Principles of HIV Drug Resistance: Resistance to New Drug Classes Mark A Wainberg McGill University AIDS Centre Montreal, Quebec, Canada
Why Is It Important to Understand HIV Drug Resistance? 1. Resistance is an important factor in treatment failure 2. Resistance limits drug activity 3. Cross-resistance can limit the use of therapies 4. An understanding of resistance may permit the sequencing of drugs within the same class Hirsch MS, et al, Clin Infect Dis. 2003 Jul 1;37(1):113 28. Epub 2003 Jun 23.
Drug Resistance Approximately 20 billion mutations can occur each day in an infected person All possible point mutations including those responsible for drug resistance occur daily Minority mutated populations are generally less fit than wild-type viruses but can be selected by antiviral drugs, if viral replication is not fully suppressed Hammer SM, et al, International AIDS Society-USA panel. Treatment for adult HIV infection: 2006 recommendations of the International AIDS Society-USA panel. JAMA. 2006 Aug 16;296(7):827 43.
Resistance Is a Result of HIV Replication in the Presence of Drug Outgrowth of mutated viruses Drug CA B pressure No drug pressure Wild type Drug-resistant virus Double drug-resistant virus Hammer SM, et al, JAMA. 2006, Aug 16; 296(7):827 43. Wainberg MA, et al, JAMA. 1998 Jun 24;279(24):1977 83.
HIV Drug Resistance Is Influenced by: 1. The genetic barrier for a drug, i.e.: number of mutations needed for resistance to occur and the speed with which they are likely to occur 2. Ability of a resistant virus to grow out and to become predominant in the presence of drug (i.e.: increased fitness compared to wild-type virus in the presence of drug) 3. Development of additional compensatory and/or secondary mutations Martinez-Cajas JL, et al, Antiviral Res. 2007 Dec;76(3):203 21. Epub 2007 Jul 16.
The Enzymes of HIV All are now targets of therapy Reverse Transcriptase Converts the HIV RNA into HIV DNA Protease Cleaves HIV polyproteins into functional protein pieces Integrase Processes this HIV DNA and incorporates it into the host genome
Thus, Clinical Resistance a. Begins with a mutation (or a change in the genotype) that increases the IC 50 for a given drug, i.e.: establishment of a resistant phenotype. b. This, in turn, leads to increased plasma viral load or virological failure. Hammer SM, et al, JAMA. 2006, Aug 16; 296(7):827 43.
Role of Host Factors Maintenance of adequate drug levels both in plasma and/or inside cells is key to prevention of drug resistance. Individual variations may occur in regard to drug levels. These may be influenced by host genetic composition. Martinez-Cajas JL et al, Antiviral Res. 2007 Dec;76(3):203 21. Epub 2007 Jul 16.
Prevalence of Resistance 1. Between 20 and 30% of patients who start triple therapy may develop resistance to one, two, or three classes of drugs within a few years. 2. Transmission of drug-resistant viruses is a major problem in regard to both public health and care. Harrigan PR, et al, J Infect Dis. 2005 Feb 1;191(3):339 47. Epub 2004 Dec 22. Wainberg MA, et al, JAMA. 1998 Jun 24;279(24):1977 83.
Some Examples of Transmission of MDR Viruses in Primary HIV-1 Infection Patient 1 Codon substitutions associated with drug resistance NRTI mutations NNRTI mutations PI mutations 41L; 67N; 69N; 70R; 74V; 184V; 215F; 219Q 100I; 103N 2 41L; 184V; 215Y 103N, 179E 3 None 103N 10I; 36I; 54V; 63P; 71V; 73S; 82V; 90M 48V; 63P; 71V; 73S; 77I; 82A; 90M 10I; 54V; 63P; 71V; 82T; 84V; 90M 4 184V; 215Y 103N 77I 5 184V 108I 20R, 77I 6 41L; 67N; 210W; 215Y None 63P; 71V; 73S; 90M 7 41L; 215Y 101E None 8 41L; 215Y 101E None Brenner B, et al, AIDS. 2004 Aug 20;18(12):1653 60.
Independent Predictors of Drug Resistance in Individuals after Beginning HAART High baseline plasma HIV RNA Low CD4 cell count Moderate levels of adherence Harrigan PR, et al, J Infect Dis. 2005 Feb 1;191(3):339 47. Epub 2004 Dec 22.
