Platinum Resistance is in the Eye of the Beholder

I was recently apprised of an online conversation surrounding the treatment of platinum refractory and platinum resistant ovarian cancer. To clarify our terminology, platinum refractory disease refers to cancer that progresses during platinum therapy. This would be considered the most platinum resistant of the ovarian patients. The term “platinum resistant” developed over the last two decades, by Markman and others, is used to describe patients who initially respond to platinum-based chemotherapy and then relapse within six months of treatment.

While platinum refractory seems intuitively obvious, it has been suggested that platinum resistance is somewhat more arbitrary.  That is, what if one relapses one month versus five months, or seven months after treatment. In fact, studies conducted by investigators at Memorial Sloane-Kettering under Dr. David Spriggs, suggest that platinum resistance is a continuum extending from six months continuing out to 24 months and beyond. The longer the “platinum-free interval” the better the chance of response to combinations like carboplatin plus Taxol. Within the scope of this discussion I am in general agreement. However, as I describe below, this is, by far, not the whole story.

I am composing this particular blog in response to a comment that I encountered in a recent chat room discussion. The individual took an extremely strong stance stipulating that no medical oncologist should re-challenge a patient with a platinum-based regimen if they fall within the category of platinum refractory or platinum resistant. This statement is absolutely, positively WRONG.

Platinum resistance is mediated by DNA repair enzymes. These enzymes recognize and respond to platinum adducts and excise the DNA residues, replacing them with the appropriate base pairs. While this confers resistance to single agent platins, a degree of resistance which is largely is unaffected by the addition of taxanes, platinum resistance actually opens up an Achilles heel for treatment of these patients. Drugs like the antimetabolites (Gemcitabine, 5-FU), as well as the topoisomerase inhibitors become collaterally more active in those tumors with the most active DNA repair capacities. This is the reason why we have consistently observed responses in both platinum resistant and platinum refractory patients utilizing the combination of cisplatin and gemcitabine, as we reported in the original paper describing this combination in 2003 (Nagourney, R et al, Gyn Onc, 2003). Our response rate of 50 percent in heavily pre-treated and platinum resistant patients was confirmed by investigators in Ohio who reported similarly good results in patients with p-glycoprotein positive/platinum resistant disease (Rose, P, Gyn Onc 2003).  To formally test this hypothesis we conducted a national clinical trial with the GOG, which treated platinum resistant and platinum refractory patients with the combination of cisplatin plus gemcitabine. This trial provided the longest-time-to-progression for this population (six months) in the history of the GOG (Brewer et al, Gyn Onc 2006). These observations were subsequently reported in our textbook (Deoxynucleoside Analogs in Cancer Therapy, GPeters [ed] Humana Press 2006).

Similar results have been reported for Folfox in recurrent ovarian patients by Greek investigators (Pectasides, D et al, Gyn Onc 2004). To examine this phenomenon, one of the great investigators of antimetabolite chemistry, William Plunkett, conducted an instructive series of experiments in which they showed that platinum resistant ovarian cell lines expressed high levels of the DNA repair enzyme ERCC1. When these investigators blocked the ERCC1 expression with siRNA, the cell lines became resistant to the cisplatin plus gemcitabine combination, indicating beyond a shadow of a doubt, that it is the cells’ own DNA repair capacity that makes it sensitive to this drug doublet.

I write this blog because it is critically important for patients and doctors alike, to understand the chemistry of these agents and their interactions. While platinum resistance may indeed confer clinical resistance to platinum, carboplatin plus Taxol and related combinations, platinum resistant tumors may actually be more sensitive to intelligently administered drug combinations. Using our laboratory platform to measure the chemosensitivity and synergy for drug combinations we have identified numerous platinum resistant and platinum refractory patients who have had dramatic and durable response to re-challenge with platinum based therapies that employ these synergistic combinations. This is why we are extremely interested to study platinum resistant patients. After all, platinum resistance is in the eye of the beholder.

The Avastin Saga Continues

We previously wrote about bevacizumab (Avastin) and its approval for breast cancer. The early clinical trials revealed evidence of improved time to disease progression. This surrogate measure for survival benefit had, over recent years, gained popularity, as time to disease progression is a measure of the impact of a given treatment upon the patient’s response durability. It was hoped and believed that time to progression would be an early measure of survival.

Unfortunately, the survival advantage for the Avastin-based therapies in breast cancer has not met statistical significance. As such, careful review by the oncology drug committee of the FDA lead to a unanimous decision to remove Avastin’s indication in breast cancer. Avastin has not been removed from the market, but instead, cannot be promoted or advertised, nor do insurers necessarily reimburse it. This decision, however, will have a very big impact on Medicare patients and many others who are in managed care programs (HMOs).

