Cytosine arabinoside affects multiple cellular factors and induces drug resistance in human lymphoid cells

Mechanisms of resistance to hypomethylating agents and BCL-2 inhibitors

Myeloid malignancies such as myelodysplastic syndrome (MDS) & acute myeloid leukemia (AML) are clonal diseases that emerge and progress due to the expansion of disease-initiating aberrant hematopoietic stem cells, that are not eliminated by conventional cytotoxic therapies. Hypomethylating agents(HMA), azacytidine and decitabine are the first line agents for treatment of MDS and a combination with BCL-2 inhibitor, venetoclax, is approved for AML induction in patients above 75 years and is also actively being investigated for use in high risk MDS. Resistance to these drugs has become a significant clinical challenge in treatment of myeloid malignancies. In this review, we discuss molecular mechanisms underlying the development of resistance to HMA and venetoclax. Insights into these mechanisms can help identify potential biomarkers for resistance prediction, aid in the development of combination therapies and strategies to prevent resistance and advance the field of cancer therapeutics.

Quinacrine-CASIN combination overcomes chemoresistance in human acute lymphoid leukemia

Chemoresistance posts a major hurdle for treatment of acute leukemia. There is increasing evidence that prolonged and intensive chemotherapy often fails to eradicate leukemic stem cells, which are protected by the bone marrow niche and can induce relapse. Thus, new therapeutic approaches to overcome chemoresistance are urgently needed. By conducting an ex vivo small molecule screen, here we have identified Quinacrine (QC) as a sensitizer for Cytarabine (AraC) in treating acute lymphoblastic leukemia (ALL). We show that QC enhances AraC-mediated killing of ALL cells, and subsequently abrogates AraC resistance both in vitro and in an ALL-xenograft model. However, while combo AraC+QC treatment prolongs the survival of primary transplanted recipients, the combination exhibits limited efficacy in secondary transplanted recipients, consistent with the survival of niche-protected leukemia stem cells. Introduction of Cdc42 Activity Specific Inhibitor, CASIN, enhances the eradication of ALL leukemia stem cells by AraC+QC and prolongs the survival of both primary and secondary transplanted recipients without affecting normal long-term human hematopoiesis. Together, our findings identify a small-molecule regimen that sensitizes AraC-mediated leukemia eradication and provide a potential therapeutic approach for better ALL treatment.

Response and Toxicity to Cytarabine Therapy in Leukemia and Lymphoma: From Dose Puzzle to Pharmacogenomic Biomarkers

Simple Summary

In this review, the authors propose a crosswise examination of cytarabine-related issues ranging from the spectrum of clinical activity and severe toxicities, through updated cellular pharmacology and drug formulations, to the genetic variants associated with drug-induced phenotypes. Cytarabine (cytosine arabinoside; Ara-C) in multiagent chemotherapy regimens is often used for leukemia or lymphoma treatments, as well as neoplastic meningitis. Chemotherapy regimens can induce a suboptimal clinical outcome in a fraction of patients. The individual variability in clinical response to Leukemia & Lymphoma treatments among patients appears to be associated with intracellular accumulation of Ara-CTP due to genetic variants related to metabolic enzymes. The review provides exhaustive information on the effects of Ara-C-based therapies, the adverse drug reaction will also be provided including bone pain, ocular toxicity (corneal pain, keratoconjunctivitis, and blurred vision), maculopapular rash, and occasional chest pain. Evidence for predicting the response to cytarabine-based treatments will be highlighted, pointing at their significant impact on the routine management of blood cancers.

Abstract

Cytarabine is a pyrimidine nucleoside analog, commonly used in multiagent chemotherapy regimens for the treatment of leukemia and lymphoma, as well as for neoplastic meningitis. Ara-C-based chemotherapy regimens can induce a suboptimal clinical outcome in a fraction of patients. Several studies suggest that the individual variability in clinical response to Leukemia & Lymphoma treatments among patients, underlying either Ara-C mechanism resistance or toxicity, appears to be associated with the intracellular accumulation and retention of Ara-CTP due to genetic variants related to metabolic enzymes. Herein, we reported (a) the latest Pharmacogenomics biomarkers associated with the response to cytarabine and (b) the new drug formulations with optimized pharmacokinetics. The purpose of this review is to provide readers with detailed and comprehensive information on the effects of Ara-C-based therapies, from biological to clinical practice, maintaining high the interest of both researcher and clinical hematologist. This review could help clinicians in predicting the response to cytarabine-based treatments.