Mutations Selected by NRTIs K L Y M Abacavir 65 74 115 184 R V F V Didanosine Emtricitabine Lamivudine K 65 74 R K R L V 65 184 K 65 184 R M V I M V I M D K L T K Stavudine 41 67 70 210 215 219 L N R W Y Q F E Tenofovir K K 65 70 R E M D K L T K Zidovudine 41 67 70 210 215 219 L N R W Y Q F E
Mutations Selected by NRTIs Multi-NRTI Resistance: 69 Insertion Complex (affects all NRTIs currently approved by the US FDA) M A K L T K 41 62 69 70 210 215 219 L V Insert R W Y Q F E Multi-NRTI Resistance: 151 Complex (affects all NRTIs currently approved by the US FDA except tenofovir) A V F F Q 62 75 77 116 151 V I I Y M Multi-NRTI Resistance: Thymidine Analogue-associated Mutations (TAMs; affects all NRTIs currently approved by the US FDA) M D K L T K 41 67 70 210 215 219 L N R W Y Q F E Johnson VA, et al, 2007. Top HIV Med. 2007 Aug-Sep;15(4):119 25.
Mutations Selected by NNRTIs Efavirenz L K V V 100 103 106 108 I N M I Y C I Y G 181 188 190 225 L S A P H Etravirine (expanded access) V I A L K V 90 98 100 101 106 179 181 190 G I E I P V D F Y C I V G S A Nevirapine L K V V Y Y G 100 103 106 108 181 188 190 I N A I M C I C L H A Johnson VA, et al, 2007. Top HIV Med. 2007 Aug-Sep;15(4):119 25.
Mutations Selected by PIs Atazanavir +/- ritonavir Fosamprenavir/ ritonavir Darunavir/ ritonavir Indinavir/ ritonavir Lopinavir/ ritonavir L G K L D L E M M G I F L D I I A G V I I N L I 10 16 20 24 32 33 34 36 46 48 50 53 54 60 62 64 71 73 82 84 85 88 90 93 I E R I I I Q I I V L L L E V L V C A V V S M L F M F L L Y V M I S T M V I V V M V T T F C T T L A I V A L V M I I I G L V I L 10 32 46 47 50 54 73 76 82 84 90 F I I V V L S V A V M I L V F R M S V T L 10 V V L I I I G L I L 11 I I F V V M S V V V L K L V M M I A G L V V I 20 24 32 32 33 36 46 71 73 76 77 82 84 I M I I I I V S S V I A V M R R L T A F V M T 10 20 24 32 33 46 47 50 53 54 63 71 73 76 82 84 90 F M I I F I V V L V P V S V A I R L A L T F R A T V M S V T S 47 L K L V L M I I F I L A G L V I L 50 54 54 73 76 84 V 89 L 90 M
Mutations Selected by PIs (cont) Nelfinavir L D M M A V V I N L 10 30 36 46 71 77 82 84 88 90 F N I I V I A V D I L T F S T S M Saquinavir/ ritonavir L 10 L 24 G I I A 48 54 62 71 73 77 82 84 90 G V V I L I R V I V V L V V T S I A F T S V M Tipranavir/ ritonavir L I K L E M M M I I Q H T V N I L 10 13 20 33 35 36 43 46 47 54 58 69 74 82 83 84 90 V V M F G I T L V A E K P L D V M R M V Johnson VA, et al, 2007. Top HIV Med. 2007 Aug-Sep;15(4):119 25.
