Therefore, modulation of ABC efflux transporters at the BBB forms a novel strategy to enhance the penetration of drugs into the brain and may yield new therapeutic options for drug-resistant CNS diseases.

Blood-brain barrier transport of synthetic adenosine A1 receptor agonists in vitro: structure transport relationships

Transport of 11 structurally related adenosine A(1) receptor agonists was determined in an in vitro BBB model of brain-capillary-endothelial-cells and astrocytes. Inhibitor S-(4-nitrobenzyl)-6-thioinosine (NBTI) was used to quantify the contribution of the es nucleoside transporter to the overall transport. The N(6)-substituted adenosine analogues N(6)-cyclobutyladenosine (CBA), N(6)-cyclopentyladenosine (CPA) and N(6)-cyclohexyladenosine (CHA) showed concentration-dependent clearance and their transport could be inhibited by NBTI. The V(max) was 1.5+/-0.2 pmol min(-1) and the Km values were 2.2+/-0.2, 1.8+/-0.3 and 15+/-4 microM for CBA, CPA and CHA, respectively. Further chemical modification such as substitution in the C8-position or modification at the ribose-moiety resulted in loss of affinity for the es nucleoside transporter. Transport by passive diffusion was slow with clearances ranging from 0.21+/-0.01 microl min(-1) for 8-(methylamino)-CPA (MCPA) to 1.8+/-0.18 microl min(-1) for 5'-deoxy-CPA (5'dCPA). Regression analysis showed no relationship between transport clearance by passive diffusion and the GTP-shift, a non-linear relationship between the transport clearance by passive diffusion and the dynamic polar surface area (Cl=0.469e(-0.071DPSA); R2=0.88) and a linear relationship between transport clearance and prediction of BBB transport on basis of the Abraham equation (logCl=1.53logBB-1.56; R2=0.83). It is concluded that the transport of synthetic A(1) adenosine derivatives across the blood-brain barrier is generally quite slow. In addition, transport by the es nucleoside transporter may contribute to the transport of certain structurally distinct analogues.

The Plasma and Intracellular Steady-State Pharmacokinetics of Lopinavir/Ritonavir in HIV-1-Infected Patients

Therapeutic drug monitoring of protease inhibitors (PIs) is usually performed on plasma samples although their antiretroviral effect takes place inside cells. Little is known, however, about the intracellular accumulation and related plasma pharmacokinetics of PIs such as lopinavir/ritonavir (LPV/RTV). Therefore, we studied the plasma and intracellular (cell-associated) steady-state pharmacokinetics of this PI combination in a dosage of 400/100 mg twice daily in a non-randomized cohort of HIV-1-infected individuals. Plasma (0–12 h) and peripheral blood mononuclear cell (PBMC; 0–8 h) samples were drawn during a 12-h dosing interval in 11 subjects.

The plasma concentrations versus time curves of LPV and RTV were characterized by an irregular absorption phase showing double-peaks (Cmax) in most subjects and single-peaks in the remaining patients between 1 and 3 h after drug intake. Pre-dose concentrations of both agents in plasma were significantly higher than the concentrations at the end of the dosing interval indicating the presence of a circadian rhythm in their pharmacokinetics. The course of the intracellular concentrations versus time curves appeared to be similar to the plasma concentration curves, with the highest intracellular concentration measured 3 h after drug intake. The intracellular RTV concentrations were higher than reported in vitro EC50 values and might therefore contribute to the antiretroviral effect of LPV/RTV. The median intracellular-to-plasma concentration ratios (interquartile range) were 1.18 (0.74–2.06) and 4.59 (3.20–7.70) for LPV and RTV, respectively.

In conclusion, both LPV and RTV accumulate to potential therapeutic concentrations in PBMCs. Irregular absorption and circadian plasma clearance patterns were observed for the PI combination LPV/RTV.

Anti-HIV effects of chloroquine: inhibition of viral particle glycosylation and synergism with protease inhibitors

OBJECTIVE

We tested the effects of chloroquine (CQ) on glycosylation of HIV particles and in combination with protease inhibitors (PIs) on HIV replication and on P-glycoprotein (P-gp)/multidrug resistance protein-1 (MRP1).

DESIGN

CD4 cell lines were infected with laboratory strains and peripheral blood mononuclear cells were infected with primary isolates for evaluation of the anti-HIV effects. Peripheral blood lymphocytes were evaluated for of P-gp and MRP1 functions.