Expression of the nucleoside transporters hENT1 (SLC29) and hCNT1 (SLC28) in pediatric acute myeloid leukemia

Cellular uptake of clinically important deoxynucleoside analogs is mediated by nucleoside transporters including the human equilibrative nucleoside transporter 1 (hENT1) and the concentrative nucleoside transporter-1 (hCNT1). These transporters are responsible for influx of cytarabine and reduced hENT1 expression is a major resistance mechanism in acute myeloid leukemia. We determined hENT1 and hCNT1 protein expression by immunocytochemistry in 50 diagnostic pediatric acute myeloid leukemia patient samples. All samples expressed hENT1 [9/43 (21%) low; 26/43 (60%) medium and 8/43 (19%) high] and hCNT1 [2/42 (5%) low; 35/42 (83%) medium and 5/42 (12%) high] at the cell membrane and cytoplasm. Statistical analysis showed a non-significant relationship between survival and transporter expression and in vitro drug sensitivity. In conclusion, the nucleoside transporters hENT1 and hCNT1 are broadly expressed in pediatric acute myeloid leukemia at diagnosis.

miR-134 increases the antitumor effects of cytarabine by targeting Mnks in acute myeloid leukemia cells

The relapse and resistance to cytarabine (Ara-C) therapy is still a dominating obstacle to the successful clinical treatment of acute myeloid leukemia (AML). Recent studies have shown that dysregulation of miRNAs might modulate the resistance of cancer cells to anticancer drugs; yet, the mechanism is not fully understood. In this study, we showed a significant downregulation of miR-134 in human multidrug-resistant leukemia cells and relapsed/refractory AML patient samples. Overexpression of miR-134 sensitized K562/A02 and HL-60/ADM cells to Ara-C, inhibited cell colony formation, and enhanced the ability of Ara-C to induce apoptosis. Mechanistic analyses revealed that Mnks was a putative target of miR-134, which was inversely correlated with miR-134 expression in human multidrug-resistant leukemia cells and relapsed/refractory AML patient samples. Further investigation showed that miR-134 increased the anti-tumor effects of Ara-C through inhibiting phosphorylation of eukaryotic initiation factor 4E and downregulating Mcl-1 and bcl2, which was independent of p38 and Erk1/2 activation. Taken together, our results demonstrate that miR-134 plays a pivotal role in AML Ara-C resistance through increasing cell sensitivity to Ara-C and promoting apoptosis by targeting Mnks.

Inhibition of Mnk enhances apoptotic activity of cytarabine in acute myeloid leukemia cells

Cytarabine (Ara-C) is a first line clinical therapeutic agent for treatment of acute myeloid leukemia (AML). However, this therapy is limited due to high rate of resistance and relapse. Recent research has revealed that the poor prognosis and resistance to Ara-C in AML were associated with its abnormally activated MAPK pathways. In this study, we showed a strong synergistic effect of Ara-C with either our Mnk inhibitor (MNKI-8e) or short hairpin RNA (shRNA) mediated knockdown of Mnks in MV4-11 AML cells. We investigated the underlying mechanisms for this synergism. We showed that both MNKI-8e and Mnk shRNAs enhanced the ability of Ara-C to induce apoptosis. We found that Ara-C increased the phosphorylation of Erk1/2, p38 and eIF4E, which correlated with an enhanced level of anti-apoptotic Mcl-1 protein. Inhibition of Mnk activity suppressed the Ara-C-induced MAPK activity, and thus enhanced apoptosis in MV4-11 cells. Taken together, our study suggests that MAPK-Mnk-eIF4E pathway plays a critical role in Ara-C-treated MV4-11 cells and targeting Mnk may be a promising therapeutic strategy for sensitizing leukemic cells to Ara-C therapy.

Impact of single nucleotide polymorphisms of cytarabine metabolic genes on drug toxicity in childhood acute lymphoblastic leukemia

BACKGROUND

Cytarabine (cytosine arabinoside, ara-C) is a chemotherapeutical agent used in the treatment of pediatric acute lymphoblastic leukemia (ALL). Adverse drug reactions, such as interpatient variability in sensitivity to ara-C, are considerable and may cause difficulties during chemotherapy. Single nucleotide polymorphisms (SNPs) can play a significant role in modifying nucleoside-drug pharmacokinetics and pharmacodynamics and thus the development of adverse effects. Our aim was to determine whether polymorphisms in genes encoding transporters and enzymes responsible for the metabolism of ara-C are associated with toxicity and clinical outcome in a patient population with childhood ALL.