PI NNRTI NRTI/NtRTI Drugs Fold change 1 Cut-off 2 Resistance analysis 3 NRTI/NtRTI mutations 4 : 41L, 184V, 210W, 215Y Retrovir Zidovudine 5.6 1.5 11.4 Reduced response Epivir Lamivudine 45.4 1.2 4.6 Minimal response Videx Didanosine 1.1 0.9 2.6 Reduced response Zerit Stavudine 1.0 1.0 2.3 Maximal response Ziagen Abacavir 2.8 0.9 3.5 Reduced response Emtriva Emtricitabine 43.5 3.1 Resistant Viread Tenofovir DF 1.2 1.0 2.3 Reduced response NNRTI mutations 4 : 103N Viramune Nevirapine 43.3 6.0 Resistant Sustiva, Stocrin Efavirenz 16.4 3.3 Resistant PI mutations 4 : 10I, 32I, 54L. 71T, 77I, 84V, 90M Crixivan Indinavir 5.8 1.0 5.4 Minimal response Crixivan ; boosted Indinavir / r 5.8 2.3 27.2 Reduced response Viracept Nelfinavir 27.4 1.2 9.4 Minimal response Invirase ; boosted Saquinavir / r 9.7 3.1 22.6 Reduced response Lexiva, Telzir ; boosted Fosamprenavir / r 12.8 1.5 19.5 Reduced response Kaletra Lopinavir / r 7.8 6.1 51.2 Reduced response Reyataz ; boosted Atazanavir / r 16.2 2.5 32.5 Reduced response Aptivus ; boosted Tipranavir / r 0.9 1.5 7.0 Maximal response 1 Prezista TM ; boosted Darunavir / r 5.3 10.0 106.9 Maximal response 1. Predicted fold change in 50% inhibitory concentration (IC 50 ), relative to susceptible reference virus. 2. Cut-off values for maximal and minimal clinical response (clinical cut-off) or for normal susceptibility range in vitro (biological cut-off). Biological cut-offs are printed in italic. 3. Resistance analysis based on the magnitude of the fold change relative to the clinical or the biological cut-offs.4. Mutations printed on page 1 are those reported on public lists (ANRS, Stanford, IAS-USA) or by drug development sponsors. A complete list of all differences from the reference wild type is given on page 2.
Adding New Drugs Is Useful When: 1. A new member of a previously used class can help to suppress mutated viruses, i.e.: sequencing of drugs within a class. 2. The genetic barrier of a newly added drug is higher than that of a previously used member of that class. 3. The new combination suppresses viral replication and can prevent new mutations from arising; this pre-supposes that the drugs in the new combination can reinforce each other so that viral replication and mutagenesis do not occur. Martinez-Cajas JL, et al, Antiviral Res. 2007 Dec;76(3):203 21. Epub 2007 Jul 16.
Lessons in Drug Resistance 1. All drugs select for resistance-conferring mutations 2. Combination therapy with different drugs can select for novel patterns of mutations 3. Availability of new drug classes (i.e.: integrase and CCR5 inhibitors) offer potential for treating people who have resistance to older agents Harrigan PR, et al, Clin Infect Dis. 2000 Jun;30 Suppl 2:S117 22. Correll T, et al, Pharmacotherapy. 2008 Jan;28(1):90 101. Van Lunzen J, Eur J Med Res. 2007 Oct 15;12(9):435 40.
Treatment Failure 1. Will first result in resistance to those drugs that have a low genetic barrier (e.g., 3TC, NVP, EFV) 2. Will result in the preferential outgrowth of mutated viruses 3. Will often potentiate an accumulation of mutations that increase both levels of resistance as well as viral fitness 4. Will not commonly result in resistance to all of the drugs in a regimen Harrigan PR, et al, Clin Infect Dis. 2000 Jun;30 Suppl 2:S117 22. Maguire M, et al, AIDS. 2000 Jun 16;14(9):1195 201.
Overcoming Drug Resistance The use of newer generation boosted PIs (darunavir and tipranavir) can be shown to be useful in treatment of patients resistant to earlier generations of boosted PIs. However, an accumulation of limited numbers of additional mutations may result in resistance to these newer agents. Thus, the genetic barrier of a new generation boosted PI in the case of PI-experienced patients is not as high as would be the case for a treatment naïve patient. Indeed, in some cases involving treatment-experienced patients, only 1 or 2 new mutations might now suffice in order for resistance to occur. Hammer SM, et al, JAMA. 2006 Aug 16;296(7):827 43 Morand-Joubert L, et al, Antivir Ther. 2006;11(2):143 54.
Genetic Barrier for Resistance 1. Refers to number of mutations required in order for resistance to develop and the speed with which such mutations are likely to accumulate. 2. A high genetic barrier for resistance is always advantageous for a drug, but is not less of an important consideration than potency. 3. There is a need to protect drugs with a low or moderate genetic barrier for resistance by using other potent drugs in combination with them. Sometimes, long drug half-lives can help to forestall resistance in the event of non-adherence. This is sometimes referred to as the forgiveness of a regimen. Mo H, et al, J Virol. 2005 Mar;79(6):3329 38. Hoefnagel JG, et al, Antivir Ther. 2005;10(8):879 92. Morand-Joubert L, et al, Antivir Ther. 2006;11(2):143 54.
