The story behind this compound perhaps explains the considerable skepticism surrounding Immunogen’s platform, and should serve as a demonstration for the risks and uncertainties in drug development. huC242-DM4 is comprised of the humanized antibody huC242 , which binds to CanAg, which is expressed on colorectal, pancreatic and gastric cancer cells. The first ADC based on huC242 was huC242-DM1, which is identical to huC242-DM4 with one difference – the linker.
huC242-DM1 entered 2 phase I clinical trials, in which the vast majority of patients had colorectal cancer. Unfortunately, the compound’s performance was rather weak, leading to some minor responses but no objective responses (50% regression in tumor burden lasting at least 6 weeks). There were two possible explanations for the disappointing activity. First, it is well known that colorectal cancer is relatively resistant to antimicrotubule agents, such as paclitaxel (Taxol®) and maytansine, (which is the drug Immunogen uses for its compounds). Second, the compound had a rather poor stability in the bloodstream, which led to lower drug concentrations shortly after administration. In parallel to the phase I, the company started evaluating ADCs based on the same antibody, but with a different linker – huC242-DM4. These evaluations revealed that the substitution of the linker led to an increase of more than 2-fold in the ADC’s stability in comparison to huC242-DM1.
Subsequently, on October 2004, the company announced that it had decided to replace huC242-DM1 by huC242-DM4. That meant, of course, a new phase I trial must take place, in order to assess the safety of the new compound. The phase I clinical trial was initiated in mid 2005, with results announced in June of 2007. In this trial, the vast majority of patients were also advanced-stage colorectal cancer patients, so the chances didn’t look too good in the first place. Although there were no objectives responses, huC242-DM4 indeed proved to be more stable than the earlier version of this ADC. It was quite clear that huC242-DM4 will not be effective against colorectal cancer, so the company decided to assess the compound’s activity for the treatment of gastric cancer, as approximately half of all gastric cancer tumors express CanAg. The company recently initiated a phase II trial in advanced gastric cancer patients, who have failed front-line chemotherapy. During the second half of the 20th century, the incidence of gastric cancer has dramatically decreased in developed countries, but it is still a leading cause of death, responsible for over 11,000 deaths annually in the US alone. Just like many other types of cancer, gastric cancer is curable in its early stages, however, unfortunately, in many cases it is diagnosed in its advanced stages, where the only option is chemotherapy treatments. The company takes pride in the fact that gastric cancer has been found to be highly sensitive to huC242-DM4 in preclinical studies, but we also know that was the case for several colorectal cancer cell lines. Fortunately, last year, the antimicrotubule agent, docetaxel (Taxotere®) was approved by the FDA for the treatment of gastric cancer in combination with other drugs. Additional clinical trials, including trials published this year, also support the use of Taxotere for the treatment of gastric cancer. This is in striking contrast to colorectal cancer, for which antimicrotubule agents are not approved. In addition, both Taxotere and Taxol demonstrated clinical activity in gastric cancer as single agents. These are very encouraging news, since this time, huC242-DM4 actually stands a chance of showing some sort of clinical response, especially in light of the higher potency of DM4 relatively to Taxotere. Nevertheless, although Taxotere was the first FDA approved drug to demonstrate a survival advantage in gastric cancer in more than a decade, it did not improve survival dramatically. The fact that patients enrolled to the phase II trial had failed at least one chemotherapy treatment makes it even more challenging.
In order to get a reliable assessment of huC242-DM4’s efficacy with minimal allocation of resources, Immunogen will first evaluate only 17 patients, hoping that there will be at least one objective response (50% regression in tumor size lasting at least 6 weeks). Statistically, if no clinical response is observed among these 17 patients, it is likely that the drug won’t be very effective for gastric cancer. That means that interim results from this study may be published in the coming months. In our opinion, this trial is the most important one for Immunogen, as even one partial response can open the door for recruitment of additional patients and possibly for a phase III trial already in 2009. The market expectations regarding this compound are pretty low, which will make any positive indication an extremely positive surprise. Bearing in mind this compound is fully owned by Immunogen, clinical success in the huC242-DM4 front, will lead to substantial appreciation in the stock price.