What is drug resistance? Musings of a clinician

What is drug resistance? Musings of a clinician William Burman MD Denver Public Health Tuberculosis Trials Consortium

Financial disclosures Tibotec (developer of TMC207 and several antiretroviral drugs) Chair of Data Monitoring Committee for TMC207 trials Scientific consultant Otsuka (developer of OPC67683) Si Scientific consultant t Both companies reimburse my employer for my time

Scientific disclosures Clinician / public health official, not a laboratorian HIV therapeutics > TB therapeutics Interest in the association between in vitro measures of drug activity, pharmacokinetics, and treatment outcomes

Scientific disclosures Clinician / public health official, not a laboratorian HIV therapeutics > TB therapeutics Interest in the association between in vitro measures of drug activity, pharmacokinetics, and treatment outcomes Purpose pose, not answer questions

Naïve clinician s view of drug resistance testing results

Situations in which the definition of drug resistance is challenging hll New drug (e.g., TMC207) Availability of different members of the same class that have different in vitro potency Fluoroquinolones Lack of clinicalvalidationof of drug resistance Most 2 nd and 3 rd line drugs New resistance testing methods genotypic tests Multidrug therapy

Possible definitions of drug resistance MIC that is beyond or at the extremes of the MIC distribution of presumed wild type isolates Comparison of the MIC to a PK parameter peak/mic, AUC/MIC, time over MIC MIC that is associated with an increased risk of failure/relapse fil l (clinical (lii lvalidation) i Change in MIC (or genotype) from baseline to time of treatment failure

Distribution of ethambutol MIC for M. Heiftet tuberculosis in agar and broth Heifets L, et al. Antimicrob Heifets L, et al. Antimicrob Agents Chemother. 1986;30:927 32.

Distribution of ethambutol MIC for M. tuberculosis in agar and broth Heiftet Possible breakpoint based on outlier method Caveats: Lack of clinical validation Challenge of getting clinical validation in the context of multidrug therapy Heifets L, et al. Antimicrob Heifets L, et al. Antimicrob Agents Chemother. 1986;30:927 32.

INH PAS vs. INH EMB (6 mg/kg, 15 mg/kg): comparison of sputum culture positivity during treatment Regimen 8 wks 12 wks 16 wks 20 wks Trial 1 INH PAS 36 19 9 3 INH EMB(6) 34 17 13 12 Trial 2 INH PAS 44 16 11 5 INH EMB(15) 43 27 9 8 EMB MICs were not done EMB is an overrated drug its weakness is one of the roots of MDR TB Am Rev Respir Dis 1975; 107: 177 90

Distribution of ethambutol MIC for M. tuberculosis in agar and broth Heiftet Possible breakpoint based on outlier method Caveats: Lack of clinical validation Suggestion compare to PK parameters Heifets L, et al. Antimicrob Heifets L, et al. Antimicrob Agents Chemother. 1986;30:927 32.

Ethambutol concentrations in TB patients (children, adults) and healthy h volunteers Volunteers Children with TB Adults with TB Range of peak concentrations in adults: 1.0 5.5 micrograms/ml Szhu M, et al. Int J Tuberc Lung Dis 2004; 8: 1360 7

Distribution of ethambutol MIC for M. tuberculosis in agar and broth Heiftet Possible breakpoint based on achieving peak/mic of > 1 Problems Comparing median PK values to MIC values How valid are pharmacodynamic comparisons in TB Rx? Heifets L, et al. Antimicrob Agents Chemother. 1986;30:927 32.

Choosing a 2 drug regimen for TB treatment: in vitro activity ii and PK Drug MIC90 Cmax Half life (hours) Cmax/MIC A < 0.05 3 5 1 4 80 B < 2.0 25 50 50 2 3 20 C 15 45 8 10 3 D < 0.25 4 12 2 3 26 Burman W. Am J Med Sci 1997;313:355 63

Choosing a drug for tuberculosis treatment: in vitro synergy studies Drug A B C D A NA + - - B + NA unknown C - unknown NA D - + = synergy, - = antagonism

Choosing a 2 drug regimen for TB treatment: in vitro activity ii and PK Drug MIC90 Cmax Half life (hours) Cmax/MIC INH < 0.05 3 5 1 4 80 Strep < 2.0 25 50 50 2 3 20 PZA 15 45 8 10 3 RIF < 0.25 4 12 2 3 26 Burman W. Am J Med Sci 1997;313:355 63

Choosing a drug for tuberculosis treatment - II: in vitro synergy studies Drug INH Strep PZA Rif INH + - - Strep + unknown 0 PZA - unknown unknown RIF - 0 unknown + = synergy, - = antagonism, 0 = additive