METHODS

HIV replication was assessed by enzyme-linked immunosorbent assay. HIV glycosylation was measured by metabolic labeling of viral particles with [H] glucosamine. Synergism was tested using isobolograms. P-gp and MRP1 functions were assayed using rhodamine 123 (Rh123) and carboxyfluorescein (CF) efflux assays, respectively.

RESULTS

CQ alone inhibited HIV replication and glycosylation in a dose-dependent manner. In combination with indinavir (IDV), ritonavir, or saquinavir (SQV), CQ had a synergistic effect at concentrations found in plasma of subjects receiving malaria prophylaxis. CQ decreased the 50% effective concentration of IDV in primary isolates from Africa and restored the response to IDV or SQV in 3 PI-resistant isolates. CQ increased the block of Rh123 and CF efflux activity exerted by PIs.

CONCLUSION

The inhibitory effects of CQ on HIV glycosylation are associated with synergistic effects in combination with PIs. The CQ/PI combination exerts combined inhibitory effects on P-gp and MRP1 function.

Atp Binding Cassette Multidrug Transporters Limit the Anti-HIV Activity of Zidovudine and Indinavir in Infected Human Macrophages

Objectives

To investigate whether P-glycoprotein (P-gp) and multidrug resistance proteins (MRPs), which limit the bioavailability of HIV protease inhibitors (PIs) and nucleoside reverse transcriptase inhibitors (NRTIs), modulate the anti-HIV activity of NRTIs, non-NRTIs and PIs in vitro. Design: We used primary cultures of major HIV target cells: human monocyte-derived macrophages (MDMs) and lymphocytes.

Methods

P-gp and MRP expression in response to long-term zidovudine (3′-azido-3′-deoxythymidine; AZT) or indinavir treatment was quantified by RT-PCR. MDM and lymphocytes were infected in vitro with HIV-1/Ba-L and HIV-1-LAI, respectively, and treated with antiretroviral drugs. We evaluated the activity of these drugs in combination with PSC833, a P-gp inhibitor, and/or probenecid, an MRP1 inhibitor. Intracellular AZT triphosphate derivative (AZT-TP) was quantified by HPLC-MSMS. P-gp ATPase activity was measured with inside-out native membrane vesicles enriched in P-gp.

Results

Levels of MDR1, mrp4 and mrp5 mRNA were high following AZT treatment. In infected MDM, PSC833 and probenecid increased the anti-HIV activity of AZT and indinavir. AZT (5 nM) decreased HIV replication by 34% alone and by 72% in combination with P-gp/MRP inhibitors. Indinavir (10 nM) gave 14% inhibition alone and 81% in combination. The increase in anti-HIV activity of AZT was correlated with an increase in intracellular AZT-TP concentration. However, unlike PIs, neither AZT nor its metabolites interacted with P-gp.

Conclusion

AZT increases the expression of multidrug transporters, thereby decreasing its pharmacological activity. The cellular efflux of AZT probably involves MRP4 or MRP5. In contrast, increases in indinavir anti-HIV activity require the inhibition of both P-gp and MRP1.

Cellular localization and functional expression of P-glycoprotein in rat astrocyte cultures

We investigated the cellular/subcellular localization and functional expression of P-glycoprotein, an ATP-dependent membrane-associated efflux transporter, in astrocytes, a brain parenchyma compartment that is poorly characterized for the expression of membrane drug transporters. Analyses were carried out on primary cultures of astrocytes isolated from the cerebral cortex of neonatal Wistar rats and CTX TNA2, an immortalized rat astrocyte cell line. Both cell cultures display morphological features typical of type I astrocytes. RT-PCR analysis revealed mdr1a and mdr1b mRNA in primary cultures of astrocytes and in CTX TNA2 cells. Western blot analysis using the P-glycoprotein monoclonal C219 antibody detected a single band of appropriate size in both cell systems. Immunocytochemical analysis using the monoclonal antibodies C219 and MRK16 labeled P-glycoprotein along the plasma membrane, caveolae, coated vesicles and nuclear envelope. Immunoprecipitation studies using the caveolin-1 polyclonal H-97 antibody demonstrated that P-glycoprotein is physically associated with caveolin-1 in both cell culture systems. The accumulation of [(3)H]digoxin (an established P-glycoprotein substrate) by the astrocyte cultures was significantly enhanced in the presence of standard P-glycoprotein inhibitors and an ATP depleting agent. These results demonstrate the cellular/subcellular location and functional expression of P-glycoprotein in rat astrocytes and suggest that this glial compartment may play an important role in the regulation of drug transport in the CNS.

