All tumors gone – a cancer therapy delivers a welcome surprise
Cancer therapy Colorectal Cancer Checkpoint Inhibitor

All tumors gone – a cancer therapy delivers a welcome surprise

Dr. Talia Henkle
Dr. Talia Henkle

In 10 seconds? A cancer-fighting drug called an immune checkpoint inhibitor (specifically, anti-PD-1) was found to successfully treat 100% of rectal cancer patients in a study of 12 patients without the need for any other cancer treatments.

Yeah, I saw this in the news. Are the results as good as they say? Well, they are certainly exciting and showcase the improved scientific understanding of which patients will respond successfully to certain cancer treatments. (More on this below) But, although the results are impressive, there are a few things to keep in mind when interpreting them. First of all, the drug used in the study, dostarlimab, is an anti-PD1 antibody that exists as a cancer treatment. And while its formulation is new (FDA approved in 2021), other very similar anti-PD-1 antibodies have been successfully used to treat cancer since 2014. So, the treatment type is not new, per se. Second, this was a small study of only 12 patients who were carefully selected for treatment based on tumor-specific genetic factors. Overall this study, exemplifies how researchers can now select patients they ‘expect’ to successfully respond to treatment.

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Stop for a sec. Remind me what “anti-PD-1” treatments do? Of course! (And if you’re extra curious you should check out our previous digests on the topic). In short, anti-PD-1 therapies can re-activate immune cells called T cells, so that they can resume their attack on the cancer (which had 'turned off' the T cells in the first place). Immune checkpoint therapies usually work better when tumors have a lot of mutations. To that point, dostarlimab was FDA-approved to treat tumors that develop high numbers of mutations due to genetic defects in their cell’s DNA repair process (AKA mismatch repair deficiency).

OK. So, what made this new study so great? Immune checkpoint inhibitors have revolutionized cancer treatment, but usually, only a percentage of patients benefit. And since these treatments are expensive and frequently accompanied by nasty side effects, cancer researchers have been actively investigating the defining factors to pinpoint which patients are most likely to benefit. And this study is proof that they are making serious progress!

And what exactly did they do in this study? They treated 12 patients with stage 2 or 3 mismatch repair-deficient rectal cancer (AKA tumors that form at the end of the colon that haven't spread and have lots of mutations). Another condition was that the patients did not receive any prior cancer treatments. This is notable because many clinical trials aim to treat patients with more advanced cancers who have no other treatment options left and whose cancers are much harder to treat. Patients with rectal cancers like these are usually treated with a combination of chemotherapy and surgery, which are both associated with a lot of uncomfortable side effects (Think: bowel, urinary, and sexual dysfunction). The novelty of this study is that they first treated the patients with anti-PD1, every 3 weeks for six months (a longer treatment regimen than is usually done). Then they planned to have the patients continue with chemotherapy and surgery, but that wasn’t necessary.

Why? What happened? Amazingly, ALL of the 12 patients’ tumors went away after the anti-PD1 treatment and have stayed away (although doctors will continue to follow up with the patients to conclude if these results will endure)! That means none of the patients had to undergo chemotherapy or surgery. What’s more is that all of the patients tolerated the treatment quite well, having no serious side effects. This is fantastic news as this means that cancer researchers have revealed a safer and more effective treatment strategy for certain types of rectal cancers!

Complete regression of a rectal tumor in a patient after receiving 6-months of anti-PD-1 antibody treatment. Source: Cercek A, et al. N Eng J Med. 2022

So, colorectal cancer is cured now? Hold up and don’t put that champagne on ice because we’re not quite there yet. Only about 5-10% of all rectal cancers have these genetic features (DNA mismatch repair deficiencies) that were necessary for inclusion in this study. And, similar anti-PD-1 treatments among patients with metastatic mismatch-repair deficient rectal cancer (a more advanced stage of cancer than those included in this study) only resulted in 11.1% of patients’ tumors going away. That means cancer researchers still have a lot of work to do before these cancers are gone for good (especially since rates of early-onset colorectal cancer are increasing!).

How many anti-PD-1 treatments are FDA-approved?

So far there are at least 7 different versions of anti-PD-1 antibodies used for cancer treatments. They all have the same putative mechanism of action but differ slightly in their formulation (e.g. the exact place where the antibody binds to the PD-1 protein).

It’s hard to discern whether one of them is 'better' than another since pharmaceutical companies typically do not perform head-to-head comparison trials (lest their drug doesn’t ‘win’). But with more comparison trials combined and more options, time, and availability, hopefully, cancer researchers can begin to uncover more about which anti-PD-1 antibodies are best for which patients (and cancers)… or at the very least, maybe market competition can drive these different anti-PD1 treatments to become more affordable.

Dr. Talia Henkle has distilled 2 research papers, saving your 7 hours of reading time

The Science Integrity Check of this 3-min Science Digest was performed by Dr. Jacquelyn Bedsaul.

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