PROCEDURE

We studied 8 SNPs in the CDA, DCK, DCTD, SLC28A3, and SLC29A1 genes in 144 patients with childhood acute lymphoblastic leukemia treated according to ALLIC BFM 1990, 1995 and 2002 protocols.

RESULTS

DCK rs12648166 and DCK rs4694362 SNPs were associated with hematologic toxicity (OR = 2.63, CI 95% = 1.37-5.04, P = 0.0036 and OR = 2.53, CI 95% = 1.34-4.80, P = 0.0044, respectively).

CONCLUSIONS

Our results indicate that DCK polymorphisms might be important genetic risk factors for hematologic toxicity during ALL treatment with ara-C. Individualized chemotherapy based on genetic profiling may help to optimize ara-C dosing, leading to improvements in clinical outcome and reduced toxicity.

Variegated clonality and rapid emergence of new molecular lesions in xenografts of acute lymphoblastic leukemia are associated with drug resistance

The use of genome-wide copy-number analysis and massive parallel sequencing has revolutionized the understanding of the clonal architecture of pediatric acute lymphoblastic leukemia (ALL) by demonstrating that this disease is composed of highly variable clonal ancestries following the rules of Darwinian selection. The current study aimed to analyze the molecular composition of childhood ALL biopsies and patient-derived xenografts with particular emphasis on mechanisms associated with acquired chemoresistance. Genomic DNA from seven primary pediatric ALL patient samples, 29 serially passaged xenografts, and six in vivo selected chemoresistant xenografts were analyzed with 250K single-nucleotide polymorphism arrays. Copy-number analysis of non-drug-selected xenografts confirmed a highly variable molecular pattern of variegated subclones. Whereas primary patient samples from initial diagnosis displayed a mean of 5.7 copy-number alterations per sample, serially passaged xenografts contained a mean of 8.2 and chemoresistant xenografts a mean of 10.5 copy-number alterations per sample, respectively. Resistance to cytarabine was explained by a new homozygous deletion of the DCK gene, whereas methotrexate resistance was associated with monoallelic deletion of FPGS and mutation of the remaining allele. This study demonstrates that selecting for chemoresistance in xenografted human ALL cells can reveal novel mechanisms associated with drug resistance.

The role of human equilibrative nucleoside transporter 1 on the cellular transport of the DNA methyltransferase inhibitors 5-azacytidine and CP-4200 in human leukemia cells

The nucleoside analog 5-azacytidine is an archetypical drug for epigenetic cancer therapy, and its clinical effectiveness has been demonstrated in the treatment of myelodysplastic syndromes (MDS) and acute myelogenous leukemia (AML). However, therapy resistance in patients with MDS/AML remains a challenging issue. Membrane proteins that are involved in drug uptake are potential mediators of drug resistance. The responsible proteins for the transport of 5-azacytidine into MDS/AML cells are unknown. We have now systematically analyzed the expression and activity of various nucleoside transporters. We identified the human equilibrative nucleoside transporter 1 (hENT1) as the most abundant nucleoside transporter in leukemia cell lines and in AML patient samples. Transport assays using [¹⁴C]5-azacytidine demonstrated Na⁺-independent uptake of the drug into the cells, which was inhibited by S-(4-nitrobenzyl)-6-thioinosine (NBTI), a hENT1 inhibitor. The cellular toxicity of 5-azacytidine and its DNA demethylating activity were strongly reduced after hENT1 inhibition. In contrast, the cellular activity of the 5-azacytidine derivative 5-azacytidine-5'-elaidate (CP-4200), a nucleoside transporter-independent drug, persisted after hENT1 inhibition. A strong dependence of 5-azacytidine-induced DNA demethylation on hENT1 activity was also confirmed by array-based DNA methylation profiling, which uncovered hundreds of loci that became demethylated only when hENT1-mediated transport was active. Our data establish hENT1 as a key transporter for the cellular uptake of 5-azacytidine in leukemia cells and raise the possibility that hENT1 expression might be a useful biomarker to predict the efficiency of 5-azacytidine treatments. Furthermore, our data suggest that CP-4200 may represent a valuable compound for the modulation of transporter-related 5-azacytidine resistances.

