Tipranavir (TPV, Aptivus), is a non-peptidic protease inhibitor (PI) that demonstrates high-level antiretroviral activity against HIV isolates resistant to all other currently available PIs. Two pivotal phase III companion trials were conducted to determine the potential safety and efficacy of ritonavir (RTV, Norvir)-boosted tipranavir in comparison with a standard-of-care, ritonavir-boosted comparator PI (CPI) in individuals with multidrug-resistant virus who were failing antiretroviral therapy. These trials together comprise one of the largest study programs examining an investigational antiretroviral agent to date. RESIST-1 (Randomized Evaluation of Strategic Intervention in Multi-Drug ReSistant Patients with Tipranavir) was conducted at multiple sites throughout the United States and Australia, while RESIST-2 was conducted throughout Europe and Latin America.

To be included in these sister trials, participants had to be triple-class experienced and undergoing virologic failure (HIV RNA of more than 1,000 copies/mL) with at least one primary PI mutation. Patients whose baseline resistance test showed more than two PI-associated mutations were excluded. The doses used in these trials were tipranavir 500 mg twice daily boosted by ritonavir 200 mg twice daily.¹ Standard doses were given for the ritonavir-boosted CPI, as well as the components of the optimized background regimen.

As new drugs such as tipranavir join our clinical armamentarium, clinical cutoffs need to be defined by studying drug activity in large patient populations. Clinical cutoffs describe the activity of a drug against a mutated virus relative to the activity against wild-type virus, thus allowing particular viral isolates to be defined as susceptible or resistant to a given drug. These reference values serve as clinical guidelines that help clinicians assess HIV drug resistance. The RESIST database was thoughtfully designed so that patients treated with tipranavir could be evaluated for drug efficacy and toxicity, as well as enabling clinical cutoffs to be defined in the laboratory.

Eoin Coakley, from Monogram Biosciences, Inc. (Monogram), and colleagues set out to define the upper clinical cutoff (UCCO) and lower clinical cutoff (LCCO) for tipranavir/ritonavir, lopinavir/ritonavir (LPV/r, Kaletra), saquinavir (SQV, Fortovase, Invirase)/ritonavir, and amprenavir (APV, Agenerase)/ritonavir by evaluating virologic outcomes at week four among patients included in the RESIST database.²

Samples were selected for analysis if the new regimen did not include enfuvirtide (T-20, Fuzeon) and if the CPI was not being continued from pre-study. The tipranavir/ritonavir samples were selected such that 50% had zero to four tipranavir-associated mutations and 50% had four or more tipranavir-associated mutations. Linear regression was used to quantify the correlation between the baseline tipranavir fold-change (FC) and the absolute HIV RNA reduction from baseline to week four.

For any given antiretroviral, the biological cutoff was defined at the 99th percentile for the fold-change distribution for isolates within the Monogram database with no genotypic resistance in the protease or reverse transcriptase sequences. Data for 556 isolates were used to define the biological cutoff for tipranavir as 2.1.

Using phenotype testing, the LCCO was defined as the fold-change at which the HIV RNA response was first observed to decline relative to the wild-type reference population. Both moving and fixed-window approaches were used to determine the LCCO, and the respective LCCOs for each approach were 1.5 and 1.7. Since these values were lower than the biological cutoff, the LCCO was set at 2.0 so as to avoid incorrectly classifying wild-type viruses as resistant to tipranavir.

