Of Cells, Proteins and Cancer Drug Development

Our recent presentation at the American Association for Cancer Research meeting reported our work with a novel class of compounds known as the HSP90 inhibitors. AACR 2015-HSP90 Abstract

The field began decades earlier when it was found that certain proteins in cells were required to protect the function of other newly formed proteins hormone receptors and signaling molecules. Estrogen and androgen receptors, among others, require careful attention following their manufacture or they will find themselves in the cellular waste bin.

230px-Geldanamycin.svgAs each new protein is formed it risks digestion at the hands of a garbage disposal-like device known as a proteasome (named for its protein digesting capabilities). To the rescue comes HSP90 that chaperones these newly created proteins through the cell and protects them until they can assume their important roles in cell function and survival.

Recognizing that these proteins were critical for cell viability, investigators at Sloan-Kettering and others developed a number of molecules to block HSP90. The original compounds known as ansamycins underwent clinical trials with evidence of activity in some breast cancers. The next generation of compounds was tested in other diseases. Though the clinical results have been mixed, the concept remains attractive.

We compared two drugs of this type and showed that they shared similar function but had different chemical properties and that the concentrations required to kill cells differed. What is interesting is the activity of these drugs seems to be patient-specific. That is, each patient, whether they had breast or lung cancer, showed a unique profile that was not directly connected to the type of cancer they had. This has important implications.

Today, pharmaceutical companies develop drugs by disease type. Compounds enter Phase II trials with 30 to 50 lung cancer patients treated, then 30 to 50 breast cancer patients treated and so on. This continues until (it is hoped) one of the diseases provides a favorable profile and the data is submitted to the FDA for a disease-specific approval. As home runs are rare, most drugs never see the light of day failing to provide sufficient response in any disease to warrant the enormous expense of bringing them to market.

What we found with the HSP90 inhibitors is that some breast cancers are extremely sensitive while others are not. Similarly some lung cancers are extremely sensitive while others are resistant. This forces us once again to confront the fact that cancer patients are unique.

Pharmaceutical companies exploring the role of targeted agents like the HSP90 inhibitors must learn to incorporate patient individuality into the drug development process. Failing to do so not only risks the loss of billions of dollars but more importantly denies patients access to active novel agents.

The future of drug development can be bright if the pharmaceutical industry embraces the concept that each patient’s profile of response is unique and that these responses reflect patient-specific, not diagnosis-based drivers. Clinical trials must incorporate individual patient profiles. Drugs could be made more available once Phase I studies were complete by using biomarkers for response, such as the EVA-PCD assay, which has the capacity to enhance access and streamline drug development.

Future (Cancer) Shock

Two related clinical trials were reported in the last several months describing the use of heat shock protein 90 (HSP90) inhibitors in lung cancer. Both trials fell short of their pre-specified endpoints casting a pall upon these drugs. However, the study of HSP90 inhibitors should not be abandoned based on these finding, as this is a fertile area of investigation and offers opportunities for the future.

Human cells marshal many defenses against stress. Thermal injury can damage basic cellular functions by denaturing (inactivating) proteins. The machinery of cells is largely comprised of protein enzymes. Excessive heat coagulates proteins much the way the albumin of an egg turns white during cooking. The loss of fluidity and function ultimately results in cell death. The heat shock proteins come to the rescue by shepherding these proteins away from injury and protecting them from denaturation.

220px-Hsp90There are many different heat shock proteins found in human cells, but one of the most abundant and active in cancer cells is known as HSP90 for its molecular weight in the range of 90-kilodaltons. Over the last two decades investigators have explored the use of small molecules to inhibit these important proteins. Among the first compounds to be isolated and applied were derivatives of geldenamycin. Although geldenamycin itself is a poison that causes severe liver damage, its derivative 17-AAG, also known as tanespimycin, has successfully entered clinical trials.

The current studies examined two other HSP90 inhibitors. One retaspimycin, has been developed by Infinity Pharmaceuticals. This clinical trial combined retaspimycin with docetaxel and compared results with docetaxel alone in 226 patients with recurrent lung cancer. None of the patients had received docetaxel prior to the trial. Drugs were administered every three weeks and the efficacy endpoint was survival with a subset analysis focused on those with squamous cell cancer. The trial fell short of its pre-designated endpoint. Interestingly, the study failed to provide benefit even in patients who were specifically targeted by their tumor’s expression of the K-Ras, p53 or by elevated blood levels of HSP90, the putative biomarkers for response.

The second trial examined a different HSP90 inhibitor developed by Synta Pharmaceuticals. The drug ganetespib was combined with docetaxel and the combination was compared with docetaxel alone. The results just reported indicate that the combination provided a median survival of 10.7 months, while docetaxel alone provided a median survival of 7.4 month. Although this represented a three month improvement, it did not meet the pre-specified target.

Taken together, these results could dampen enthusiasm for these agents. This would be unfortunate, for this class of drugs is active in a number of human tumors. We observed favorable activity and synergy for the HSP90 inhibitor geldenamycin and its derivative 17-AAG as we reported (Nagourney RA et al Proc. AACR, 2005). More importantly, 17-AAG (tanespimycin) provided objective responses in 22% and clinical benefit in 59% of patients with recurrent HER2 positive breast cancer after these patients had failed therapy with Herceptin. This clearly supports the role of HSP90 inhibition in breast cancer and would suggest that other more carefully selected target diseases could benefit as well.

