A predictive biomarker is a measurement associated with response or lack of response (e.g., resistance) to a particular therapy (5
). Biomarkers of toxicity could also be viewed as a type of predictive biomarker for which the prediction is for harm that is to be avoided. Perhaps the best-known predictive biomarker is estrogen receptor status for prediction of response to endocrine therapy for breast cancer. Estrogen receptor-negative breast tumors are unlikely to respond to endocrine therapy, whereas a substantial percentage of estrogen receptor-positive breast tumors will respond to endocrine therapy. For molecularly targeted therapies, biomarkers related to the target are natural candidates for predictive biomarkers. Ideally, one would like to have some knowledge of potential predictive biomarkers before testing a new therapy in phase II trials, but often a predictive biomarker will not be clearly identified or there will not be a suitably well-developed assay available for measuring the biomarker at the start of a phase II trial.
If information is available to suggest subgroups of patients who are more likely to benefit from a therapy, it may be reasonable to conduct the phase II trial only in those patients. Factors used to limit the study population to patients believed more likely to benefit from the experimental therapy are termed enrichment factors. Enrichment factors may be predictive biomarkers, or they may be biomarkers or clinicopathologic characteristics (such as squamous cell lung cancer for pemetrexed) or demographic characteristics (such as females and never-smokers with lung adenocarcinoma for epidermal growth factor receptor inhibitor therapy) associated with a predictive biomarker or with the target of a therapeutic agent. The enrichment factors considered in this article are assumed to be biomarker-based. The lower the proportion of truly benefiting patients, the more advantageous it is to consider studying an enriched population (6
) even if the enrichment biomarker can only approximately identify the benefiting patient group. Biomarkers that could be useful as enrichment biomarkers during the drug development process might still need further refinement before they are ready for clinical use as predictive factors. This is because many enrichment biomarkers used in drug development do not have sufficiently high positive or negative predictive value to justify clinical use or the assay used to measure the biomarker during the drug development process might not be sufficiently robust and reproducible for routine clinical use. The main purpose of using an enrichment biomarker in drug development is to improve the chances that the drug will show benefit in the tested subgroup of patients to more quickly establish that the drug is worth pursuing further. Once it has been shown that there is some group of patients who benefit, the enrichment biomarker and its assay can be further developed into a clinically useful predictive biomarker test.
Trastuzumab is an example of a targeted therapeutic for which enrichment strategies were used in the clinical development process. Trastuzumab is a monoclonal antibody that targets the HER-2/neu
receptor. Preclinical studies (7
) provided evidence that trastuzumab was most likely to be effective against tumor cells that overexpressed the HER-2/neu
receptor. Pivotal phase II studies of trastuzumab as monotherapy in patients with metastatic breast cancer (9
) required evidence of HER-2 overexpression by immunohistochemistry for eligibility and this enrichment strategy was maintained in subsequent clinical trials evaluating trastuzumab in other settings. Further studies suggested that HER-2 gene amplification as measured by fluorescence in situ
hybridization may be a more reliable indicator of benefit from trastuzumab therapy (11
). Given that the positivity rate for HER-2 (immunohistochemistry 3 positive or fluorescence in situ
hybridization positive) is ~25% to 30%, the benefit of trastuzumab might not have been detected in metastatic patients had the drug development occurred in a nonenriched (all comers) setting. When trastuzumab was tested in the adjuvant setting, the patient population was also enriched for patients whose tumors were immunohistochemistry 3 positive or fluorescence in situ
hybridization positive. Interestingly, a current controversy is whether the benefit of trastuzumab delivered as adjuvant therapy is limited to this enriched group of patients or whether patients with tumors that are immunohistochemistry 1 or 2 positive without amplification (HER-2-low) might also derive some benefit from trastuzumab (12
). Should further studies confirm that HER-2-low patients benefit from trastuzumab in the adjuvant setting, possible explanations would include variations in assay methodology or alternative mechanisms of action of trastuzumab in early-stage disease.
The story of epidermal growth factor receptor targeting agents is far more confusing than the story of HER-2 and trastuzumab (4
). Small-molecule inhibitors and the monoclonal antibodies targeting epidermal growth factor receptor have been studied in several cancers, with a wide range of results both for the effectiveness of the treatment and for biomarkers that predict treatment benefit. In colorectal cancer, for example, the monoclonal antibody cetuximab has been shown to have clinical benefit, but there is no clear association between epidermal growth factor receptor overexpression (as measured by immunohistochemistry) and benefit. In contrast, the presence of certain activating KRAS mutations may confer resistance to cetuximab through dysregulation of downstream signaling pathways (13
In some instances, the putative target of the agent in early clinical testing is found to be wrong. Thus, using this target as a biomarker would lead to major errors in the development of that agent. A recent example is sorafenib, which was originally developed and tested as an inhibitor of the kinase activity of c-Raf but was later found to be an inhibitor of the kinase activity of angiogenic receptors, particularly, vascular endothelial growth factor receptors (14
). Misspecification of a target biomarker can have significant consequences. If an agent truly benefits all patients equally without regard to target biomarker status, then studying only those patients whose tumors are positive for the biomarker will only slow accrual to trials and increase expense while producing no improvement in the chance of detecting a benefit of the new therapy and unnecessarily limiting the size of the patient population offered the agent. If an agent benefits a certain subset of patients but the wrong subgroup of patient is studied because of a faulty biomarker, then a good agent could mistakenly be abandoned in the drug development process. These examples show the potential pitfalls in pursuing enrichment strategies when the biological pathways and mechanism of action of the therapeutic agent are not well understood.
Economic considerations will likely play a role in determining how biomarkers will be used in the drug development process. Assay development, biomarker screening for trial entry, and eventual market size for the drug all have cost implications. If the proportion of patients identified by an enrichment biomarker as having an increased likelihood of benefiting from the therapy is large, 85% or 90% of the general patient population, it might not be cost-effective to spend the time or resources to develop a biomarker assay for enrichment in phase II trials. In contrast, if the proportion of patients judged likely to benefit is modest, it may be essential to have in place a reasonably well-developed assay at the phase II trial stage even if the magnitude of benefit for that minority group of patients is fairly large or a beneficial therapeutic may be overlooked in a trial that includes all patients. The total sample size required for an enriched trial (number of subjects screened for eligibility using the biomarker) compared with the sample size required for a similar trial design without enrichment will depend on the proportion of patients in the enriched subgroup and the magnitude of treatment benefit in that enriched subset. The goal at the end of the drug development process is to have a drug that works in some group of patients and to be able to reasonably accurately identify that group of benefiting patients. Decisions that have to be made during the development process include whether an enrichment or predictive biomarker is needed at all and, if needed, at what point resources should be committed to refining an enrichment biomarker assay into a clinically usable predictive biomarker test. There exists a tension between the goal of rapid therapeutic development and the goal of developing a reasonably robust and accurate assay that will be useful for identifying individual patients who will or will not benefit from a particular therapy.