Preclinical Success to Clinical Failure: Do We Have a Model Problem or an Endpoint Problem?

As the AACR (American Association for Cancer Research) Annual Meeting is fast approaching, many industry and academic scientists are busy preparing talks and posters for what they hope will be the next new wave in cancer therapy or the next “new and improved” preclinical model. However, while these may be long shots, it’s the scientific drive for improvement that keeps us moving forward. As, patients and clinicians, we are desperately looking for new therapies, even if the odds are against us. The desire to fulfill that unmet medical need is exactly how the one new drug is discovered. New oncology drugs only have a 5% success rate once making it from Phase I clinical trials to FDA approval. This is the lowest success rate among the 21 major disease indications.¹

As, patients and clinicians, we are desperately looking for new therapies, even if the odds are against us. The desire to fulfill that unmet medical need is exactly how the one new drug is discovered.

This poor success rate leaves everyone, from the patients to the benchtop scientists, questioning the validity of the models and the resulting data. This is nothing new in the world of cancer research; we are all attempting to treat a disease that has the ability to fool its host and adapt to survive. To say the least this is very challenging.

It has been well published that there is a fundamental disconnect between preclinical data and clinical results. Nine out of ten attempts to bring a new oncology therapy to the clinic will fail. This is about half as successful as other therapeutic areas; such as cardiovascular and inflammation. The very low success rate in oncology leads drug companies and the FDA to be more lenient when it comes to getting new therapies to the desperate patient population. Unfortunately, this has resulted in drugs that quickly fail in the clinical setting. In a span of ~10 years there were 9,985 new drugs entering Phase I clinical trials with 31.2% (3,163) for oncology indications alone.^1,2 The high failure rate leads to industry criticism that the traditional preclinical animal models are, at best, limited in their power of predictability and, at worst, grossly inaccurate. This may be the easy scapegoat. Improving and evolving the current preclinical animal models is essential to understanding their potential and, equally as important, their limitations. Traditional preclinical (xenograft and syngeneic) models have worked in the past and continue to be highly valuable tools. However, the interpretation of the data that is produced by all of these models and how it is used to predict clinical response may be where the biggest discrepancy lies.

In an effort to more closely mimic the human disease, many human and murine tumor cell lines have been further validated as orthotopic implants. Implanting the human or murine tumor cells in the tissue of origin can result in a pathological profile that recapitulates human disease and can increase the rate of metastatic involvement when compared to the traditional subcutaneous models.³ As with all models, there are limitations with orthotopic implants; primarily monitoring disease progression can be limited to survival endpoints which are not ideal. The optimal situation is to take advantage of the orthotopic implant by using either clinically translatable imaging technology (MRI or CT) or optical imaging (BLI or FMT). The ability to use imaging technology (clinically translatable or optical) allows for evaluation of a solid tumor or hematological cancer in the same animal over time as it is done in the clinic. Tracking disease burden and response in this manner has the potential to be a very powerful tool in translating preclinical activity (response) of a new drug into clinical success.

If we can collectively raise the bar to more stringent preclinical criteria for the evaluation of novel cancer treatments, we could possibly reduce the failure rate in the clinic. Downstream of this, it would then boost the confidence of clinicians and patients when they are reviewing preclinical data.

In recent years there have been significant enhancements in the utilization of patient derived xenografts (PDXs). In a retrospective analysis of cytotoxic and targeted therapies, PDX models were clearly predictive (~90% accurate) of clinical outcome when dosed at clinically relevant dosage levels.³ This is a significant improvement in the cancer research field, but there are limitations with these models as well. Obtaining fresh human tissue is challenging and chances of successful engraftment, even in a severely immunocompromised mouse, is approximately 30%.³ If engraftment is successful, maintaining these PDX lines as low passage models for future use presents even more complications. The logistics of obtaining and maintaining models, from tissue acquisition to running efficacy studies, along with the overall cost, may be the most significant hindrances to the wide spread use and acceptance of these PDX models.

Clinically, the most commonly utilized endpoints to evaluate the effectiveness of a therapy are an industry standardized set of terms and definitions (RECIST criteria). Paramount to this list is the responsiveness of the disease to treatment; complete response (CR), partial response (PR), and overall increase in survival. CRs are defined as complete regressions of the primary tumor mass. PRs are defined as a partial reduction in the primary tumor by at ~30%. Therapies are considered successful if they are able to induce either CRs or PRs which can lead to a positive impact on survival. However, in the preclinical setting the commonly used endpoints are tumor growth inhibition and tumor growth delay; both are defined as a slowing of disease progression. Unfortunately, tumor growth inhibition does not directly correlate to an overall increase in survival. This is a critical difference; the evaluation of preclinical efficacy data within the research community is less rigorous and held to a different set of standards than its clinical counterparts. The lower standards allow for more drugs to get through to clinical trials which results in a greater number of clinical failures. This idea of aligning our preclinical and clinical standards is not novel. In an editorial published in the Journal of the National Cancer Institute, the authors called for a consensus among drug developers that unequivocally define what successful preclinical endpoints are.⁴ If we can collectively raise the bar to more stringent preclinical criteria for the evaluation of novel cancer treatments, we could possibly reduce the failure rate in the clinic. Downstream of this, it would then boost the confidence of clinicians and patients when they are reviewing preclinical data.

Now we all know that change is hard and expecting anything to happen overnight is naïve, but small changes in perceptions and practices now could have significant impact later. The preclinical models that we have all invested years and years in developing are effective if we use them correctly. This starts with concise protocol design and ends with consistent data evaluations. In clinical trials new drugs will be facing patients with established disease. The research community can use this assumption to design more rigorous preclinical experiments. In the clinic, tumors are well established within the origin tissue with a vascular bed in place to ensure survival at the time treatment is initiated. We can mimic this environment preclinically with either subcutaneous or orthotopic implants by just allowing the tumor to grow and become more established. The time that it takes for tumors to become established is highly dependent on the tumor line and the implant location. Subcutaneous tumors can be monitored easily with standard calipers to ensure progressive growth. Orthotopic models are a bit trickier to ensure that the tumor is actively growing unless you have the ability to image the tumors over time. This relatively straightforward step would save time and money that is wasted on false positive results generated from studies designed to treat tumors that are not at all or only barely established. At the time of final data analysis scientist and drug developers need to also alter how they define an active new drug. Using the clinical standards as a guideline for activity would decrease the number of new drugs being pushed into the clinic that inevitably fail. This starts with a shift away from tumor growth inhibition endpoints to the clinically translatable endpoints CRs, PRs, and overall all increase in survival.

This is no small request; trying to convince an entire field of scientists and drug developers to look at their preclinical efficacy data more diligently and hold it to a more robust set of standards will clearly impact the perceived success rate. However, if more effort is invested into optimizing these new drugs preclinically rather than pushing them into the clinic too early their chances of real success would increase. All of this, in the long run would save time, money, and potential lives.

¹Thomas TW, Burns J, Audette J, Corrol A, Dow-Hygelund C, Hay M. Clinical development success rate 2006-2015.  BIO Industry Analysis June 2016.

²Kamb A. What’s wrong with our caner models? Nature Reviews February 2005; Vol. 4.

³Ruggeri BA, Camp F, Miknyoczki S. Animal models of disease: Pre-clinical animal models of cancer and their applications and utility in drug discovery. Biochemical Pharmacology 2014; Vol. 87, 150-161.

⁴Berttotti A, Trusolino L. From bench to bedside: dose preclinical practices in translational oncology need some rebuilding? JNCI 2013; Vol. 105, Issue 19.