On Surrogate Endpoints in HIV Vaccine Efficacy Trials

Steven Self, Peter Gilbert, Michael Hudgens

FHCRC/UW

FDA/Industry Statistics Workshop, Sept 18-19, 2003“Statistics: From Theory to Regulatory Acceptance”

Outline

1. HIV Vaccine Trials: Current Status

2. Clinical Endpoints in Vaccine Trials

3. Endpoints in HIV Vaccine Trials

4. A Simulation Approach1. Goal2. Approach3. Example

5. Conclusions/Discussion

HIV Vaccine Trials: Current Activity

• Phase I and II trials (ongoing)• 27 trials involving ~1,800 participants• 16 different vaccine candidates• 10 sponsors

• Phase III trials• 1 completed• 1 to be completed in Q4 ’03• 1 planned to start in ’04

• HVTN trials (Ph I, II) starting by Q3 ’04• 9 different vaccine candidates• 1,453 participants

HIV Vaccine Trials: Current Results

• Immune Responses• Measurable cellular response in ~50%• No broadly neutralizing Ab in sera, mucosa

• Non-human Primate Trials• Amelioration of disease course but no protection

from infection upon challenge• Body of NHP literature difficult to assess

• Efficacy• No overall efficacy in 1 completed efficacy trial

HIV Vaccine Trials: Summary

• Immune correlate of protection unknown• Many candidate vaccines but full range of desired

immune responses poorly covered • Multiple efficacy trials will be required* • Plan for long-term, iterative development program*

* Klausner et. al. (2003) Science

Classical Measure of Vaccine Efficacy

VE = % reduction in population incidence or morbidity/mortality rate

Classical Endpoint for Vaccine Efficacy

• Clinically significant morbidity and mortality• Pathogen specificity• Standard of care• For treatable infections:

– Prevent/delay constellation of signs/symptoms sufficient to trigger treatment initiation (save cost/toxicity assoc with treatment)

– Interact w/ treatment to improve risk/benefit profile of vaccine/tmt vs tmt alone

Measures of Vaccine Efficacy*

VE = % reduction in population incidence or morbidity/mortality rate

VES = % reduction in population infection rate

VEP = % reduction in rate of morbidity/mortality

VEI = % reduction in rate of 2o transmission

* Halloran, Longini, Struchiner

VE: % Reduction in Transition Intensities

Uninfected/Seronegative

Infected/Seropositive

Morbidity/Mortality

VE

VES VEP

VEI

2o Transmission

Endpoints in HIV Vaccine Efficacy Trials

– “Infection” Endpoint (A biomarker-based surrogate)

• Operationally: presence of Ab and detectable HIV RNA• Aligned with one primary objective of HIV vaccine• Acceptable by all• However captures only one aspect of potential vaccine

effects on clinical outcomes

Endpoints in HIV Vaccine Efficacy Trials

“Post-infection” Endpoints: Some Issues• Long-term FU required for morbidity/mortality endpoints

esp with ARV treatment• Complicated dynamical process likely dominated by

treatment effects• Uncertainty of optimal treatment initiation triggers• Variability in treatment initiation• Analytics

– Key biomarker trajectories “dependently censored” by treatment initiation

– Conditional vs unconditional analyses– Combination of analyses

Post-Infection Endpoints: Current Approach

Provide treatment within trial – standardized treatment initiation guidelines (e.g. DHHS,

UNAIDS)– standardized treatment monitoring/management

Develop complementary array of endpoints to cover key aspects of post-infection outcomes– Early Endpoints - pre-ART– Mid-term Endpoints - peri-ART– Long-term Endpoints - post-ART

“Reasonable conservatism” for interpretation of vaccine effects on surrogates

HIV Vaccine Efficacy Trial Endpoints

Uninfected/Seronegative

Infected/Seropositive

Treatment Initiation

Morbidity/Mortality

Infection

Short-term Endpoints: - Pre-ART VL

Mid-term Endpoints: - Composite (VL, tmt init) - Biomarker trajectories (VL, CD4)

Long-term Endpoints: - vaccine/tmt effects - CD4 - Morbidity/Mortality

• Definition: First event of ART initiation or virologic failure (VL > X cps/ml)

• Composite endpoint directly tied to clinical events• virologic failure places a subject at risk for

progression/transmission• starting ART exposes a subject to drug toxicities,

resistance, loss of future drug options

• Assess with standard statistical methods (Kaplan-Meier, Cox regression)

A Composite Endpoint

• Surrogate vaccine efficacy parameter:

VEVLC(T;X) = percent reduction (vaccine vs. placebo) in the risk of the composite endpoint by T months post infection diagnosis

• X calibrates the magnitude of virologic control (e.g., X = 1,500 copies/ml)

• T calibrates the durability of virologic control (e.g., use T 18 months)

A Composite Endpoint

Example Analysis of VEVLC(18;X)

A Numerical Study*: Goal

Provide an approach to facilitate the discussion of how to use surrogate endpoints– specific to trial design – specific to particular surrogate endpoints– accommodate statistical uncertainties– accommodate model uncertainties with desired

degree of conservatism

* Gilbert et al (2003) JID

A Numerical Study: Approach

adopt empirically-based joint model of biomarker process and clinical outcomes as “true” prediction model*

modify model to incorporate degrees of “reasonable conservatism”– proportion vaccine effect explained** (attenuate log RR relating

surrogate to clinical outcome by f percent )– selection bias for conditional analyses*** (attenuate observed vaccine

effect on surrogate outcome)

RCT simulation to identify minimum observed effects on specific surrogate endpoints that would generate 95% prediction intervals for VE parameters exceeding 40%, say

* Albert et al (1998) Stat in Med** Freedman et al (1992) Stat in Med*** Hudgens et al (2003) Stat in Med; Gilbert et al (2003) Biometrics

Numerical Study: An Example

• Question: • What inference on VEVLC(18;X) “reasonably” predicts

a clinically significant VEP?

• Numerical study based on the following predictions:

• from the MACS*: Predicted(VEP) = VEVLC(18;X)

for X 5,000-10,000 cps/ml

* Albert et al (1998) Stat in Med

The numerical study is based on the following hypothetical trial:

Hypothetical Efficacy Trial

• A lower 95% confidence bound for VEVLC(18;X) > 50% predicts VEP > 40% with f = 0.375

Prediction of VEP

Summary/Conclusions

Use of surrogate endpoints in HIV vaccine efficacy trials is question of how not whether

A framework is proposed to help interpret observed effects on surrogate endpoints that is– specific to particular trial designs/endpoints – captures relevant aspects of magnitude and durability of

effect on surrogates– uses available empirical information relating biomarkers to

clinical outcomes – is tunable with respect to degree of conservatism w/r/t use

of empirical information – flexible to evolve with development program

Summary/Conclusions

HIV vaccines showing strong and durable effects on post-infection endpoints should be licensed– use of standardized ART guidelines important– use simulation studies to assist in building agreement

about defining “sufficiently strong” and “sufficiently durable”

– design trials to detect significant levels of either VES or VEVLC(T;X)

– use supporting data on other endpoints

Long-term follow-up needed– for assessing VE and VEP directly

– better understanding of surrogate endpoints