Prodrugs of HIV protease inhibitors—saquinavir, indinavir and nelfinavir—derived from diglycerides or amino acids: synthesis, stability and anti-HIV activity

With the aim of improving the pharmacological properties of current protease inhibitors (PIs), the synthesis of various acyl and carbamate amino acid- or diglyceride-containing prodrugs derived from saquinavir, indinavir and nelfinavir, their in vitro stability with respect to hydrolysis and their anti-HIV activity in CEM-SS and MT4 cells have been investigated. l-Leucine (Leu) and l-phenylalanine (Phe) were connected through their carboxyl to the PIs while l-tyrosine (Tyr) was conjugated through its aromatic hydroxyl via various spacer units. Hydrolysis of the prodrug with liberation of the active free drug was crucial for antiviral activity. The Leu- and Phe-PI prodrugs released the active free drug very rapidly (half-lives of hydrolysis in buffer at 37 degree C of 3-4 h). The Tyr-PI conjugates with a -C(O)(CH(2))(4)- linker exhibited half-lives in the 40-70 h range and antiviral activities in the 21-325 nM range (from 2 to 22 nM for the free PIs). The chemically very stable carbamate "peptidomimetic" Tyr-PI prodrugs (no hydrolysis detected after 7 days in buffer) displayed a very low anti-HIV activity or were even inactive (EC(50) from 2300 nM to >10 microM). A very low antiviral activity was measured for the diglyceride-substituted saquinavir and for all of the disubstituted indinavir and nelfinavir prodrugs. All these prodrugs probably released the active parent PI too slowly under the antiviral assay conditions. These results combined with those from transepithelial transport studies (Rouquayrol et al., Pharm. Res., 2002, 19, 1704-1712) indicate that conjugation of amino acids (through their carboxyl) to the PIs constitutes a most appealing alternative which could improve the intestinal absorption of the PIs and reduce their recognition by efflux carriers.

The Distribution of the HIV Protease Inhibitor, Ritonavir, to the Brain, Cerebrospinal Fluid, and Choroid Plexuses of the Guinea Pig

Anti-human immunodeficiency virus (HIV) drug penetration into the brain and cerebrospinal fluid (CSF) is necessary to tackle HIV within the CNS. This study examines movement of [(3)H]ritonavir across the guinea pig blood-brain and blood-CSF barriers and accumulation within the brain, CSF, and choroid plexus. Ritonavir is a protease inhibitor, used in combination therapy (often as a pharmacoenhancer) to treat HIV. Drug interactions at brain barrier efflux systems may influence the CNS penetration of anti-viral drugs, thus the influence of additional protease inhibitors, nucleoside reverse transcriptase inhibitors, and non-nucleoside reverse transcriptase inhibitors on [(3)H]ritonavir CNS distribution was explored. Additionally, the involvement of transporters on [(3)H]ritonavir passage across the brain barriers was assessed. Results from in situ brain perfusions and capillary depletion analysis demonstrated that [(3)H]ritonavir uptake into the guinea pig brain was considerable (6.6 +/- 0.7 ml/100 g at 30 min, vascular space corrected), although a proportion of drug remained trapped in the cerebral capillaries and did not reach the brain parenchyma. CSF uptake was more limited (2.2 +/- 0.4 ml/100 g at 30 min), but choroid plexus uptake was abundant (176.7 +/- 46.3 ml/100 g at 30 min). [(3)H]Ritonavir brain and CSF uptake was unaffected by neither inhibitors of organic anion transport (probenecid and digoxin) or P-glycoprotein (progesterone), nor by any additional anti-HIV drugs, indicating that brain barrier efflux systems do not significantly limit brain or CSF [(3)H]ritonavir accumulation in this model. [(3)H]Ritonavir uptake into the perfused choroid plexus was significantly reduced by nevirapine and abacavir, additional perfusion studies, and isolated incubated choroid plexus experiments were carried out in an attempt to further characterize the transporter involved.

