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Re-Examination of the Design of Early Clinical Trials for Molecularly Targeted Drugs

This article explores the design considerations and objectives for early clinical trials of molecularly targeted drugs, including dose/schedule determination and assessing anti-tumor activity. It also discusses the use of genomic classifiers and the importance of characterizing tumor sensitivity to the drug in phase II studies.

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Re-Examination of the Design of Early Clinical Trials for Molecularly Targeted Drugs

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  1. Re-Examination of the Design of Early Clinical Trials for Molecularly Targeted Drugs Richard Simon, D.Sc. National Cancer Institute linus.nci.nih.gov/brb

  2. Objectives of Phase I Trials • Develop dose/schedule • Determine whether the drug inhibits the targeted pathway

  3. Dose/Schedule • Ideal is to have a drug and target so specific for cancer cells that the drug can be delivered repeatedly at doses that completely shut down the de-regulated pathway without toxicity to normal cells • Because most current targets are not specific to cancer cells, most targeted drugs are toxic

  4. Dose/Schedule • Few examples of drugs whose effectiveness at inhibiting target decreases with dose after maximum • Titrating dose for maximum inhibition of target is difficult due to assay variability and need for tumor biopsies • Titrating dose to plasma concentration at which target is inhibited in pre-clinical systems is more feasible

  5. Dose/Schedule • Determining dose just below MTD which can be delivered repeatedly is often the most appropriate and practical approach • Accrue an additional cohort of patients at that selected dose to determine whether the target is inhibited

  6. Objectives of Phase II Trials of Targeted Agents • Determine whether there is a population of patients for whom the drug demonstrates sufficient anti-tumor activity to warrant a phase III trial • Optimize the regimen in which the drug will be used in the phase III trial • Optimize the target population for the phase III trial

  7. Traditional Phase II Trials • Estimate the proportion of tumors that shrink by 50% or more when the drug is administered either singly or in combination to patients with advanced stage tumors of a specific primary site

  8. Endpoints for Phase II • Tumor shrinkage • Inhibition of target pathway • Time to progression or proportion of patients without progression at a specified time

  9. Using Time to Progression as Endpoint in Phase II Trials • Requires comparison to distribution of progression times for patients not receiving drug • Proportion of patients without progression at a specified time also requires comparison for evaluation • Historical control vs randomized comparison • Phase 2.5 trial design

  10. Phase 2.5 Trial Design • Simon R et al. Clinical trial designs for the early clinical development of therapeutic cancer vaccines. Journal of Clinical Oncology 19:1848-54, 2001 • Korn EL et al. Clinical trial designs for cytostatic agents: Are new approaches needed? Journal of Clinical Oncology 19:265-272, 2001

  11. Phase 2.5 Trial Design • Randomization to chemotherapy alone or with new drug • Endpoint is progression free survival regardless of whether it is a validated surrogate of survival • One-sided significance level can exceed .05 for analysis and sample size planning

  12. Total Sample SizeRandomized Phase 2.52 years accrual, 1.5 years followup

  13. Randomized Discontinuation Design (RDD) • The RDD starts all patients on the drug • Patients with early progression go off study • Patients with objective response continue on the drug • Other patients are randomized to continue the drug or stop administration and be observed • PFS from time of randomization is the endpoint

  14. Randomized Discontinuation Design (RDD) • The RDD can facilitate observing an effect of the drug on PFS compared to a standard randomized phase 2.5 design • The RDD requires a large sample size • The RDD is not a phase III trial because it does not establish the clinical utility of administering the drug to the patient compared to not administering it

  15. Conducting a phase III trial in the traditional way with tumors of a specified site/stage/pre-treatment category is likely to result in a false negative trial • Unless a sufficiently large proportion of the patients have tumors driven by the targeted pathway

  16. Positive results in phase III trials may be driven by a subset of patients whose tumors are driven by the targeted pathway • Such trials may result in treatment of the majority with very expensive drugs for the benefit of the minority