The function of HSP90 is not completely understood as it influences the intracellular trafficking of dozens ofHsp90cycle proteins. One of the complexities of this class of drugs is that they protect and enhance the function of both good and bad proteins. After all, the HSP90 protein doesn’t know which proteins we, as cancer doctors, would like it to protect.

When we apply the EVA-PCD analysis to these and related classes of compounds we focus our attention upon the downstream effects, namely the loss of cell survival. That is, whatever proteins are influenced, the important question remains “did that effect cause the cells to die?” Classes of compounds with nonspecific targets like the HSP90 inhibitors will surely be the most difficult to characterize at a genomic or proteomic level: What protein? What gene?

Functional platforms like the EVA-PCD offer unique opportunities to study these classes of agents. We are convinced that the HSP90 inhibitors have a role in cancer therapy. It would be unfortunate if these setbacks led us to “throw the baby out with the (hot) bathwater,” thus slowing or preventing their use in cancer treatment.

Future Cancer Shock: Two Lung Cancer Trials Fall Short of Goal

Hsp90 pathwayTwo related clinical trials were reported in the last several months describing the use of heat shock protein 90 (HSP90) inhibitors in lung cancer. Both trials fell short of their pre-specified endpoints casting a pall upon these drugs. However, the study of HSP90 inhibitors should not be abandoned based on these finding, as this is a fertile area of investigation and offers opportunities for the future.

Human cells marshal many defenses against stress. Thermal injury can damage basic cellular functions by denaturing (inactivating) proteins. The machinery of cells is largely comprised of protein enzymes. Excessive heat coagulates proteins much the same way the albumin of an egg turns white during cooking. The loss of fluidity and function ultimately results in cell death. The heat shock proteins come to the rescue by shepherding these proteins away from injury and protecting them from denaturation. There are many different heat shock proteins found in human cells, but one of the most abundant and active in cancer cells is known as HSP90 for its molecular weight in the range of 90-kilodaltons. Over the last two decades, investigators have explored the use of small molecules to inhibit these important proteins. Among the first compounds to be isolated and applied were derivatives of Geldanamycin. Although Geldanamycin itself is a poison that causes severe liver damage, its derivative 17-AAG, also known as Tanespimycin, has successfully entered clinical trials.

The current studies examined two other HSP90 inhibitors. One Retaspimycin, has been developed by the Infinity Pharmaceuticals. This clinical trial combined Retaspimycin with Docetaxel and compared results with Docetaxel alone in 226 patients with recurrent lung cancer. None of the patients had received Docetaxel prior to the trial. Drugs were administered every three weeks and the efficacy endpoint was survival with a subset analysis focused upon those with squamous cell cancer. The trial fell short of its pre-designated endpoint. Interestingly, the study failed to provide benefit even in patients who were specifically targeted by their tumor’s expression of the K-RAS, p53 or by elevated blood levels of HSP90, the putative biomarkers for response.

The second trial examined a different HSP90 inhibitor developed by Synta Pharmaceuticals. The drug Ganetespib was combined with Docetaxel and the combination was compared with Docetaxel alone. The results just reported indicate that the combination provided a median survival of 10.7 month, while Docetaxel alone provided a median survival of 7.4 month. Although this represented a three-month improvement, it did not meet the pre-specified target.

Taken together these results could dampen enthusiasm for these agents. This would be unfortunate, for this class of drugs is active in a number of human tumors.

Through our EVA-PCD functional profile we have observed favorable activity and synergy for the HSP90 inhibitor Geldanamycin and its derivative 17-AAG as we reported at the American Association for Cancer Research meeting in 2005 (Nagourney RA et al Proc. AACR, 2005). More importantly, 17-AAG (Tanespimycin) provided objective responses in 22 percent and clinical benefit in 59 percent of patients with recurrent HER2 positive breast cancer after these patients had failed therapy with Herceptin (Modi S. et al, Clinical Cancer Research August 2011). This clearly supports the role of HSP90 inhibition in breast cancer and would suggest that other more carefully selected target diseases could benefit as well.

The function of HSP90 is not completely understood as it influences the intracellular trafficking of dozens of proteins. One of the complexities of this class of drugs is that they protect and enhance the function of both good and bad proteins. After all, the HSP90 protein doesn’t know which proteins we as cancer doctors would like it to protect.

When we apply EVA-PCD analysis to these and other related classes of compounds, we focus our attention upon the downstream effects, namely the loss of cell survival. That is, whatever proteins are influenced, the important question remains “did that effect cause the cells to die?”

Classes of compounds with nonspecific targets like the HSP90 inhibitors will surely be the most difficult to characterize at a genomic or proteomic level: What protein? What gene? Functional platforms like the EVA-PCD offer unique opportunities to study these classes of agents. We are convinced that the HSP90 inhibitors have a role in cancer therapy. It would be unfortunate if these setbacks led us to “throw the baby out with the (hot) bathwater,” thus, slowing or preventing their use in cancer treatment.