There are no villains here. Instead, dedicated physicians empowered to scrutinize the best data could not prove beyond any doubt that the drug improved survival. The time to progression data was favorable and the survival data also trended in a favorable direction. But, the final arbiter of clinical approval — statistically significant survival — was not met.

The physicians who want to provide this for the patients, the company that produces the drug and the patients who believe it offers benefit all have legitimate positions. As Jerome Groopman, MD, once said, in a similar situation with regard to the FDA approval of interleukin 2 (a biological agent with profound activity in a small minority of melanoma and renal cell cancer patients), “I am confronted with a dilemma of biblical proportions, how to help the few at the expense of the many.”

The Avastin saga is but one example of what will occur repeatedly. The one-size-fits-all paradigm is crumbling as individual patients with unique biological features confront the results of the blunt instrument of randomized clinical trials. Our laboratory has been deeply involved in these stories for 20 years. When we first observed synergy for purine analogs (2CDA and fludarabine) with cytoxan, and then recommended and used this doublet in advanced hematologic malignancies (highly successfully, we might add) we were a lone voice in the woods. Eventually, clinical trials conducted at M.D. Anderson and other centers confirmed the activity establishing these treatments as the standards of care for CLL and low-grade lymphoma.

The exact same experience occurred in our solid tumor work when we combined cisplatin plus gemcitabine in pancreatic, ovarian, breast, bladder, lung and other cancers. While our first patient (presumably the first patient in the world) received cisplatin plus gemcitabine for drug-resistant recurrent ovarian cancer in 1995 — providing her an additional five years of life — it wasn’t until 2006 that the FDA approved the closely related carboplatin plus gemcitabine for this indication.

We now confront an even greater hurdle. With our discoveries, using novel combinations of targeted agents, we are years (perhaps decades) ahead of the clinical trial process. We know that patients evaluated in our laboratory with favorable profiles can respond to some of the newest drugs, many of which have already completed Phase I of clinical trials. It is our fervent belief that we could accelerate the drug development process if we could join with the pharmaceutical companies and the FDA to put these hypotheses to a formal test.

Again, there are no villains here. Patients want, and should, receive active drugs. Doctors should be allowed to give them. The drug companies want to sell their agents and the FDA wants to see good therapies go forward.

The rancor that surrounds these emotionally charged issues will best be resolved when we introduce techniques that match patients to active therapies. We believe that the primary culture platform used in our laboratory, and a small number of dedicated investigators like us, may be the answer to this dilemma.

We will redouble our efforts to apply these methods for our patients and encourage our patients to lobby their health care insurers and representatives to sponsor these approaches. To date, we have been unsuccessful in convincing any cooperative group to test the predictive ability of these selection methodologies. In response, I reiterate that I will gladly participate and, to the best of my ability, support at least the laboratory component of any fair test of our primary culture methodologies.

We stand at the ready for the challenge.

The Role of the Platinum Derivatives in Cancer Therapy

The discovery of cisplatinum and the subsequent development of its derivatives (carboplatin and oxaliplatin) represent an interesting saga in modern oncology. When Rosenberg observed in 1960s that platinum electrodes in salt water baths inhibited the growth of bacteria and fungi it lead to the isolation of cis-dichloro diamine platinum (cisplatin). Its application in testicular cancer provided a dramatic leap forward for this heretofore-lethal disease. Subsequent applications in ovary and lung cancers lead to some of the most effective therapies in modern oncology. Although the exact mechanisms of action continue to be investigated, the platination of guanine residues in DNA constitutes the principle mechanism of cytotoxicity.

The use of the human tumor laboratory model has provided us the luxury of exploring the platinum drugs in a wide variety of diseases. Among our published discoveries has been the relative equivalence of the platinum derivatives, as well as their profound synergy with agents like gemcitabine. It is of significant interest that this broadly effective class of compounds — extensively applied in the treatment of lung, colorectal, ovarian and breast cancers, as well as others — remains less active in the hematologic neoplasms. This is in striking counter distinction to nearly all other classes of chemotherapeutics.

Among our most gratifying observations, from the early 1990s, was the clear and profound activity of the platinum derivatives in breast cancers. We feel that our discoveries, outlined in an editorial published in 2000 (The Once and Future Role of Platinum Agents in Advanced Breast Cancer), in no small part have influenced the broad application of platinum in modern breast cancer management.

It was not genius or divine intervention that lead us to these important discoveries, but, quite simply, the use of a validated human tumor model that accurately probed tumor types, leading us to these findings. It is virtually impossible for an unbiased observer to review these contributions and not recognize that the human tumor model has been the conduit by which these discoveries were made.

The proper study of human cancer is human cancer. Our results speak for themselves when it comes to ovarian, breast and hematologic neoplasms, treatments for which can be traced directly to our laboratories.