Cross-resistance 1. Occurs when mutational changes selected by a given drug also confer resistance to other drugs that are members of the same class 2. Is closely associated with specific mutations or combinations of mutations 3. Can be conferred by some single mutations in the case of NRTIs and NNRTIs but never in the case of PIs Harrigan PR, et al, Antimicrob Agents Chemother. 2002 Mar;46(3):909 12.
Drug Resistance Is Relative and Not Absolute In many cases, an increase in IC 50 may not necessarily preclude use of a drug in therapy Some mutations may actually enhance sensitivity to certain drugs, e.g., some NRTI mutations can enhance sensitivity to some NNRTIs The M184V, L74V, and K65R mutations can enhance sensitivity to ZDV Sometimes, CD4 counts continue to rise despite virological failure Kempf DJ, et al, Antivir Ther. 2002. 7: 165 74. Larder BA, et al, Science. 1995 Aug 4;269(5224):696 9. St Clair MH, et al, Science. 1991 Sep 27;253(5027):1557 9. Parikh UM, et al, J Virol. 2006 May;80(10):4971 7.
Treatment Interruptions 1. Can lead to non-detectability of previously detectable mutations and re-emergence of more fit wild-type viruses 2. Do not prevent archiving of resistance mutations and their subsequent reappearance upon introduction of relevant drugs El-Sadr WM, et al. N Eng J Med. 2006.
Allele Specific PCR or Ultrasensitive Resistance Assays 1. Are more sensitive than traditional bulk sequencing for detection of mutated viruses 2. Can sometimes predict treatment failure for certain drug classes, even if minority species of mutated viruses are detected Johnson VA, et al, Top HIV Med. 2007 Aug-Sep;15(4):119 25.
New Agents in Existing Classes Mutations to older agents are likely to be present Majority variants Minority variants Some degree of cross resistance can be anticipated Cross resistance increases with the number and type of mutations The activity of a new agent from a new class is likely to be more predictable even with state of the art resistance testing Hammer SM, et al, JAMA. 2006, Aug 16; 296(7):827 43 Martinez-Cajas JL, et al, Antiviral Res. 2007 Dec;76(3):203 21. Epub 2007 Jul 16.
Steps of Integrase Activity att 3 processing 1) Endonucleotide specifically cleaves 2 bases from 3 ends of HIV DNA takes place in cytosol, leaving CA ends HIV DNA Strand transfer 2) HIV DNA strands are transferred onto host DNA at fairly non-specific sites Host DNA 5 5 5 5 5 5 3) Host DNA gaps are filled and repaired, leaving fully integrated HIV provirus Thomas M, et al, Trends Biotechnol. 1997 May;15(5):167 72.
Mutations in the Integrase Gene Associated with Resistance to Integrase Inhibitors Raltegravir (expanded access) Q 148 H K R N 155 H Johnson VA, et al, 2007. Top HIV Med. 2007 Aug-Sep;15(4):119 25.
Protocol 005: Raltegravir + OBR in Patients with Triple Class-resistant Virus Randomized, double-blind, placebo-controlled, dose-ranging, phase IIb study Patients with triple-class resistant virus, HIV-1 RNA >5000 copies/ml and CD4+ cell count >50 cells/mm 3 (N = 178) Double-blind phase Raltegravir 200 mg BID + OBR* (n = 43) Raltegravir 400 mg BID + OBR* (n = 45) Raltegravir 600 mg BID + OBR* (n = 45) Placebo + OBR* (n = 45) Week 24 Open-label phase* Raltegravir 400 mg BID + OBR* (n = 100) Week 48 *Patients allowed to receive open-label raltegravir 400 mg BID after at least 24 weeks of double-blind therapy. Median duration of double-blind phase was 40 weeks for the raltegravir groups and 24 weeks for the placebo group. *DRV/RTV was not available for use in OBR Grinsztejn B, et al, Lancet. 2007 Apr 14;369(9569):1261 9.
