Drug Selection Pressure Research Articles

Yeast cells as a tool for analysis of HIV-1 protease susceptibility to protease inhibitors, a comparative study

HIV develops drug resistance at a high rate under drug selection pressure. Resistance tests are recommended to help physicians optimize antiretroviral drug therapies. For this purpose, genotypic and phenotypic tests have been developed. In order to propose a new phenotypic test that will be less laborious, expensive, and time consuming than the standard ones, a new procedure to measure HIV-1 protease susceptibility to protease inhibitor (PIs) in Saccharomyces cerevisiae yeast cells was developed. This procedure is based on HIV-1 protease expression in yeast. While the viral protein induces yeast cell death, its inhibition by PIs in the culture medium allows the cell to grow in a dose-dependent manner. In a comparative study of standard genotypic analysis vs. yeast cell-based phenotypic tests, performed on HIV-1 protease coding DNA in 17 different plasma samples from infected individuals, a clear match was found between the results obtained using the two technologies. This suggests that the yeast-based procedure is at least as accurate as standard genotypic test in defining susceptibility to protease inhibitors. This encouraging result should be the basis for large-scale validation of the new phenotypic resistance test.

Substrate Envelope-Designed Potent HIV-1 Protease Inhibitors to Avoid Drug Resistance

The rapid evolution of HIV under selective drug pressure has led to multidrug resistant (MDR) strains that evade standard therapies. We designed highly potent HIV-1 protease inhibitors (PIs) using the substrate envelope model, which confines inhibitors within the consensus volume of natural substrates, providing inhibitors less susceptible to resistance because a mutation affecting such inhibitors will simultaneously affect viral substrate processing. The designed PIs share a common chemical scaffold but utilize various moieties that optimally fill the substrate envelope, as confirmed by crystal structures. The designed PIs retain robust binding to MDR protease variants and display exceptional antiviral potencies against different clades of HIV as well as a panel of 12 drug-resistant viral strains. The substrate envelope model proves to be a powerful strategy to develop potent and robust inhibitors that avoid drug resistance.

Research development of HIV drug resistance

Highly active antiretroviral combination therapy significantly reduced the mortality, but in the high-speed copying, high genetic variation and drug selection pressure under the effect of the increasingly serious problem of drug resistance greatly weakened the role of HAART inhibit viral replication and reduce antiviral treatment. This paper reports the latest trends in HIV drug-resistance in order to develop anti-HIV drugs in clinical programs, research and development of new guidance anti-HIV-1 strategy to bring guidance.

Persistence of HIV-1 Transmitted Drug Resistance Mutations

There are few data on the persistence of individual human immunodeficiency virus type 1 (HIV-1) transmitted drug resistance (TDR) mutations in the absence of selective drug pressure. We studied 313 patients in whom TDR mutations were detected at their first resistance test and who had a subsequent test performed while ART-naive. The rate at which mutations became undetectable was estimated using exponential regression accounting for interval censoring. Most thymidine analogue mutations (TAMs) and T215 revertants (but not T215F/Y) were found to be highly stable, with NNRTI and PI mutations being relatively less persistent. Our estimates are important for informing HIV transmission models.

Viral Resistance in Hepatitis C Virus Genotype 1-Infected Patients Receiving the NS3 Protease Inhibitor Faldaprevir (BI 201335) in a Phase 1b Multiple-Rising-Dose Study

Faldaprevir (BI 201335) is a selective NS3/4A protease inhibitor under development for the treatment of chronic hepatitis C virus (HCV) infection. NS3/4A genotyping and NS3 protease phenotyping analyses were performed to monitor the emergence of resistance in patients with HCV genotype 1 infection receiving faldaprevir alone or combined with pegylated interferon alfa 2a and ribavirin (PegIFN-RBV) during a phase 1b study. Among all baseline variants, a maximum 7-fold reduction in in vitro sensitivity to faldaprevir was observed for a rare NS3 (V/I)170T polymorphism. During faldaprevir monotherapy in treatment-naive patients, virologic breakthrough was common (77%, 20/26) and was associated with the emergence of resistance mutations predominantly carrying NS3 substitutions R155K in GT1a and D168V in GT1b. D168V conferred a greater reduction in faldaprevir sensitivity (1,800-fold) than R155K (330-fold); however, D168V was generally less fit than R155K in the absence of selective drug pressure. Treatment-experienced patients treated with faldaprevir-PegIFN-RBV triple therapy showed higher viral load reductions, lower rates of breakthrough (8%, 5/62), and less frequent emergence of resistance-associated variants compared with faldaprevir monotherapy. (This study has been registered at ClinicalTrials.gov under registration no. NCT00793793.).