The role of drug transporters at the blood-brain barrier

The blood-brain barrier (BBB) is a dynamic interface between the blood and the brain. It eliminates (toxic) substances from the endothelial compartment and supplies the brain with nutrients and other (endogenous) compounds. It can be considered as an organ protecting the brain and regulating its homeostasis. Until now, many transport systems have been discovered that play an important role in maintaining BBB integrity and brain homeostasis. In this review, we focus on the role of carrier- and receptor-mediated transport systems (CMT, RMT) at the BBB. These include CMT systems, such as P-glycoprotein, multidrug-resistance proteins 1-7, nucleoside transporters, organic anion transporters, and large amino-acid transporters; RMT systems, such as the transferrin-1 and -2 receptors; and the scavenger receptors SB-AI and SB-BI.

abcb1ab P-glycoprotein is involved in the uptake of citalopram and trimipramine into the brain of mice

The phenomenon of a heterogeneous response to the same drug in different patients is well-known. An important reason is that, even at equal concentrations, the bioavailability of a drug depends on the interaction of the drug with the blood-brain barrier (BBB). In part, this is due to the drug-transporting P-glycoprotein (P-gp), a product of the multiple drug resistance gene (ABCB1), which can transport drugs against a concentration gradient across the BBB back into the plasma and thereby reduce the bioavailability in the brain. In the present study, we have examined the uptake of the antidepressants citalopram and trimipramine into the brain of abcb1ab knockout mice compared with controls. One hour after s.c. injection of the drugs, concentrations of the two drugs and of the metabolite d-trimipramine in brain, spleen, kidney, liver and plasma were measured with HPLC. Significantly higher brain concentrations in knockout mice, showing that these drugs are substrates of P-gp and that the presence of P-gp reduces the effective bioavailability of these substances in the brain. The results of our study contradict an earlier report that citalopram is not actively transported from endothelial cells. These results were derived from an in vitro study, showing that due to the complexity of the BBB-drug interaction, it is difficult to transfer results from in vitro studies to the in vivo situation. We hypothesize that inter-individual differences in the activity of the ABCB1 gene can account in part for the great variation in clinical response to antidepressants in psychiatric patients, even at comparable plasma levels.

Neurologic aspects of HIV infection in infants and children: therapeutic approaches and outcome

Central nervous system (CNS) HIV-related disorders frequently have devastating consequences. Significant progress has been made in the early diagnosis and treatment of the HIV-infected patient. As a result, the prevalence and natural history of neurologic illnesses have changed. This paper reviews the epidemiology, clinical manifestations, and neuropathogenesis of HIV-related CNS disorders. Advances in antiretroviral therapy, neuroprophylaxis, and neuroprotective strategies are also discussed.

Assessing safety and efficacy of directed P-glycoprotein inhibition to improve the pharmacokinetic properties of saquinavir coadministered with ritonavir

Using a mouse model, we tested the effects of in vivo P-glycoprotein inhibition to enhance the oral uptake and penetration into pharmacological sanctuary sites of the human immunodeficiency virus protease inhibitor (HPI) saquinavir. The HPI ritonavir is frequently coadministered with saquinavir to improve saquinavir plasma levels since it strongly reduces the cytochrome P450 3A4-mediated metabolism of saquinavir. Previously, we demonstrated that ritonavir is not an efficient P-glycoprotein inhibitor in vivo, evidenced by the limited oral uptake of saquinavir and its penetration into brain and fetus. Increasing drug concentrations in these sites using more effective P-gp inhibitors might improve therapy but could also lead to toxicity. We orally coadministered ritonavir and saquinavir to mice, with or without the potent P-glycoprotein inhibitor N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918). Upon GF120918 coadministration, two of seven P-glycoprotein-deficient animals died. Using a decreased ritonavir dose, GF120918 coadministration led to a 4.4-fold increase in the saquinavir plasma area under the curve in wild-type mice, whereas no such effect was observed in P-glycoprotein-deficient mice. Despite the decreased ritonavir dose, all mice did suffer from impaired gastric emptying. Including GF120918 in a multiple (twice daily) dosing regimen, we found continued accumulation of saquinavir in brain over several days, resulting in 10-fold higher levels compared with vehicle-treated mice. Transient ritonavir-related neurotoxicity, however, was observed after the fourth and final drug dosing. Clinical attempts to efficiently inhibit P-glycoprotein function for improved HPI disposition may therefore be feasible, but they should be performed without ritonavir and monitored carefully for unexpected toxicities.