HIV-1 RNA <50 copies/ml (%) CD4+ cell count change (cells/mm 3 ) Protocol 005: 48-Week Results Raltegravir 200 mg BID 100 Double-blind only ITT, NC = F Raltegravir 400 mg BID Double-blind plus open label 200 Raltegravir 600 mg BID Placebo Baseline Carried Forward for VFs Double-blind only Double-blind plus open label 80 150 60 100 40 50 20 0 0 0 2 4 8 12 16 24 32 40 48 Week No. of Contributing Patients 43 43 42 45 45 44 45 45 45 45 45-50 0 2 4 8 12 16 24 32 40 48 Week No. of Contributing Patients 43 41 37 45 43 44 45 42 43 45 43 Grinsztejn B, et al, Lancet. 2007 Apr 14;369(9569):1261 9.
Analysis of Raltegravir Resistance in PN005 Phase 2 Study in patients w/triple class resistance: Genotype of first time point, 24 week results Virologic failure was observed in 38/133 (28.6%) patients on RAL Integrase mutations were observed in 35 of 38 patients failing RAL RAL failure was usually associated with either of two genetic pathways (N155 or Q148) Additional mutations were frequently observed with both pathways E92Q, L74M, T97A (predominately N155 pathway) G140A/S, E138A/K (predominately Q148 pathway) Q148 containing mutants were more common than N155 N155H pathway Total n=14 Q148 pathway Total n=20 Y143 pathway Total n=1 N155H (n=2) Q148H + secondary change (n=19) Y143R (n=1) N155H + secondary change (n=12) Q148R (n=1) Hazuda DJ, et al, June 2007, 16th International HIV Drug Resistance Workshop, Barbados, Abstract #8.
Raltegravir Resistance Data Protocols 005, 018, and 019 Paired sequence analysis of baseline and on-treatment samples from 77 subjects with evidence of virologic failure 75/77 (97%) genotypic mutations in the HIV-1 integrase coding region 3 key mutations, Y143C/H/R, Q148H/K/R, or N155H Observed in 65/75 subjects (87%) susceptibility in cell culture to raltegravir Q148 H/K/R N155H 24- to 46-fold 13-fold Grinsztejn B, et al, Lancet. 2007 Apr 14;369(9569):1261 9.
Phenotypic Effects of Selected Integrase Mutations on Raltegravir Sensitivity (Avg +/- SEM) 600 Fold-change IC 50 400 300 200 100 0 Hazuda DJ, et al, June 2007, 16th International HIV Drug Resistance Workshop, Barbados, Abstract #8.
Raltegravir Summary The highest levels of resistance were seen when three or more mutations were present in Integrase. Single mutations were rarely seen and conferred only low-level resistance as well as decreased fitness. Data from the Merck 005 study are consistent with tissue culture results. Hazuda DJ, et al, June 2007, 16th International HIV Drug Resistance Workshop, Barbados, Abstract #8.
Elvitegravir in Treatment-experienced Patients Randomized, active-control, partially-blinded (dose of elvitegravir), phase II dose-finding study Primary endpoint: time-weighted average change from baseline in HIV RNA through 24 weeks (DAVG 24 ) HIV-infected patients with HIV-1 RNA 1000 copies/ml, any CD4+ cell count, and 1 PI mutation (n=278) Stratified by ENF in OBR Elvitegravir/RTV 20/100 mg + OBR* (n=71) Elvitegravir/RTV 50/100 mg + OBR* (n=71) Elvitegravir/RTV 125/100 mg + OBR* (n=73) CPI/RTV + OBR* (n=63) Week 16 Week 24 *OBR = NRTIs ± ENF (NNRTIs excluded). TPV and DRV permitted after Week 16. Discontinued at Week 16 by DMSB. Zolopa A, et al. ICAAC 2007. Abstract H-714.
Resistance to Elvitegravir Phase 2 study involving 278 patients Most common mutations were E92Q and T66I/A/K Other mutations included E92Q E138K Q148R/K/H N155H Levels of resistance 150-fold Zolopa A, et al. ICAAC 2007. Abstract H-714. McColl DJ, et al, 2007, 16th International HIV Drug Resistance Workshop, Barbados, Abstract #9.