Antiviral Effect, Safety, and Pharmacokinetics of Five-Day Oral Administration of Deleobuvir (BI 207127), an Investigational Hepatitis C Virus RNA Polymerase Inhibitor, in Patients with Chronic Hepatitis C

Deleobuvir (BI 207127) is an investigational oral nonnucleoside inhibitor of hepatitis C virus (HCV) NS5B RNA polymerase. Antiviral activity, virology, pharmacokinetics, and safety were assessed in HCV genotype 1-infected patients receiving 5 days' deleobuvir monotherapy. In this double-blind phase 1b study, treatment-naive (TN; n = 15) and treatment-experienced (TE; n = 45) patients without cirrhosis received placebo or deleobuvir at 100, 200, 400, 800, or 1,200 mg every 8 h (q8h) for 5 days. Patients with cirrhosis (n = 13) received deleobuvir at 400 or 600 mg q8h for 5 days. Virologic analyses included NS5B genotyping and phenotyping of individual isolates. At day 5, patients without cirrhosis had dose-dependent median HCV RNA reductions of up to 3.8 log10 (with no placebo response); patients with cirrhosis had median HCV RNA reductions of approximately 3.0 log10. Three patients discontinued due to adverse events (AEs). The most common AEs were gastrointestinal, nervous system, and skin/cutaneous tissue disorders. Plasma exposure of deleobuvir was supraproportional at doses ≥ 400 mg q8h and approximately 2-fold higher in patients with cirrhosis than in patients without cirrhosis. No virologic breakthrough was observed. NS5B substitutions associated with deleobuvir resistance in vitro were detected in 9/59 patients; seven encoded P495 substitutions, including P495L, which conferred 120- to 310-fold-decreased sensitivity to deleobuvir. P495 variants did not persist in follow-up without selective drug pressure. Deleobuvir monotherapy was generally well tolerated and demonstrated dose-dependent antiviral activity against HCV genotype 1 over 5 days.

P1.015 On the Mechanisms of Drug Resistance in HIV-1 RT

Human Immunodeficiency Virus Reverse Transcriptase (HIV-1RT), catalysing synthesis of proviral cDNA has been widely exploited as a most suitable target for attack by anti-AIDS chemotherapeutics. The antiHIV-1RT molecules involve dideoxy...

Latent HIV-1 Can Be Reactivated by Cellular Superinfection in a Tat-Dependent Manner, Which Can Lead to the Emergence of Multidrug-Resistant Recombinant Viruses

The HIV-1 latent reservoir represents an important source of genetic diversity that could contribute to viral evolution and multidrug resistance following latent virus reactivation. This could occur by superinfection of a latently infected cell. We asked whether latent viruses might be reactivated when their host cells are superinfected, and if so, whether they could contribute to the generation of recombinant viruses. Using populations of latently infected Jurkat cells, we found that latent viruses were efficiently reactivated upon superinfection. Pathways leading to latent virus reactivation via superinfection might include gp120-CD4/CXCR4-induced signaling, modulation of the cellular environment by Nef, and/or the activity of Tat produced upon superinfection. Using a range of antiviral compounds and genetic approaches, we show that gp120 and Nef are not required for latent virus reactivation by superinfection, but this process depends on production of functional Tat by the superinfecting virus. In a primary cell model of latency in unstimulated CD4 T cells, superinfection also led to latent virus reactivation. Drug-resistant latent viruses were also reactivated following superinfection in Jurkat cells and were able to undergo recombination with the superinfecting virus. Under drug-selective pressure, this generated multidrug-resistant recombinants that were identified by unique restriction digestion band patterns and by population-level sequencing. During conditions of poor drug adherence, treatment interruption or treatment failure, or in drug-impermeable sanctuary sites, reactivation of latent viruses by superinfection or other means could provide for the emergence or spread of replicatively fit viruses in the face of strong selective pressures.