Drug efflux transporters in the CNS

The central nervous system (CNS) contains important cellular barriers that maintain homeostasis by protecting the brain from circulating toxins and through the elimination of toxic metabolites generated in the brain. The barriers that limit the concentration of toxins and xenobiotics in the interstitial fluids of the CNS are the capillary endothelial cells of the blood-brain barrier (BBB) and the epithelial cells of the blood-cerebrospinal fluid barrier (BCSFB). Both of these barriers have cellular tight junctions and express transport systems which serve to actively transport nutrients into the brain, and actively efflux toxic metabolites and xenobiotics out of the brain. This review will focus on the expression and function of selected drug efflux transporters in these two barriers, specifically the multidrug resistance transporter, p-glycoprotein, and various organic anion transporters, such as multidrug resistance-associated proteins, organic anion transporter polypeptides, and organic anion transporters. These transport systems are increasingly recognized as important determinants of drug distribution to, and elimination from, different compartments of the CNS. Consequences of drug efflux transporters in barriers of the CNS include limiting the distribution of substrates that are beneficial to treat CNS diseases, and increasing the possibility of drug-drug interactions that may lead to untoward toxicities. Therefore, the study of these transporters is important in examining the various determinants of drug delivery to the CNS.

Mammalian ABC transporters in health and disease

The ATP-binding cassette (ABC) transporters are a family of large proteins in membranes and are able to transport a variety of compounds through membranes against steep concentration gradients at the cost of ATP hydrolysis. The available outline of the human genome contains 48 ABC genes; 16 of these have a known function and 14 are associated with a defined human disease. Major physiological functions of ABC transporters include the transport of lipids, bile salts, toxic compounds, and peptides for antigen presentation or other purposes. We review the functions of mammalian ABC transporters, emphasizing biochemical mechanisms and genetic defects. Our overview illustrates the importance of ABC transporters in human physiology, toxicology, pharmacology, and disease. We focus on three topics: (a) ABC transporters transporting drugs (xenotoxins) and drug conjugates. (b) Mammalian secretory epithelia using ABC transporters to excrete a large number of substances, sometimes against a steep concentration gradient. Several inborn errors in liver metabolism are due to mutations in one of the genes for these pumps; these are discussed. (c) A rapidly increasing number of ABC transporters are found to play a role in lipid transport. Defects in each of these transporters are involved in human inborn or acquired diseases.

Multidrug resistance protein 2 (MRP2) transports HIV protease inhibitors, and transport can be enhanced by other drugs

BACKGROUND

Various drug transporters of the ATP-binding cassette (ABC) family restrict the oral bioavailability and cellular, brain, testis, cerebrospinal fluid and fetal penetration of substrate drugs. MDRI P-glycoprotein (P-gp) has been demonstrated to transport most HIV protease inhibitors (HPI) and to reduce their oral bioavailability and lymphocyte, brain, testis and fetal penetration, possibly resulting in major limiting effects on the therapeutic efficacy of these drugs.

OBJECTIVES

To investigate whether the ABC transporters MRP1, MRP2, MRP3, MRP5 and breast cancer resistance protein 1 (Bcrp1) are efficient transporters of the HPI saquinavir, ritonavir and indinavir.

METHODS

Polarized epithelial non-human (canine) cell lines transduced with human or murine complementary DNA (cDNA) for each of the transporters were used to study transepithelial transport of the HPI.

RESULTS

MRP2 efficiently transported saquinavir, ritonavir and indinavir and this transport could be enhanced by probenecid. Sulfinpyrazone was also able to enhance MRP2-mediated saquinavir transport. In contrast, MRP1, MRP3, MRP5, or Bcrp1 did not efficiently transport the HPI tested.

CONCLUSIONS

Human MRP2 actively transports several HPI and could, based on its known and assumed tissue distribution, therefore reduce HPI oral bioavailability. It may also limit brain and fetal penetration of these drugs and increase their hepatobiliary, intestinal and renal clearance. MRP2 function and enhancement of its activity could adversely affect the therapeutic efficacy, including the pharmacological sanctuary penetration, of HPI. In vivo inhibition of MRP2 function might, therefore, improve HIV/AIDS therapy.

Intracellular concentration of protease inhibitors in HIV-1-infected patients: correlation with MDR-1 gene expression and low dose of ritonavir

BACKGROUND

Protease inhibitors (PIs) are substrates for the P-glycoprotein (P-gp/170) encoded by the multi-drug resistance gene (MDR-1). HIV infection is associated with increased expression of P-gp. The role of MDR gene overexpression in clinical pharmacokinetics is not known.

