Authored By: B. V Waghmare

Thursday, October 28, 2010


2. Development of HIV Mutations

We strongly recommend genotypic testing on all patients who meet the protocol-defined definition of a lack or loss of virologic response, preferably while on study drug or as soon as possible after discontinuation of study drug. Studies have shown WT virus may outgrow resistant HIV strains in the absence of selective drug pressure. For this reason, it can be useful to collect and store samples for resistance testing at the same time points HIV RNA testing are done. These samples can provide important information on the development of resistance, especially for drugs that may have more than one possible resistance pathway.
The proportion of patients who develop any nucleoside analogue reverse transcriptase inhibitor- (NRTI), nonnucleoside reverse transcriptase inhibitor- (NNRTI), or PI-associated mutation and the time to development of these mutations (measured as time to virologic failure) should be presented. Both primary and secondary mutations should be evaluated. For example, for patients receiving a new PI, it is important to evaluate the development of primary and secondary PI mutations along with any other changes in the PR (protease) and RT gene, when applicable. It is also important to assess the genotypic basis of drug susceptibility changes attributable to extragenic sites, such as the protease cleavage sites.

3. Baseline Phenotype and Virologic Response
Analyses also should be conducted to define the decrease in phenotypic susceptibility that adversely affects virologic response (i.e., susceptibility breakpoints). Rather than a single breakpoint, we encourage sponsors to explore incremental subgroups associated with maximal, reduced, or minimal response rates. However, we recognize a single breakpoint may be more appropriate for some drugs. Importantly, the breakpoints determined for a given study are not meant to represent definitive clinical susceptibility breakpoints for all patient populations. Often the data in an initial NDA submission are based on a select patient population. Data displayed in package inserts are provided to give clinicians information on the likelihood of virologic success based on pretreatment susceptibility to a given agent. Additional data are needed to determine definitive susceptibility breakpoints for a given drug. An example of this analysis is presented in Table 3.

4. Genotypic and Phenotypic Correlations: Changes in Susceptibility from Baseline
Assessing changes in susceptibility over time on treatment is an important factor in the characterization of a drug’s resistance profile. For patients who meet the protocol-specified definition of a lack or loss of virologic response, evaluation of mean and median-fold changes in susceptibility from baseline for the investigational drug and other approved drugs from both inside and outside the same class is important. In addition, analyses should be conducted on patients who develop a particular new mutation during treatment, and the median-fold change in susceptibility from baseline should be presented. Table 4 shows an example of how data from this analysis can be displayed. Efforts also should be made to define relationships between genotype and phenotype as shown in Table 5.

5. Cross-Resistance
The evaluation of cross-resistance with other drugs in the same class is important. Characterization of cross-resistance of a drug can provide health care providers and patients with information on how to choose and sequence antiretroviral drugs. Evaluation of the effect of the investigational drug on subsequent use of other drugs and how previous treatment with other drugs may affect the response to the investigational drug is essential in drug development. The former can be accomplished by designing rollover studies evaluating virologic response rates in patients discontinuing study drug in clinical trials.
We encourage sponsors to incorporate prospective rollover designs to provide for assessment of virologic responses in study patients administered subsequent antiretroviral regimens. When possible, the design of a rollover study should include a randomized control. Every effort should be made to capture as much information as possible from the original studies. Resistance testing can be used to assess the genotype and phenotype of antiretroviral-experienced patients that predict success or failure after exposure to previous therapies. This testing can involve longer follow-up of study patients, perhaps continuing into the postmarketing period.

6. Additional Analyses
In addition to the analyses previously suggested, sponsors should consider conducting the following investigations:
• Genotypic sensitivity scores (GSS) and phenotypic sensitivity scores (PSS) and virologic outcome analyses can be investigated in all phases of development. Sponsors should discuss with the division in advance methods to calculate GSS and PSS for the optimized background regimens.

• Sponsors are encouraged to conduct exposure-response analyses throughout the drug development process, beginning with phase 2a studies in HIV-infected patients. One goal of exposure-response evaluations is to aid dose selection for phase 2b and phase 3 studies. Results from exposure-response evaluations also can help determine whether dose adjustments are warranted for special populations and whether therapeutic drug monitoring may be helpful in some patients.

Exposure-response evaluations require the collection of exposure data (drug concentrations) and response data (efficacy and safety). The sponsor should discuss exposure-response evaluation plans with the division and the Office of Clinical Pharmacology before study initiation. Exposure-response evaluations should indicate which drug exposure measures (e.g., area under the curve, Ctrough) are relevant to a given virologic outcome. In addition, sponsors are encouraged to evaluate the proportion of responders to a given drug regimen by inhibitory quotient (IQ), steady-state Cmin values, and other methods, as appropriate. An IQ value is the ratio of Cmin/EC50 (protein-binding corrected).

• Pharmacogenetic analyses can be conducted to determine genetic factors that may be involved in virologic response (e.g., for co-receptor inhibitors targeting a host receptor, genetic differences in the receptor may affect response).


Drug Resistance Assay and Antiretroviral drug development guidance--->

A. Mechanism of Action
B. Antiviral Activity in Cell Culture
C. Cytotoxicity and Therapeutic Indexes
D. Protein Binding
E. Selection of Drug-Resistant HIV-1 Variants in Cell Culture
1. Genotype
2. Phenotype
F. Cross-Resistance
G. Characterization of Genotypic and Phenotypic Assays
1. Genotypic Assays
2. Phenotypic Assays
A. General Considerations
B. Data Collection
C. Methods and Types of Analyses
1. Baseline Genotype and Virologic Response
2. Development of HIV Mutations
3. Baseline Phenotype and Virologic Response
4. Genotypic and Phenotypic Correlations: Changes in Susceptibility from Baseline
5. Cross-Resistance
6. Additional Analyses
D. Other Considerations
1. Role for Supporting Initial Activity and Dose-Finding Studies
2. Data Collection from Dose-Finding Trials
3. Use of Resistance Data to Establish an Indication
4. Use of Resistance Data for Study Enrollment Criteria, Background Regimen Selection, and Stratification Factors
5. Nonclade B Subtypes
E. Monitoring during Phase 4



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Last updated: 28-Oct-2010

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