Explaining Fingolimod’s Fatal Flaw
The unphosphorylated, “inactive” version of fingolimod may be responsible for one of the drug’s most serious downsides: susceptibility to viral infections
Because it’s a pill, fingolimod spares MS patients from shots, but it also boosts the risk of serious viral infections. Now, researchers may have pinpointed the cause of this side effect, showing that the drug hobbles CD8 T cells that counterattack viruses, according to a study published online ahead of print in the Journal of Neuroimmunology (Ntranos et al., 2014).
Approved by the FDA in 2010, fingolimod was the first MS drug that patients could take orally. It can produce dramatic results in relapsing-remitting MS, cutting the frequency of flare-ups and reducing the number of brain lesions (Kappos et al., 2010). But some people paid a steep price for avoiding injections. Two subjects died from viral illnesses during the phase 3 TRANSFORMS trial of the drug, one from a varicella zoster infection and one from herpes simplex encephalitis (Cohen et al., 2010). Patients’ experiences since the drug’s approval bolster the case that fingolimod increases susceptibility to viral infections (Uccelli et al., 2011; Calabresi et al., 2014). “We wanted to see if there’s a mechanism behind why these viral infections are occurring,” immunologist Anne Gocke, Ph.D., senior author of the new study, told MSDF.
One way that fingolimod works in the body is by imprisoning certain memory and naïve T cells in the lymph nodes. Among the incarcerated cells in MS patients, researchers surmise, are self-targeting T lymphocytes, which are unable to reach the central nervous system and induce damage. However, almost all of the fingolimod research has focused on its impact on the CD4 helper T cells that orchestrate immune responses and may be culprits in MS (see “Altered Immunity, Crippled Neurons”). For Gocke and colleagues at the Johns Hopkins School of Medicine in Baltimore, Maryland, the prevalence of viral infections in patients who’ve taken fingolimod suggested that the drug also impairs CD8 cells, which neutralize viruses.
To investigate whether fingolimod targets CD8 cells, the team infected mice with influenza viruses. Rodents that had been dosed with fingolimod were about twice as likely to die from flu as were controls. Compared with the untreated animals, the fingolimod group also perished sooner and carried larger amounts of the virus in their lungs, the researchers revealed.
When the researchers dissected the mice, they found that fewer CD8 T cells reached the lungs in animals that had received fingolimod. The drug detained most of the cells in the lymph nodes, and only a small number of them made it to the front line to combat the flu viruses. CD8 cells in treated mice showed another deficiency: reduced generation of granzyme B, an enzyme that helps CD8 cells purge viruses by killing infected body cells. Thus fingolimod diminishes CD8 cells’ virus-fighting ability in more than one way. “We think it’s a multifaceted problem,” Gocke said. “The cells that do get out [of the lymph nodes] are not functional.”
To delve deeper into fingolimod’s mechanism, Gocke and colleagues added the drug to cultures of mouse spleen cells, which include CD8 cells. The researchers tested two versions of fingolimod. In the body, about 30% of fingolimod molecules pick up a phosphate group, and researchers have assumed that this phosphorylated form of the drug accounted for the effects on T cells. The general view has been that unphosphorylated fingolimod molecules remain inert.
But the team found the opposite. The unphosphorylated form of fingolimod cut CD8 cells’ production of granzyme B and interferon γ, another virus-fighting protein. In contrast, phosphorylated fingolimod had no impact on levels of either molecule. So the “inactive” version of fingolimod could be responsible for one of the drug’s most serious downsides.
Gocke and colleagues may have identified a way to counteract the effect on CD8 cells. The researchers followed up on a study by another team that showed that unphosphorylated fingolimod disrupts a biochemical pathway containing the molecule arachidonic acid. Through a series of connections, the pathway ultimately may trigger cells to fashion granzyme B. So Gocke and colleagues added arachidonic acid to CD8 T cell cultures treated with unphosphorylated fingolimod. Granzyme B and interferon-γ production rebounded. Patients taking fingolimod may be able to protect their CD8 cells by also taking arachidonic acid, Gocke suggested. Although arachidonic acid is not commonly used medically, studies have found that it’s safe for humans, the researchers noted.
By detaining and disarming CD8 cells that would normally pounce on the invaders, fingolimod leaves patients vulnerable to viral infections, the team’s findings suggest. But impeding CD8 cells might provide some benefits, Gocke noted, since many researchers now think that these cells also help drive the pathology of MS (see “Altered Immunity, Crippled Neurons”). “It’s definitely a fine balance,” she said.
Researchers knew that overall blood lymphocyte levels plummet in patients who are taking fingolimod, neurologist Jeffrey Cohen, M.D., of the Cleveland Clinic Mellen Center for Multiple Sclerosis in Ohio told MSDF. But the people who show the largest declines aren’t necessarily the most likely to develop infections, said Cohen, who wasn’t connected to the research. “The paper suggests that effects on one lymphocyte subset may account for some of that discrepancy.”
For researchers who are trying to make fingolimod safer, the work points toward CD8 T cells. “This gives us an avenue to pursue to improve on the benefits and lessen the adverse effects,” Cohen said. For example, he said, more rational dosing could be one payoff of the findings. Patients now get a standard dose of the drug. But a fall in the patient’s CD8 cell numbers may indicate that the dose needs to be adjusted. “Having a better handle on which effects on which cells are important might be helpful” in determining what dose to give, he said.
Key open questions
- How similar are the doses of fingolimod in the mouse and culture experiments to those in patients?
- What effect, if any, does fingolimod have on CD8 cells in MS patients?
- What is the impact of arachidonic acid on CD8 cells in animal models and humans?
The new study was funded by a National Institutes of Health grant awarded to Dr. Calabresi to study effector memory T cells in MS. The authors report no financial conflicts of interest. Dr. Cohen reports personal compensation for consulting from EMD Serono, Genentech, Genzyme, Innate Immunotherapeutics, Novartis, and Vaccinex. Dr. Cohen received research support paid to his institution from Biogen Idec, Consortium of MS Centers, Department of Defense, Genzyme, National Institutes of Health, National MS Society, Novartis, Receptos, Synthon, Teva, and Vaccinex.