Impact of polymorphisms in drug pathway genes on disease-free survival in adults with acute myeloid leukemia

Acute myeloid leukemia (AML) is a clinically heterogeneous disease, with 5-year disease-free survival (DFS) ranging from under 10% to over 70% for distinct groups of patients. At our institution, cytarabine, etoposide and busulfan are used in first or second remission patients treated with a 2-step approach to autologous stem cell transplantation (ASCT). In this study, we tested the hypothesis that polymorphisms in the pharmacokinetic and pharmacodynamic pathway genes of these drugs are associated with DFS in AML patients. A total of 1659 variants in 42 genes were analyzed for their association with DFS using a Cox proportional hazards model. 154 genetically European patients were used for the primary analysis. An intronic SNP in ABCC3 (rs4148405) was associated with a significantly shorter DFS (HR=3.2, p=5.6 x 10(-6)) in our primary cohort. In addition a SNP in the GSTM1-GSTM5 locus, rs3754446, was significantly associated with a shorter DFS in all patients (HR=1.8, p=0.001 for 154 European ancestry; HR=1.7, p=0.028 for 125 non-European patients). Thus for the first time, genetic variants in drug pathway genes are shown to be associated with DFS in AML patients treated with chemotherapy-based autologous ASCT.

Variability in transport and biotransformation of cytarabine is associated with its toxicity in peripheral blood mononuclear cells

AIM

To adopt an individualized approach to assess cytarabine (ara-C) hematotoxicity, we studied the relationship between pharmacogenetic variability in the cytidine deaminase gene (CDA) and ara-C toxicity in native peripheral blood mononuclear cells from 100 healthy volunteers.

MATERIALS & METHODS

Peripheral blood mononuclear cells were incubated for 48 h with 3 µM ara-C, and cell viability was analyzed by flow cytometry with and without the addition of an equilibrative nucleoside transporter transport inhibitor. CDA promoter and exonic variants were genotyped to derive haplotypes for the CDA gene.

RESULTS

Significant between-subject variability was observed in ara-C toxicity (21-fold with 40.1% coefficient of variation compared with 1.2-fold within-subject variability [9.6% coefficient of variation]). Inhibition of hENT1 reversed ara-C cytotoxicity. The linked CDA promoter variants -451C>T, -92A>G, -31Del and the exonic 79A>C variant were associated with ara-C toxicity (p < 0.05). CDA*2A haplotype was associated with ara-C toxicity (p = 0.03).

CONCLUSION

Genetic polymorphisms within CDA may be risk factors for ara-C-induced hematotoxicity. Original submitted 6 October 2010; Revision submitted 29 November 2010.

Resistance to cytarabine induces the up-regulation of NKG2D ligands and enhances natural killer cell lysis of leukemic cells

Prolonged treatment of leukemic cells with chemotherapeutic agents frequently results in development of drug resistance. Moreover, selection of drug-resistant cell populations may be associated with changes in malignant properties such as proliferation rate, invasiveness, and immunogenicity. In the present study, the sensitivity of cytarabine (1-beta-D-arabinofuranosylcytosine, araC)-resistant and parental human leukemic cell lines (T-lymphoid H9 and acute T-lymphoblastic leukemia Molt-4) to natural killer (NK) cell-mediated killing was investigated. The results obtained demonstrate that araC-resistant H9 and Molt-4 (H9(r)ARAC(100) and Molt-4(r)ARAC(100)) cell lines are more sensitive to NK cell-mediated lysis than their respective parental cell lines. This increased sensitivity was associated with a higher surface expression of ligands for the NK cell-activating receptor NKG2D, notably UL16 binding protein-2 (ULBP-2) and ULBP-3 in H9(r)ARAC(100) and Molt-4(r)ARAC(100) cell lines. Blocking ULBP-2 and ULBP-3 or NKG2D with monoclonal antibody completely abrogated NK cell lysis. Constitutive phosphorylated extracellular signal-regulated kinase (ERK) but not pAKT was higher in araC-resistant cells than in parental cell lines. Inhibition of ERK using ERK inhibitor PD98059 decreased both ULBP-2/ULBP-3 expression and NK cell cytotoxicity. Furthermore, overexpression of constitutively active ERK in H9 parental cells resulted in increased ULBP-2/ULBP-3 expression and enhanced NK cell lysis. These results demonstrate that increased sensitivity of araC-resistant leukemic cells to NK cell lysis is caused by higher NKG2D ligand expression, resulting from more active ERK signaling pathway.