METHOD

We determined by HPLC, at trough and peak levels, the current PI concentrations in plasma (P) and in peripheral blood mononuclear cells (PBMCs) (intracellular concentration [IC]) from 49 HIV-infected patients receiving different treatment combinations: nelfinavir ([NFV] n = 12); indinavir ([IDV] n = 10); amprenavir ([APV] n = 5); ritonavir (RTV) 100 bid/IDV 800 mg bid (n = 6); RTV 400 bid/IDV 400 mg bid (n = 3); RTV 100 bid/saquinavir (SQV) 600 mg tid (n = 9); APV 600 bid/RTV 100 mg bid (n = 4). We determined the mean ratio of intracellular/plasma PI concentration for each treatment group. The MDR-1 gene expression was determined by a semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR). HIV viral load was simultaneously measured.

RESULTS

49 patients (mean age 41 +/- 8.7 years; mean CD4 cell count 418 [57-972]; mean HIV RNA 2.1 +/- 0.8 log(10)) were included in the study. Patients who overexpressed the MDR-1 gene had significantly lower trough intracellular PI levels (p =.02) or lower intracellular accumulation of PI (p =.042). Patients treated with low-dose RTV in combined regimens with detectable RTV intracellular concentration showed lack of MDR-1 gene expression (p =.01). Patients with HIV RNA < 40 copies/mL had significantly higher RTV intracellular accumulation (p =.029).

CONCLUSION

In HIV-infected patients, IC of PI is inversely correlated with MDR-1 gene overexpression. Undetectable viral load was associated with the use of low-dose RTV, probably linked to better intracellular accumulation of the drug. Nevertheless, further investigation is needed to confirm these results.

The protease inhibitor ritonavir inhibits the functional activity of the multidrug resistance related-protein 1 (MRP-1)

BACKGROUND

Efflux pumps situated on the plasma membrane, such as P-glycoprotein (Pgp) and the multidrug resistance related-protein 1 (MRP-1), have been shown to extrude HIV protease inhibitors from the cell. MRP-1 is present on many barrier sites throughout the body, such as the blood-brain and blood-testis interfaces and could reduce the concentration of protease inhibitors in these sanctuary sites for HIV-1 replication. Factors that modulate efflux pump function in vivo are poorly defined.

OBJECTIVE

To analyze the inhibitory potential of the anti-retroviral drugs indinavir, amprenavir, ritonavir, lamivudine or zidovudine to modulate MRP-1 function.

METHODS

Effect of anti-HIV drugs on the efflux pump activity of MRP-1 was evaluated in the presence of increasing concentrations of human plasma, using UMCC-1/VP cells which stably over-express MRP-1. MRP-1 activity was abrogated by probenecid. The potential of blocking MRP-1 function for an extended (3 day) time period, was also examined in MRP-1 over-expressing cells cultured with either probenecid or the anti-retroviral drugs and a cytotoxic compound (etoposide) that is transported by MRP-1.

RESULTS

Ritonavir inhibited the functional activity of MRP-1 similarly to probenecid, as demonstrated by re-sensitization of MRP-1 over-expressing cells to cytotoxic effects of etoposide. Inhibition by ritonavir was inversely related to the concentration of human plasma added to the cells (r2 = 0.89). Other anti-HIV drugs didn't affect the MRP-1 mediated efflux of etoposide.

CONCLUSIONS

These data may be exploitable to further improve sanctuary site concentrations of anti-HIV or anti-cancer drugs by using ritonavir as a lead compound to develop more potent MRP-1 inhibitors.

The safety, plasma pharmacokinetics, and antiviral activity of subcutaneous enfuvirtide (T-20), a peptide inhibitor of gp41-mediated virus fusion, in HIV-infected adults

Enfuvirtide (T-20) is a novel antiretroviral agent that blocks HIV-1 cell fusion. A 28-day randomized dose-comparison study was conducted to determine the safety, pharmacokinetics, and antiviral activity of enfuvirtide in 78 HIV-infected adults, most with extensive treatment experience. Patients received enfuvirtide, added to a failing regimen, either by continuous subcutaneous infusion (CSI: 12.5, 25, 50 or 100 mg/day) or by subcutaneous (SC) injection (50 or 100 mg twice daily). Dose-related decreases in viral load were observed, with a maximum mean reduction from baseline of 1.6 log(10) copies/ml (p< 0.001) seen in the 100 mg bid SC group. Most responses diminished by 28 days. Plasma pharmacokinetics and antiviral responses were more consistent for SC injection than for CSI because of technical difficulties experienced with CSI. Injection site reactions were common but generally mild. These results indicate that enfuvirtide is a promising new therapeutic agent for HIV-infected patients, including those with prior antiretroviral treatment.

