A New Target in Breast Cancer Therapy

In many ways the era of targeted therapy began with the recognition that breast cancers expressed estrogen receptors, the original work identified the presence of estrogen receptors by radioimmunoassay. Tumors positive for ER tended to be less aggressive and appear to favor bone sites when they metastasized. Subsequently, drugs capable of blocking the effects of estrogen at the estrogen receptor were developed.  Tamoxifen competes with estrogen at the level of the receptor. This drug became a mainstay with ER positive tumors and continues to be used today, decades after it was first synthesized.

Recognizing that some patients develop resistance to Tamoxifen, additional classes of drugs were developed that reduced the circulating levels of estrogen by inhibiting the enzyme aromatase, this enzyme found in adipose tissue, converts steroid precursors to estrogen.  Despite the benefits of these classes of drugs known as SERMS (selective receptor modulators), many patients break through hormonal therapies and require cytotoxic chemotherapy.

With the identification of HER-2 amplification, a new subclass of breast cancers driven by a mutation in the growth factor family provided yet a new avenue of therapy – trastuzumab (Herceptin). For HER-2 positive breast cancers Herceptin has dramatically changed the landscape. Providing synergy with chemotherapy this monoclonal antibody has also been applied in the adjuvant setting offering survival advantage in those patients with the targeted mutation.

Reports from the San Antonio breast symposium held in Texas last December, provide two new findings.

The first is a clinical trial testing the efficacy of pertuzumab. This novel monoclonal antibody functions by preventing dimerization of HER-2 (The target of Herceptin) with the other members of the human epidermal growth factor family HER-1, HER-3 and HER-4. In so doing, the cross talk between receptors is abrogated and downstream signaling in squelched.

The second important finding regards the use of everolimus. This small molecule derivative of rapamycin blocks cellular signaling through the mTOR pathway. Combining everolimus with the aromatase inhibitor exemestane, improved time to progression.

While these two classes of drugs are different, the most interesting aspect of both reports reflects the downstream pathways that they target. Pertuzumab inhibits signaling at the PI3K pathway, upstream from mTOR. Everolimus blocks mTOR itself, thus both drugs are influencing cell signaling that channel through metabolic pathways PI3K is the membrane signal from insulin, while mTOR is an intermediate in the same pathway. Thus, these are in truest sense of the word, breakthroughs in metabolomics.

Targeted Therapies — The Next Chapter

Within this blog, we have intermittently reviewed the concept of targeted therapies. To reiterate, these are classes of drugs that target specific pathways considered tumorigenic. Among the pathways initially targeted were the epidermal growth factor receptor and the closely related HER2. Shortly after the introduction of EGFr and HER2 directed therapies came the development of drugs that target another critical pathway, mTOR.

Hundreds of compounds are now under development intended to more accurately hone in on the pathways of interest in patients’ tumors. Regrettably, the medical community continues to apply old clinical trial methods to this newest era of drugs. While the selective application of drugs like: Tarceva for EGFR mutants, Herceptin for HER2 over-expressers, and Crizotinib for EML4-ALK mutants, are much more effective in patients with these gene expressions, these are a select few examples of linear thinking that bore fruit.

That is, this gene is associated with this disease state and can be treated with this drug.

Many, if not most cancers will prove to be demonstrably more complicated. Genomic trials can only succeed if we first know the gene of interest and second know that its (over) expression alone is pathogenetic for the disease entity. Even meeting these conditions is likely to result in comparatively brief partial responses due to the crosstalk, redundancy and complexity of human tumor signaling pathways — the “targets” of these new drugs.

To address these complexities, functional analytic platforms that examine outcomes, not targets, are needed. This bottom-up approach has now enabled my team to explore the activity of novel compounds. When investigators develop interesting “small molecules,” we examine the disease specificity, combinatorial potential and sequence dependence of these compounds in short-term cultures to provide meaningful insights that can then be addressed on genomic and proteomic platforms. This reduces the time required to take these new agents from bench to bedside. We cannot solve tomorrow’s questions using yesterday’s mindsets