The UCCO was defined as the fold-change above which a clinically meaningful HIV RNA response was unlikely -- that is, the fold-change at which the week 4 HIV RNA reduction from baseline was below 0.3 log₁₀ copies/mL. It is especially important to account for the contribution of background therapy when evaluating limited HIV RNA responses. To quantify the relative virologic impact of the ritonavir-boosted PI and the other components included in the background regimen, a continuous phenotypic susceptibility score (PSS) was calculated by summing the PSS for each drug in the study regimen to which the patient was naive. Thus, the HIV RNA change attributable to each new drug was determined by its relative proportion in the total PSS. The scoring used for the individual drugs was based on data from previously described PSS studies.^3,4

A total of 176 tipranavir/ritonavir, 112 amprenavir/ritonavir, 94 lopinavir/ritonavir, and 87 saquinavir/ritonavir samples were analyzed. The baseline tipranavir fold-change significantly correlated with the adjusted HIV RNA change from baseline to week four (R² = 0.27, P < .0001). With the tipranavir LCCO set at 2.0, the UCCO for tipranavir was defined at 8. The mean HIV RNA reduction at week four for "susceptible" HIV isolates with a tipranavir fold-change below 2.0 (n=78) was -1.3 log₁₀ copies/mL, while the change for "intermediate" HIV isolates with a fold-change between 2.0 and 8 (n=72) was -0.6 log₁₀ copies/mL (P < .0001). In comparison with "intermediate" HIV isolates, the mean HIV RNA reduction at week 4 for "resistant" HIV isolates with a fold-change above 8 was -0.1 log₁₀ copies/mL (P = .002).

The LCCOs and UCCOs for the PIs included in the analysis are shown below. The investigators cautioned against using the LCCO for a specific drug as a definitive guide for determining potency or activity. This is exemplified by comparing the LCCO of tipranavir/ritonavir (2.0) with that of lopinavir/ritonavir (9.0). In RESIST, the majority of baseline isolates were susceptible tipranavir/ritonavir, whereas most were fully resistant to lopinavir/ritonavir.

Among the 568 RESIST study samples, the proportions classified as sensitive, intermediate and resistant to the various PIs are shown below:

These findings highlight the unique activity profile of tipranavir among highly PI-resistant isolates. The investigators stressed that although the most precise method for defining clinical cutoffs has yet to be determined, current methods generally provide useful guidance for clinicians. Defining upper and lower cutoffs helps guide clinical decision-making by enabling providers to choose the most active drugs available while avoiding those with minimal activity against a given patient's viral strain. Ultimately, getting patients on the right drug at the right time is what good HIV care is all about.

Footnotes

1. Hicks CB, Cahn P, Cooper DA, et al. Durable efficacy of tipranavir-ritonavir in combination with an optimized background regimen of antiretroviral drugs for treatment-experienced HIV-1-infected patients at 48 weeks in the Randomized Evaluation of Strategic Intervention in multi-drug resistant patients with Tipranavir (RESIST) studies: an analysis of combined data from two randomized open-label trials. Lancet. August 5, 2006:368(9534);466-475.

2. Coakley EP, Chappey C, Flandre P, et al. Defining Lower (L) and Upper (U) Phenotypic Clinical Cutoffs (CCO's) for Tipranavir/r (TPV), Lopinavir/r (LPV), Saquinavir (SQV) and Amprenavir (APV) Co-Administered with Ritonavir (r) Within the RESIST Dataset by the PhenoSense Assay. In: Program and abstracts of the 46th Annual Interscience Conference on Antimicrobial Agents and Chemotherapy; September 27-30, 2006; San Francisco, Calif. Abstract H-995.
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3. De Luca A, Weidler J, Di Giambenedetto S, et al. Incremental and Continuous Phenotypic Drug Susceptibility Scores more Accurately Predict Virologic and Immunologic Treatment Outcomes in HIV+ Patients Starting Salvage Therapy: Findings from the Argenta Trial. In: Program and abstracts of the 13th Conference on Retroviruses and Opportunistic Infections; February 5-8, 2006; Denver, Colo. Poster 650.

4. Lawrence J, Huppler Hullsiek K, Mayers D, et al. Antiretroviral Phenotypic Susceptibility Score as a Predictor of Treatment Response in Persons with Multi-drug-resistant HIV-1. In: Program and abstracts of the 13th Conference on Retroviruses and Opportunistic Infections; February 5-8, 2006; Denver, Colo. Poster 651.