Balancing long-term risks vs benefits
Long-term data have been accumulating for these novel therapies since Olson’s treatment in 2010. This is particularly important for CAR T-cell therapy, because of its longevity. Because these are living cells and are expected to persist in the body for years, there is great interest in longer-term data, especially the risks for toxicity.
The FDA requires clinical follow-up for at least 15 years for patients treated with CAR T-cell therapy or any other genetically modified cells.
So far, most of the experience with CAR T cells comes from anti-CD19-directed therapy, which has shown “remarkable” remission rates in the 50% to 85% range, said Nirali Shah, MD, head of the hematologic malignancies section of the Pediatric Oncology Branch at the National Cancer Institute (NCI).
The most recent results presented at this year’s annual meeting of the American Society of Clinical Oncology support earlier efficacy data, she noted. In the longest follow-up to date, researchers reported remissions lasting over 9 years in patients with relapsed/refractory B-cell lymphoma or CLL treated with Kite›s axicaptagene cilleucel (Yescarta), one of two anti-CD19-directed CAR T-cell therapies approved by the FDA in 2017 (the other is Novartis’ tisagenlecleucel [Kymriah]).
This study included 43 patients and showed an overall remission rate of 76%. Complete remission was achieved in 54% of patients, and 22% had partial remission.
The other focus is long-term safety. Although some of the long-term adverse effects are known and are manageable, others fall into the theoretical realm. In early May 2020, the NCI held a multidisciplinary virtual conference on CAR T-cell therapy «to encourage collaborative research about the subacute and potentially long-term toxicity profile of these treatments.»
“We know just a little at this point about late- and long-term effects of CAR-T therapy, because we are relatively early in the era of CAR T cells,” said Merav Bar, MD, from the Fred Hutchinson Cancer Research Center in Seattle, Washington.
B-cell aplasia and risk for new infections
What is known is that B-cell aplasia represents the most common long-term adverse effect of CAR T-cell therapy. B-cell aplasia results when anti-CD19 CAR-T therapy wipes out healthy B cells as well as the malignant ones responsible for leukemia/lymphoma.
As major players in the immune system, B cells are a key defense against viruses. So B-cell aplasia represents a very specific type of immunosuppression. It is generally less severe than immunosuppression that occurs after organ transplant, which hits the immune system pretty much across the board and carries a much higher risk for infection.
The main concern is what happens when someone with B-cell aplasia encounters a new pathogen, such as SARS-CoV-2.
After infection, B cells generate memory cells, which are not killed off by anti-CD19 therapy and that stick around for life. So a patient such as Olson would still make antibodies that fight infections they experienced before receiving CAR-T therapy, such as childhood chickenpox. But now they are unable to make new memory cells, so these patients receive monthly immunoglobulin infusions to protect against pathogens they have not previously encountered.
Olson takes this in stride and says he isn’t overly worried about COVID-19. He follows the recommended precautions for a man his age. He wears a mask, washes his hands frequently, and tries to maintain social distancing. But he doesn’t stay locked up in his New Hampshire home.
“I took the attitude when I was diagnosed with cancer that I’m going to live my life,” he said. “Quality of life to me is more important than quantity.”