Development of a sensitive and selective LC/MS/MS method for the simultaneous determination of intracellular 1-beta-D-arabinofuranosylcytosine triphosphate (araCTP), cytidine triphosphate (CTP) and deoxycytidine triphosphate (dCTP) in a human follicular lymphoma cell line

A method was developed for the quantification of araCTP, CTP and dCTP in a human follicular lymphoma cell line. This method involves solid phase extraction (SPE) using a weak anion-exchanger (WAX) cartridge, a porous graphitic carbon high-performance liquid chromatography (HPLC) column separation, and tandem mass spectrometry (MS/MS) detection. By using a triple quadrupole mass spectrometer operating in negative ion multiple reaction monitoring (MRM) mode, the method was able to achieve a lower limit of quantification (LLOQ) of 0.1 microg mL(-1) for araCTP and of 0.01 microg mL(-1) for both CTP and dCTP. The method was validated and used to determine the amount of araCTP, CTP and dCTP formed after incubation of araC and an araCMP prodrug in the human follicular lymphoma cell line RL.

Adenoviral-mediated overexpression of human equilibrative nucleoside transporter 1 (hENT1) enhances gemcitabine response in human pancreatic cancer

Nucleoside-derived anticancer agents must be transported across the plasma membrane as a preliminary step to their conversion into active drugs. Hence, modulation of a specific nucleoside transporter may affect bioavailability and contribute significantly to sensitizing tumor cells to these anticancer agents. We have generated and functionally characterized a new recombinant adenovirus (Ad-hENT1) that has allowed us to overexpress the equilibrative nucleoside transporter hENT1 and to analyze its effects in human pancreatic tumor cells. Overexpression of hENT1 is associated with changes in cell cycle profile, in a variable manner depending on the particular cell type, thus suggesting a metabolic link between hENT1-mediated transport processes and the enzymatic machinery responsible for intracellular nucleoside metabolism. When assayed in vivo in a human pancreatic adenocarcinoma xenograft, intratumoral Ad-hENT1 injection improved the therapeutic response to gemcitabine. In summary, hENT1 overexpression is associated with alterations in nucleoside enzymatic machinery and cell cycle progression in cultured cells and enhances gemcitabine action in vivo.

Arsenic trioxide/ascorbic acid therapy in patients with refractory metastatic colorectal carcinoma: a clinical experience

Arsenic trioxide (As2O3) has demonstrated effectiveness in treating acute promyelocytic leukemia (APL). Therefore the FDA has approved it to treat APL. In patients with refractory metastatic colorectal carcinoma (CRC), we assessed the efficacy and toxicity of As2O3/AA (ascorbic acid) as the outcome of this trial. Five patients with refractory metastatic CRC who failed all previous standard chemotherapy were enrolled in this study. They were treated with 0.25 mg/kg body weight/day As2O3 and 1000 mg/day of ascorbic acid for 5 days a week for 5 weeks. Each treatment cycle extended for 7 weeks with 5 weeks of treatment and 2 weeks of rest. All the patients developed moderate to severe toxic side effects to arsenic trioxide/AA therapy and therefore the study was discontinued. No CR (complete remission) or PR (partial remission) was observed. CT scans demonstrated stable or progressive disease. Three of the five patients died within 2 to 5 months after cessation of the therapy. None of the deaths could be related to this clinical trial. Two years of follow-up study showed that two patients were alive with stable disease. Under the current treatment regimen all patients developed moderate to severe side effects with no clinically measurable activity. As an alternate, efforts may be made to reduce the dose and arsenic trioxide may be combined with other standard regimen in reversing the chemo resistance.