Elucidating a Relationship between Conformational Sampling and Drug Resistance in HIV-1 Protease

Enzyme targets in rapidly replicating systems, such as retroviruses, commonly respond to drug-selective pressure with mutations arising in the active site pocket that limit inhibitor effectiveness by introducing steric hindrance or by eliminating essential molecular interactions. However, these primary mutations are disposed to compromising pathogenic fitness. Emerging secondary mutations, which are often found outside of the binding cavity, may or can restore fitness while maintaining drug resistance. The accumulated drug pressure selected mutations could have an indirect effect in the development of resistance, such as altering protein flexibility or the dynamics of protein-ligand interactions. Here, we show that accumulation of mutations in a drug-resistant HIV-1 protease (HIV-1 PR) variant, D30N/M36I/A71V, changes the fractional occupancy of the equilibrium conformational sampling ensemble. Correlations are made among populations of the conformational states, namely, closed-like, semiopen, and open-like, with inhibition constants, as well as kinetic parameters. Mutations that stabilize a closed-like conformation correlate with enzymes of lowered activity and with higher affinity for inhibitors, which is corroborated by a further increase in the fractional occupancy of the closed state upon addition of inhibitor or substrate-mimic. Cross-resistance is found to correlate with combinations of mutations that increase the population of the open-like conformations at the expense of the closed-like state while retaining native-like occupancy of the semiopen population. These correlations suggest that at least three states are required in the conformational sampling model to establish the emergence of drug resistance in HIV-1 PR. More importantly, these results shed light on a possible mechanism whereby mutations combine to impart drug resistance while maintaining catalytic activity.

Progressive emergence of an oseltamivir-resistant A(H3N2) virus over two courses of oseltamivir treatment in an immunocompromised paediatric patient

A minor viral population of oseltamivir-resistant A(H3N2) viruses (E119V neuraminidase mutation) was selected and maintained in a continually infected immunocompromised child following initial oseltamivir treatment. A subsequent course of oseltamivir given 7weeks later rapidly selected for the E119V variant resulting in a near-pure population of the resistant virus. The study highlights the challenges of oseltamivir treatment of immunocompromised patients that are continually shedding virus and demonstrates the ability of the E119V oseltamivir-resistant virus to be maintained for prolonged periods even in the absence of drug-selective pressure.

HIV-1 fitness landscape models for indinavir treatment pressure using observed evolution in longitudinal sequence data are predictive for treatment failure

We previously modeled the in vivo evolution of human immunodeficiency virus-1 (HIV-1) under drug selective pressure from cross-sectional viral sequences. These fitness landscapes (FLs) were made by using first a Bayesian network (BN) to map epistatic substitutions, followed by scaling the fitness landscape based on an HIV evolution simulator trying to evolve the sequences from treatment naïve patients into sequences from patients failing treatment.In this study, we compared four FLs trained with different sequence populations. Epistatic interactions were learned from three different cross-sectional BNs, trained with sequence from patients experienced with indinavir (BNT), all protease inhibitors (PIs) (BNP) or all PI except indinavir (BND). Scaling the fitness landscape was done using cross-sectional data from drug naïve and indinavir experienced patients (Fcross using BNT) and using longitudinal sequences from patients failing indinavir (FlongT using BNT, FlongP using BNP, FlongD using BND). Evaluation to predict the failing sequence and therapy outcome was performed on independent sequences of patients on indinavir. Parameters included estimated fitness (LogF), the number of generations (GF) or mutations (MF) to reach the fitness threshold (average fitness when a major resistance mutation appeared), the number of generations (GR) or mutations (MR) to reach a major resistance mutation and compared to genotypic susceptibility score (GSS) from Rega and HIVdb algorithms.In pairwise FL comparisons we found significant correlation between fitness values for individual sequences, and this correlation improved after correcting for the subtype. Furthermore, FLs could predict the failing sequence under indinavir-containing combinations. At 12 and 48weeks, all parameters from all FLs and indinavir GSS (both for Rega and HIVdb) were predictive of therapy outcome, except MR for FlongT and FlongP. The fitness landscapes have similar predictive power for treatment response under indinavir-containing regimen as standard rules-based algorithms, and additionally allow predicting genetic evolution under indinavir selective pressure.