The medicinal chemistry of multidrug resistance (MDR) reversing drugs

Multidrug resistance (MDR) is a kind of resistance of cancer cells to multiple classes of chemotherapic drugs that can be structurally and mechanistically unrelated. Classical MDR regards altered membrane transport that results in lower cell concentrations of cytotoxic drug and is related to the over expression of a variety of proteins that act as ATP-dependent extrusion pumps. P-glycoprotein (Pgp) and multidrug resistance protein (MRP1) are the most important and widely studied members of the family that belongs to the ABC superfamily of transporters. It is apparent that, besides their role in cancer cell resistance, these proteins have multiple physiological functions as well, since they are expressed also in many important non-tumoural tissues and are largely present in prokaryotic organisms. A number of drugs have been identified which are able to reverse the effects of Pgp, MRPI and sister proteins, on multidrug resistance. The first MDR modulators discovered and studied in clinical trials were endowed with definite pharmacological actions so that the doses required to overcome MDR were associated with unacceptably high side effects. As a consequence, much attention has been focused on developing more potent and selective modulators with proper potency, selectivity and pharmacokinetics that can be used at lower doses. Several novel MDR reversing agents (also known as chemosensitisers) are currently undergoing clinical evaluation for the treatment of resistant tumours. This review is concerned with the medicinal chemistry of MDR reversers, with particular attention to the drugs that are presently in development.

Complementary antiviral efficacy of hydroxyurea and protease inhibitors in human immunodeficiency virus-infected dendritic cells and lymphocytes

Dendritic cells are susceptible to human immunodeficiency virus (HIV) infection and may transmit the virus to T cells in vivo. Scarce information is available about drug efficacy in dendritic cells because preclinical testing of antiretroviral drugs has been limited predominantly to T cells and macrophages. We compared the antiviral activities of hydroxyurea and two protease inhibitors (indinavir and ritonavir) in monocyte-derived dendritic cells and in lymphocytes. At therapeutic concentrations (50 to 100 microM), hydroxyurea inhibited supernatant virus production from monocyte-derived dendritic cells in vitro but the drug was ineffective in activated lymphocytes. Concentrations of hydroxyurea insufficient to be effective in activated lymphocytes cultured alone strongly inhibited supernatant virus production from cocultures of uninfected, activated lymphocytes with previously infected monocyte-derived dendritic cells in vitro. In contrast, protease inhibitors were up to 30-fold less efficient in dendritic cells than in activated lymphocytes. Our data support the rationale for testing of the combination of hydroxyurea and protease inhibitors, since these drugs may have complementary antiviral efficacies in different cell compartments. A new criterion for combining drugs for the treatment of HIV infection could be to include at least one drug that selectively targets HIV in viral reservoirs.

A co-culture-based model of human blood-brain barrier: application to active transport of indinavir and in vivo-in vitro correlation

The growing array of in vitro models of the blood-brain barrier (BBB) which have been used makes it difficult to draw firm conclusions concerning the BBB penetration of HIV-1 protease inhibitors. What is needed is a combined in vivo and in vitro study on biological models that mimic as closely as possible the normal human BBB, to establish whether and how indinavir crosses the BBB. We developed a new human BBB model using primary endothelial cells and astrocytes. The biological relevance of this model was checked with respect on the one hand, to the close relationship between the log of drug permeability coefficient normalized to molecular weight and the log of the 1-octanol/water partition coefficient, and on the other hand to the functional P-glycoprotein (P-gp) expression. We employed this model to perform transport studies with indinavir and showed that the rate of in vitro indinavir transport from the basal to apical compartment was higher than the rate of apical to basal transport. Pretreatment of the BBB model with the P-gp inhibitor, quinidine, significantly increased apical to basal transport. Intracellular indinavir accumulation was increased in BBB as a result of inhibition of active transport. These data were correlated with the indinavir-mediated P-gp ATPase modulation showing that indinavir specifically interacted with a binding site on P-gp. Moreover, the activation of P-gp ATPase by indinavir was inhibited by quinidine. In addition, the in vivo brain to plasma concentration ratio of indinavir into mice showed that indinavir concentration was up to five times higher in the brain of mdr1a(-/-) mice than in the brain of mdr1a(+/+) mice. All these results confirm the role of P-gp in preventing the passage of indinavir across BBB and thus its entry into the central nervous system (CNS). Our human BBB model represents a useful tool for the evaluation of drug penetration into the CNS.