  17. It is important to characterize in phase II studies which tumors are most likely to be sensitive to the drug

  18. Strategies for Development of Genomic Classifiers • Type of Classifier • Single gene or protein based on knowledge of therapeutic target • HER2 amplification • EGFR mutation or amplification • Empirically determined based on correlating gene expression or genotype to patient outcome after treatment. • When to develop Classifier • During phase I/II • After failed phase III trial or after broad indication drug approval

  19. Single gene or protein based on knowledge of therapeutic target • Often there will be several assays that can be used • Phase II development is a good time to establish which assay should be used in phase III • The assay to be used for either selecting patients for phase III trial or for testing hypotheses in the phase III trial should be determined before starting the phase III trial

  20. Refining the target based single gene or protein assay to use in phase III can be accomplished either from traditional single arm phase II trials with response endpoint or from randomized phase 2.5 trials with PFS endpoint

  21. Guiding Principle • The data used to develop the classifier must be distinct from the data used to test hypotheses about treatment effect in subsets determined by the classifier • Developmental studies are exploratory • Studies on which treatment effectiveness claims are to be based should be definitive hypothesis testing studies based on completely pre-specified classifiers

  22. Develop Predictor of Response to New Drug Using phase II data, develop predictor of response to new drug Patient Predicted Responsive Patient Predicted Non-Responsive Off Study New Drug Control

  23. Randomized Clinical Trials Targeted to Patients Predicted to be Responsive to the New Treatment Can Be Much More Efficient than Traditional Untargeted Designs • Simon R and Maitnourim A. Evaluating the efficiency of targeted designs for randomized clinical trials. Clinical Cancer Research 10:6759-63, 2004. • Maitnourim A and Simon R. On the efficiency of targeted clinical trials. Statistics in Medicine 24:329-339, 2005. • reprints at http://linus.nci.nih.gov/brb

  24. Comparison of Targeted to Untargeted DesignSimon R,Development and Validation of Biomarker Classifiers for Treatment Selection, JSPI

  25. Develop Predictor of Response to New Rx Predicted Responsive To New Rx Predicted Non-responsive to New Rx New RX Control New RX Control Using the Classifier in Evaluation of a New Therapeutic (II)

  26. Using Genomics in Development of a New Therapeutic (II) • Do not use the diagnostic to restrict eligibility, but to structure a prospectively planned analysis strategy of a randomized trial of the new drug • Compare the new drug to the control overall for all patients ignoring the classifier • If the treatment effect on the primary pre-specified endpoint is significant at the 0.04 level, then claim effectiveness for the eligible population as a whole. • If the overall test is not significant at the 0.04 level, then perform a single subset analysis evaluating the new drug in the classifier + patients • If the treatment effect is significant at the 0.01 level, then claim effectiveness for the classifier + patients

  27. Patient Accrual in Phase II • If the phase II trial for a particular primary site is not enriched for patients thought responsive to the drug, an initial stage of 10-15 patients may contain very few responsive patients. • Single stage design of 25-30 patients may be better • Accrual of separate cohort of 25-30 patients whose tumors express target gives best chance to evaluate drug

  28. Development of Empirical Gene Expression Based Classifier • 20 phase II responders are needed to compare to non-responders in order to develop signature for predicting response • Dobbin KK, Simon RM. Sample size planning for developing classifiers using high dimensional DNA microarray data, Biostatistics (In Press); available at http://linus.nci.nih.gov

  29. Development of Empirical Gene Expression Based Classifier • Primary site may be less relevant to predicting response • Rather than running phase II trials separately in a large number of primary sites, it may be more efficient to run a broad eligibility multi primary site phase II trial in patients for whom pre-treatment tumor biopsy is available

  30. Summary • Phase I trials of molecularly targeted agents should generally be based on determining a dose that can be delivered repeatedly over time with acceptable toxicity and then evaluating whether the targeted pathway is inhibited at that dose • Phase II trials are most efficient if anti-tumor effect can be evaluated in individual tumors rather than use of PFS

  31. Summary • Phase II trials should include establishment and refinement of classifiers of which tumors are most likely to respond • Such classifiers should be completely specified prior to launching of phase III trials • Phase III trials should incorporate prospectively specified plans for the utilization of biomarker classifiers of patients with sensitive tumors

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