Structural and Thermodynamic Basis of Amprenavir/Darunavir and Atazanavir Resistance in HIV-1 Protease with Mutations at Residue 50

Drug resistance occurs through a series of subtle changes that maintain substrate recognition but no longer permit inhibitor binding. In HIV-1 protease, mutations at I50 are associated with such subtle changes that confer differential resistance to specific inhibitors. Residue I50 is located at the protease flap tips, closing the active site upon ligand binding. Under selective drug pressure, I50V/L substitutions emerge in patients, compromising drug susceptibility and leading to treatment failure. The I50V substitution is often associated with amprenavir (APV) and darunavir (DRV) resistance, while the I50L substitution is observed in patients failing atazanavir (ATV) therapy. To explain how APV, DRV, and ATV susceptibility are influenced by mutations at residue 50 in HIV-1 protease, structural and binding thermodynamics studies were carried out on I50V/L-substituted protease variants in the compensatory mutation A71V background. Reduced affinity to both I50V/A71V and I50L/A71V double mutants is largely due to decreased binding entropy, which is compensated for by enhanced enthalpy for ATV binding to I50V variants and APV binding to I50L variants, leading to hypersusceptibility in these two cases. Analysis of the crystal structures showed that the substitutions at residue 50 affect how APV, DRV, and ATV bind the protease with altered van der Waals interactions and that the selection of I50V versus I50L is greatly influenced by the chemical moieties at the P1 position for APV/DRV and the P2 position for ATV. Thus, the varied inhibitor susceptibilities of I50V/L protease variants are largely a direct consequence of the interdependent changes in protease inhibitor interactions.

T-705 (Favipiravir) Induces Lethal Mutagenesis in Influenza A H1N1 Viruses In Vitro

Several novel anti-influenza compounds are in various phases of clinical development. One of these, T-705 (favipiravir), has a mechanism of action that is not fully understood but is suggested to target influenza virus RNA-dependent RNA polymerase. We investigated the mechanism of T-705 activity against influenza A (H1N1) viruses by applying selective drug pressure over multiple sequential passages in MDCK cells. We found that T-705 treatment did not select specific mutations in potential target proteins, including PB1, PB2, PA, and NP. Phenotypic assays based on cell viability confirmed that no T-705-resistant variants were selected. In the presence of T-705, titers of infectious virus decreased significantly (P < 0.0001) during serial passage in MDCK cells inoculated with seasonal influenza A (H1N1) viruses at a low multiplicity of infection (MOI; 0.0001 PFU/cell) or with 2009 pandemic H1N1 viruses at a high MOI (10 PFU/cell). There was no corresponding decrease in the number of viral RNA copies; therefore, specific virus infectivity (the ratio of infectious virus yield to viral RNA copy number) was reduced. Sequence analysis showed enrichment of G→A and C→T transversion mutations, increased mutation frequency, and a shift of the nucleotide profiles of individual NP gene clones under drug selection pressure. Our results demonstrate that T-705 induces a high rate of mutation that generates a nonviable viral phenotype and that lethal mutagenesis is a key antiviral mechanism of T-705. Our findings also explain the broad spectrum of activity of T-705 against viruses of multiple families.

Mechanisms and insights into drug resistance in cancer

Cancer drug resistance continues to be a major impediment in medical oncology. Clinically, resistance can arise prior to or as a result of cancer therapy. In this review, we discuss different mechanisms adapted by cancerous cells to resist treatment, including alteration in drug transport and metabolism, mutation and amplification of drug targets, as well as genetic rewiring which can lead to impaired apoptosis. Tumor heterogeneity may also contribute to resistance, where small subpopulations of cells may acquire or stochastically already possess some of the features enabling them to emerge under selective drug pressure. Making the problem even more challenging, some of these resistance pathways lead to multidrug resistance, generating an even more difficult clinical problem to overcome. We provide examples of these mechanisms and some insights into how understanding these processes can influence the next generation of cancer therapies.

Susceptibility or resistance of praziquantel in human schistosomiasis: a review

Since praziquantel was developed in 1970s, it has replaced other antischistosomal drugs to become the only drug of choice for treatment of human schistosomiases, due to high efficacy, excellent tolerability, few and transient side effects, simple administration, and competitive cost. Praziquantel-based chemotherapy has been involved in the global control strategy of the disease and led to the control strategy shifting from disease control to morbidity control, which has greatly reduced the prevalence and intensity of infections. Given that the drug has been widely used for morbidity control in endemic areas for more than three decades, the emergence of resistance of Schistosoma to praziquantel under drug selection pressure has been paid much attention. It is possible to induce resistance of Schistosoma mansoni and Schistosoma japonicum to praziquantel in mice under laboratorial conditions, and a reduced susceptibility to praziquantel in the field isolates of S. mansoni has been found in many foci. In addition, there are several schistosomiasis cases caused by Schistosoma haematobium infections in which repeated standard treatment fails to clear the infection. However, in the absence of exact mechanisms of action of praziquantel, the mechanisms of drug resistance in schistosomes remain unclear. The present review mainly demonstrates the evidence of drug resistance in the laboratory and field and the mechanism of praziquantel resistance and proposes some strategies for control of praziquantel resistance in schistosomes.