Expression of P-glycoprotein and multidrug resistance-associated protein in healthy volunteers and HIV-infected patients

Increased expression of the multidrug efflux transporters P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP) has been suggested as a potential mechanism for decreased drug availability at certain intracellular sites that provide sanctuary for HIV. Here we investigate the expression of these transporters in peripheral blood mononuclear cells (PBMCs) of HIV-infected patients and healthy volunteers. Venous blood (30 ml) was taken from healthy volunteers (n = 21) and HIV-infected patients (n = 21; 4 antiretroviral drug naive, 17 antiretroviral drug experienced). PBMCs were isolated and fixed. To assess P-gp expression, PBMCs were incubated with an isotype control antibody or an antibody directed to an external epitope of P-gp (UIC2). To assess MRP expression, cells were permeabilized before incubation with either a control antibody or an antibody directed to an internal epitope of MRP (MRPm5). After washing, a secondary phycoerythrin-bound antibody was incubated. After additional wash steps, samples were fixed and analyzed by flow cytometry. The median fluorescence intensity of 5000 events was recorded. Results are expressed as fold increase between isotype control and UIC2/MRPm5 samples. Expression of P-gp in HIV-infected patients (1.42 +/- 0.36) was significantly lower (p = 0.0021; 95% CI, -0.633 to -0.164) than in healthy volunteers (1.82 +/- 0.55). However, MRP expression was similar in HIV-infected patients (1.37 +/- 0.34) and healthy volunteers (1.37 +/- 0.21; p = 0.91; 95% CI, -0.148941 to 0.165191). We conclude that in HIV infection, P-gp expression in total PBMCs is reduced whereas MRP expression appears to be unaltered.

Active efflux of the 5-HT(1A) receptor agonist flesinoxan via P-glycoprotein at the blood-brain barrier

The role of P-glycoprotein on the efflux of the 5-HT(1A) receptor agonist flesinoxan across the blood-brain barrier in vivo and in vitro was investigated. In vitro, the transport ratios (representing polarized transport) of flesinoxan (10 microg/ml) were 4.2 in the MDR1-transfected LLC-PK1 cell line, which could be inhibited by the Pgp modulators SDZ-PSC 833 and LY 335979 and 1.1 in the wild-type LLC-PK1 cell line after 4 h. Flesinoxan concentrations lower than 33 microg/ml were actively transported by Pgp, while at higher concentrations Pgp became saturated and transport in the MDR1-transfected cell line was comparable with the wild-type cell line. In the in vitro BBB co-culture model the transport ratio was 2.0 and was decreased to 1.0 in the presence of Pgp modulators. In vivo, the accumulation of flesinoxan in the brain at 3 h was much higher in the mdr1a(-/-) mice compared to mdr1a(+/+) mice (ratio 12.6 and 27.0 at dose levels of 3 mg/kg and 10 mg/kg respectively). In conclusion, both in vivo as well as in vitro results have demonstrated that Pgp is a limiting factor for the transport of the 5-HT(1A) receptor agonist flesinoxan into the CNS. This should be considered when its application in therapy is combined with other Pgp substrates.

P-glycoprotein inhibition leads to enhanced disruptive effects by anti-microtubule cytostatics at the in vitro blood-brain barrier

PURPOSE

To investigate whether P-glycoprotein (Pgp) protects the in vitro BBB against the cytotoxic effects of anti-tumour drugs.

METHODS

In an in vitro BBB coculture model the influence of the anti-microtubule drugs vinblastine, colchicine, paclitaxel and the non-anti-microtubule drugs doxorubicin, fluorouracil and etoposide in the absence or presence of Pgp modulators on the trans-endothelial electrical resistance (TEER), which is an indicator for the integrity, was investigated.

RESULTS

In the absence of Pgp modulators vinblastine, colchicine and paclitaxel dose dependently decreased TEER values to less than 20% of control. Non-anti-microtubule drugs did not affect TEER values. Following competitive inhibition of Pgp by various Pgp modulators and substrates, even low concentrations of vinblastine, colchicine and paclitaxel substantially decreased TEER. IC50 values of LY 335979, SDZ-PSC 833, cyclosporin A, and verapamil were 0.03, 0.25, 0.46, and 13.7 microM, respectively.

CONCLUSIONS

These results indicate that Pgp normally protects the in vitro BBB against the disruptive effects of anti-microtubule drugs, but its integrity is lost when anti-microtubule drugs are used in combination with potent Pgp modulators. In addition, this procedure offers the possibility to characterize Pgp modulators and substrates with respect to their efficacy and to elucidate drug interactions at the level of Pgp.