Pharmacogenetics of deoxycytidine kinase: identification and characterization of novel genetic variants

Deoxycytidine kinase (DCK) is a rate-limiting enzyme in the activation of nucleoside analogs such as cytarabine (ara-C), gemcitabine, clofarabine, and others. The present study was undertaken to identify and to determine the functional consequences of genetic variants in DCK. We sequenced 1.5 kilobases of the DCK proximal promoter and all seven coding exons in International HapMap Project panels (n = 90 each) with European (Centre d' Etude du Polymorphisme Humain; CEPH) or African (Yoruba people in Ibadan, Nigeria; YRI) ancestry. Sixty-four genetic polymorphisms, including three nonsynonymous coding changes (I24V, A119G, and P122S) were identified. Compared with DCK-wild-type (WT) protein, the activity of the recombinant DCK24Val, DCK119Gly, and DCK122Ser proteins was 85 +/- 5, 66 +/- 3, and 43 +/- 4%, respectively. DCK119Gly and DCK122Ser mutants had lower Km (p < 0.01) and Vmax (p < 0.001) compared with DCK-WT protein. Lymphoblast cell lines from subjects heterozygous for the coding changes had significantly lower DCK activity compared with homozygous WT subjects. Ethnic differences were observed, with African ancestry subjects demonstrating significantly higher DCK mRNA expression compared with subjects with European ancestry. In both CEPH and YRI subjects, the C allele of a 3'-untranslated region single-nucleotide polymorphism (SNP) (35708 C>T) was significantly associated with lower DCK mRNA expression. This SNP was strongly linked with other intronic SNPs, forming a major haplotype block in both ethnic groups. In an exploratory analysis, the 35708C allele was also associated with lower blast ara-C-5'-triphosphate (ara-CTP) levels in acute myeloid leukemia patients receiving ara-C as continuous infusion. These results suggest that genetic variation in DCK influences its activity and expression and may predict the variability observed in intracellular levels of the ara-C active metabolite ara-CTP.

Role of CNT3 in the transepithelial flux of nucleosides and nucleoside-derived drugs

We examined the role of the concentrative nucleoside transporter CNT3 in the establishment of a transepithelial flux of natural nucleosides and their pharmacologically active derivatives in renal epithelial cell lines. Murine PCT cells grown on a transwell dish showed endogenous CNT3 activity at their apical membrane that was responsible for the sodium-dependent transepithelial flux of both purine and pyrimidine nucleosides. hCNT3 was also identified in human kidney and its role in the transport of nucleosides was tested. To this end, MDCK cells, lacking endogenous CNT3 activity, were genetically engineered to express the human orthologue of CNT3 (hCNT3-MDCK cells). In these cells, hCNT3 was inserted into the apical membrane, thus generating, as for PCT cells, a transepithelial flux of both nucleosides and nucleoside-derived drugs. Apical-to-basolateral transepithelial flux was present in all cells expressing a functional CNT3 transporter and was significantly higher than that found either in PCT cells in absence of sodium or in mock-transfected MDCK cells. Nevertheless in all cases a significant amount of the transported nucleoside was retained and transformed inside cells. However release to the opposite compartment was CNT3 dependent, not only in terms of absolute flux (much higher when an apical CNT3 transporter was active) but also regarding metabolic transformations of the apically absorbed nucleosides. These results underline a critical role of CNT3 in the renal reabsorption of nucleosides and their derivatives as well as in their intracellular metabolism.

The effects of prolonged treatment with zidovudine, lamivudine, and abacavir on a T-lymphoblastoid cell line

A human T-lymphoblastoid cell line that is resistant to the antiviral activity of zidovudine (ZDV) and moderately resistant to lamivudine (3TC) has been obtained as a result of prolonged treatment with a combination of three nucleoside analogues (NA), ZDV, 3TC, and abacavir (ABV). These cells, called CEM(ZLA), are fully sensitive to ABV. The cellular resistance of the CEM(ZLA) cells to ZDV correlates with significant reductions in thymidine kinase (TK) activity and in the amount of intracellular TK protein. Interestingly, the reduction in TK activity led to impairment of the ability of CEM(ZLA) to accumulate the triphosphate metabolite of ZDV. However, the moderately 3TC-resistant phenotype of CEM(ZLA) cannot be ascribed to a similar reduction in deoxycytidine kinase activity. Compared to the parental CEM cells, CEM(ZLA) cells express a high level of multidrug resistance protein 4 (MRP4), which could reduce the intracellular concentration of 3TC. This study shows that the exposure of cells to a combination of NAs is capable of simultaneously affecting more than one target site to confer resistance and that NAs display differing abilities to select cellular resistance mechanisms.