Treatment-associated polymorphisms in protease are significantly associated with higher viral load and lower CD4 count in newly diagnosed drug-naive HIV-1 infected patients.

BackgroundThe effect of drug resistance transmission on disease progression in the newly infected patient is not well understood. Major drug resistance mutations severely impair viral fitness in a drug free environment, and therefore are expected to revert quickly. Compensatory mutations, often already polymorphic in wild-type viruses, do not tend to revert after transmission. While compensatory mutations increase fitness during treatment, their presence may also modulate viral fitness and virulence in absence of therapy and major resistance mutations. We previously designed a modeling technique that quantifies genotypic footprints of in vivo treatment selective pressure, including both drug resistance mutations and polymorphic compensatory mutations, through the quantitative description of a fitness landscape from virus genetic sequences.ResultsGenotypic correlates of viral load and CD4 cell count were evaluated in subtype B sequences from recently diagnosed treatment-naive patients enrolled in the SPREAD programme. The association of surveillance drug resistance mutations, reported compensatory mutations and fitness estimated from drug selective pressure fitness landscapes with baseline viral load and CD4 cell count was evaluated using regression techniques. Protease genotypic variability estimated to increase fitness during treatment was associated with higher viral load and lower CD4 cell counts also in treatment-naive patients, which could primarily be attributed to well-known compensatory mutations at highly polymorphic positions. By contrast, treatment-related mutations in reverse transcriptase could not explain viral load or CD4 cell count variability.ConclusionsThese results suggest that polymorphic compensatory mutations in protease, reported to be selected during treatment, may improve the replicative capacity of HIV-1 even in absence of drug selective pressure or major resistance mutations. The presence of this polymorphic variation may either reflect a history of drug selective pressure, i.e. transmission from a treated patient, or merely be a result of diversity in wild-type virus. Our findings suggest that transmitted drug resistance has the potential to contribute to faster disease progression in the newly infected host and to shape the HIV-1 epidemic at a population level.

The stepwise acquisition of fluconazole resistance mutations causes a gradual loss of fitness in Candida albicans

The pathogenic yeast Candida albicans can develop resistance to the widely used antifungal agent fluconazole, which inhibits ergosterol biosynthesis. Resistance is often caused by gain-of-function mutations in the transcription factors Mrr1, Tac1 and Upc2, which result in constitutive overexpression of multidrug efflux pumps and ergosterol biosynthesis genes respectively. It is not known how the permanently changed gene expression program in resistant strains affects their fitness in the absence of drug selection pressure. We have systematically investigated the effects of activating mutations in Mrr1, Tac1 and Upc2, individually and in all possible combinations, on the degree of fluconazole resistance and on the fitness of C. albicans in an isogenic strain background. All combinations of different resistance mechanisms resulted in a stepwise increase in drug resistance, culminating in 500-fold increased fluconazole resistance in strains possessing mutations in the three transcription factors and an additional resistance mutation in the drug target enzyme Erg11. The acquisition of resistance mutations was associated with reduced fitness under non-selective conditions in vitro as well as in vivo during colonization of a mammalian host. Therefore, without compensatory mutations, the inability to appropriately regulate gene expression results in a loss of competitive fitness of drug-resistant C. albicans strains.

HIV Drug Resistance Interpreted by Cumulative Versus Last Genotypes in HIV-Infected Patients with Multiple Treatment Failures

Genotypic resistance test has been recommended to evaluate HIV drug resistance and guide the effective regimens of antiretroviral therapy (ART) in HIV-infected patients with treatment failure. In patients with multiple treatment failures, drug resistance-associated mutations may disappear due to the loss of selective drug pressure after switching regimens. A cohort study was conducted among HIV-infected patients who had ≥2 genotypic resistance tests during 2003-2011. HIV-1 pol nucleotide sequencing of reverse transcriptase and protease region was carried out using TRUGENE HIV-1 Genotypic Assay. Sequencing data was analyzed using Stanford rule-based interpretation algorithms. Of 54 patients with mean age of 30.1 years, 46.3% were males. HIV-1 subtype A/E was observed in 88.9% of patients. At the latest failure, 55.3% were receiving protease inhibitor-based regimens. Median CD4 and HIV RNA were 167 cells/mm(3) and 22,359 copies/mL. During a median duration of ART of 38.6 months, 72.2%, 22.2%, and 5.6% had 5, 3, and 2 genotype tests, respectively. When compared between using cumulative (CG) and last genotypes (LG), CG interpreted resistance to any drug 59.3% higher than LG did. For NRTI, NNRTI, and PI drug classes, CG interpreted as resistance 42.6%, 27.8%, and 7.4% higher than LG, respectively. The most common drugs that CG interpreted resistance with the higher rate than LG were lamivudine/emtricitabine, nevirapine, efavirenz, etravirine and abacavir. In conclusion, CG interprets HIV drug resistance at a higher rate than LG and may be more accurate to use for selecting the next effective regimen of ART among HIV-infected patients with multiple treatment failures.