Clinical activity of first-line TB drugs - results of the first short-course therapy trial Regimen Duration Relapse rate INH/strep/thiacet 18 3% INH/strep INH/strep/PZA 6 6 27% 10% INH/strep/RIF 6 2% Lancet 1973;1:1331-9

INH and PZA in an animal model of TB J Exp Med 1956;104:763-802

Association between initial drug resistance and cure with ith2shrz/6he (Vietnam, n = 1403) Quy HT, et al. Int J Tuberc Lung Dis 2006; 10: 45 51

The surprises of the pharmacodynamics of the first-line TB drugs Lack of sustained bactericidal activity from INH (and strep) in vivo despite remarkable in vitro activity Relatively minor effect of INH resistance on clinical outcomes Clinical activity of PZA, allowing treatment shortening from 24 to 9 mos, despite little in vitro activity Lack of predictive value of in vitro synergy studies Ability to dose TB drug much less frequently than would be predicted

Correlation between response to therapy with INH/thiacetazone and pretreatment MIC to INH Tubercle 1968;49:48

Comparison of predicted critical concentration ti (by Cmax) and the critical concentration validated from clinical trials Drug Breakpoint Breakpoint from by Cmax clinical trials INH 3-5 1.0 (or lower) RIF 4-8 05 0.5

Words of wisdom from early in the chemotherapy era The protean character of the infectious process thus causes bacilli and drugs to come into contact in areas differing markedly in their physicochemical characteristics... As drugs are selected on the basis of their ability to function in the normal physiological [in vitro] environment, they are often ineffective within the lesions, where conditions are profoundly different from the physiological. R. Dubos Am Rev Tuberc 1954;70:398

Problems of defining drug resistance Relationships betweenin vitroactivity (MIC, MBC) and clinical activity are class specific, cannot be assumed Simple pharmacodynamic models (peak/mic) do not correlate with clinical activity Definitions of resistance based on PD models cannot be accepted without clinical validation Activity in mouse model of TB treatment: much better correlation with clinical activity

Do we underestimate the frequency of drug resistance? Activity of 12RE(7) for retreatment TB, by baseline ethambutol susceptibility Resistance to INH Resistance to INH and EMB alone Number assessed 15 73 Favorable 5 (33%) 53 (73%) Failure 10 (67%) 20 (27%) Acquired resistance in treatment failure isolates from patients with baseline susceptible isolates RIF 95%, EMB 60% Conclusions Remarkably poor performance amongpatients with baseline EMB susc. isolates undetected resistance? Hong YP. Tubercle 1988 69: 241 53

Frequency of HIV resistance among treatment naïve patients: standard dvs. ultra deep genotype B li NNRTI it di t d fil f NNRTI b d Baseline NNRTI resistance predicted failure of NNRTI based therapy (HR 2.7) J Infect Dis 2009;199:693 701

Limitations of genotypic HIV resistance testing: reversion of resistance over 16 weeks of treatment interruption Complete reversion of 1 o mutations 5/18 (28%) Partial reversion No reversion 7/18 (39%) 6/18 (33%) In the absence of drug pressure, resistance mutations revert, particularly M184V and PI resistance mutations Reversion occurs because of fitness cost of a mutation J Infect Dis 2006;194:1309 18

Frequency of mixed infection among TB patients in Taiwan Beijing, n Non Beijing, Both, n (%) n (%) (%) Prior TB treatment 51% 34% 15% No prior treatment 45% 44% 10% Drug resistance Susceptible 71% 80% 71% Non MDR 12% 17% 24% MDR 17% 4% 5% Mixed infection is one possible cause of undetected dt tdresistance it Huang HY, et al. J Clin Microbiol 2010; 48: 4474 80

Other possible reasons that resistance may be under estimated dby phenotypic testing Lack of detectionof of specificforms forms of resistance Specific rpob mutations (e.g., Asp516Tyr), low level resistance Not detected in broth systems, correlated with failure/relapse (J Clin Microbiol 2009; 47: 3501 6) Minority variants resistance mutations having a fitness cost May explain worse outcomes in patients with prior treatment, even after adjusting for degree of resistance (by phenotypic testing)

Summary TB drug resistance is complex Intracellular pathogen with the capacity for dormancy Relationships most complex for drugs with moderate activity (EMB, 2 nd and 3 rd line drugs Definitions that rely on PD relationships are problematic Drug class specific; mechanism of action Drug penetration into tissues

Summary continued Use of sensitive molecular techniques suggest undetected resistance mixed infections minority variants low level level resistance not detected in phenotypic tests, but still clinically relevant Patients with prior therapy Clean drugs, not just presence of in vitro susceptibility

Acknowledgements Randall Reves Chuck Peloquin Marc Weiner Andy Vernon Ed Gardner