RAL fold change (relative to NL4-3) Cross-resistance Between Raltegravir and Elvitegravir 1000 Correlation of EVG and RAL susceptibility among EVG/r 125 mg VF isolates (n=28) R 2 =0.66 100 10 G140S/C + Q148H/R/K G140S + Q148K mixture E138K + S147G + Q148R Other IN mutation patterns No IN mutation developed 1 0.1 1 10 100 1000 EVG fold change (relative to NL4-3) McColl DJ, et al, 2007, 16th International HIV Drug Resistance Workshop, Barbados, Abstract #9.
Levels of resistance associated with failure to elvitegravir seem higher than those associated with failure against raltegravir. Cross-resistance between raltegravir and elvitegravir is a reality. McColl DJ, et al, 2007, 16th International HIV Drug Resistance Workshop, Barbados, Abstract #9.
Resistance to Raltegravir and Elvitegravir: Observations to Date Virologic failure is generally (but not always) associated with the development of integrase mutations Multiple mutation patterns have been observed For raltegravir, at least three different genetic pathways defined by a specific signature mutation and additional secondary changes have been identified Secondary changes are required to confer high level resistance; consistent with a pharmacologic threshold (clinical cut-off needs to be established) Time to rebound and evolution of high level resistance varies Early data suggest significant potential for cross resistance with Elvitegravir, but NOT all integrase inhibitors Hazuda DJ, et al, 2007, 16th International HIV Drug Resistance Workshop, Barbados, Abstract #8; also McColl DJ, et al, 2007, 16th International HIV Drug Resistance Workshop, Barbados, Abstract #9.
Viral Escape to Maraviroc will be Different from Anything Seen Previously Maraviroc: Binds to a host protein (all other ARV have viral targets) N-terminus and Extracellular Loops Binding pocket in TM domain Cell surface Only active against CCR5-tropic strains Not a competitive inhibitor
Viral escape to CCR5 inhibitors Change in Tropism Phenotypic Markers Genotypic Markers
Selective Inhibition of R5 Viruses can Lead to 43 a Change in Tropism Result to D/M or X4 A Trofile (like all resistance tests) measures relative proportions (not absolute amounts) of different viruses (Panel A) Selective inhibition of a majority virus type, increases the sensitivity to detect the minor variant (Panel B) R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R D R R R D R R R D R R R D R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R X R R R D R R R D R R R D R R R R R R R R R R R R R R R R B D X D D D D D D R5 MVC D/M E van der Ryst et al, 47th ICAAC, September 2007, paper number H-715
Viral Load (RNA copies/ml) 10^1 10^2 10^3 10^4 10^5 10^6 10^7 CD4 + (cells/mcl) CXCR4-using Env Clones Were Detected at Low Frequency in the Baseline Sample Patient 6 R5 R5 DM DM DM DM DM DM R5 R5 1000000 500 100000 400 10000 1000 100 10 CXCR4-using clones detected at baseline (7%) No CCR5-tropic clones on treatment 300 200 100 1-100 0 100 200 300 0 Time Since First Administration (Day)
Resistance to Entry Inhibitors What about T-20 or Enfuvirtide, the fusion inhibitor that prevents HIV entry into target cells. It is important to understand that the target of this drug is the viral envelope gene. Therefore resistance is encoded by mutations within the env gene, unlike the situation with CCR5 inhibitors.
Mutations in the Envelope Gene Associated with Resistance to Entry Inhibitors Enfuvirtide G 36 37 38 39 40 42 D S I V Q V A M E R Q H N T N 43 D Johnson VA, et al, 2007. Top HIV Med. 2007 Aug-Sep;15(4):119 25. Westby M, et al, 2007. Journal of Virology. Vol 81:2359-7.
Summary and Conclusions Lack of pre-existing cross-resistance is of significant benefit for new drug classes. The recent availability of entirely new drug classes along with newer PIs and NNRTIs is very promising for experienced patients. Cross-resistance between integrase inhibitors is likely. More information about resistance to these new agents will accumulate with more experience.
Summary Complete HIV RNA suppression is now a realistic goal for most ART experienced individuals even with the most complicated resistance patterns.
Conclusions 1. The occurrence of drug resistance mutations is a continuous process, so long as viruses are able to replicate. 2. Drug resistant viruses are likely to become predominant in the presence of drug, which is when they have a growth advantage over wild-type viruses that are drug sensitive. 3. Higher viral loads in patients will favour a more rapid accumulation of mutations, because more extensive viral replication will occur in this circumstance.
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