Pfatp6 molecular profile of Plasmodium falciparum isolates in the western Brazilian Amazon

BackgroundAnti-malarial drug resistance has emerged as one of the biggest challenges confronting the worldwide effort to control malaria. The appearance of chloroquine and multi-drug resistance had devastating effects on therapeutic efficacy of former first-line agents. Artemisinin has proven to be an excellent therapeutic alternative to fill the void in chemotherapeutic options left by resistance mechanisms. At the time of introduction, no resistance to artemisinins had been recorded, and artemisinins demonstrated excellent parasite reduction rates. In an attempt to protect artemisinin efficacy, the World Health Organization (WHO) made artemisinin-based combination therapy (ACT) its official first-line treatment recommendation for uncomplicated Plasmodium falciparum in 2006. In Brazil, artemether/lumefantrine became the Brazilian Malaria Control Programme's official treatment recommendation in 2007. The sarco/endoplasmic reticulum Ca2+ - ATPase ortholog of P. falciparum (pfatp6) has been suggested as one of the targets of artemisinins. Consequently, pfatp6 gene polymorphisms are being investigated as markers of artemisinin resistance elsewhere. The goal of this work was to describe the molecular profile of pfatp6 in P. falciparum isolates from different localities in the Amazonas State.MethodsDNA polymorphisms of the pfatp6 gene in 80 P. falciparum isolates from 11 municipalities of the Amazonas State (Western Brazilian Amazon), before and after the introduction of ACT in the Brazilian anti-malarial guidelines, were analysed by automatic sequencing. Mutations in the pfatp6 gene were searched using Mutation Surveyor v3.25 software.ResultsThe P. falciparum pfatp6 gene presented polymorphisms at codons 37, 630 and 898. The R37K mutation was found in 16% of the samples, A630S in 32% and I898I in 52%. No S769N mutation, however, was detected in the analysed samples.ConclusionDespite the small number of samples, data presented here provide baseline information about polymorphisms of pfatp6 gene before and after exposure to ACT in a low transmission area, which will help to infer drug selection pressure in this area in the future.

Different Modalities of Intercellular Membrane Exchanges Mediate Cell-to-cell P-glycoprotein Transfers in MCF-7 Breast Cancer Cells

Multi-drug resistance (MDR) is a phenomenon by which tumor cells exhibit resistance to a variety of chemically unrelated chemotherapeutic drugs. The classical form of multidrug resistance is connected to overexpression of membrane P-glycoprotein (P-gp), which acts as an energy dependent drug efflux pump. P-glycoprotein expression is known to be controlled by genetic and epigenetic mechanisms. Until now processes of P-gp gene up-regulation and resistant cell selection were considered sufficient to explain the emergence of MDR phenotype within a cell population. Recently, however, "non-genetic" acquisitions of MDR by cell-to-cell P-gp transfers have been pointed out. In the present study we show that intercellular transfers of functional P-gp occur by two different but complementary modalities through donor-recipient cells interactions in the absence of drug selection pressure. P-glycoprotein and drug efflux activity transfers were followed over 7 days by confocal microscopy and flow cytometry in drug-sensitive parental MCF-7 breast cancer cells co-cultured with P-gp overexpressing resistant variants. An early process of remote transfer was established based on the release and binding of P-gp-containing microparticles. Microparticle-mediated transfers were detected after only 4 h of incubation. We also identify an alternative mode of transfer by contact, consisting of cell-to-cell P-gp trafficking by tunneling nanotubes bridging neighboring cells. Our findings supply new mechanistic evidences for the extragenetic emergence of MDR in cancer cells and indicate that new treatment strategies designed to overcome MDR may include inhibition of both microparticles and Tunneling nanotube-mediated intercellular P-gp transfers.