Details on the Failed Phase 3 Trial of the resTORbio mTORC1 Inhibitor
The short version of the story regarding the failure of resTORbio's phase 3 trial of an mTORC1 inhibitor targeting immune function and influenza infection in old people is that the FDA forced a last minute change of the phase 3 endpoint from the phase 2 endpoint of a reduction in clinically confirmed infections to a more nebulous outcome of whether or not people reported feeling better. Which is far from the worst offense that FDA staff have committed in the course of hindering the adoption of new medical technologies, but it is illustrative of the obstacle that regulators pose. We can all speculate as to what was going on under the hood here, and which influences led to this outcome.
To my eyes, the field of mTOR based therapies remains something of a sideshow when it comes to human aging and longevity. The same is true of many of the metabolic manipulation approaches based on upregulation of stress response mechanisms. These mechanisms are known to produce sizable effects in short-lived species, but not in long-lived species such as our own. Thus here, mTORC1 inhibition does not produce a startling and large effect on infection rate and immune function, and nor should we expect it to, but it is cheap and it does produce some effect. mTORC1 inhibition replicates a thin slice of the beneficial calorie restriction response, and we know what calorie restriction can achieve in humans; this sort of approach isn't the path to very large gains.
We did a phase 2b and a phase 3 double-blind, randomised, placebo-controlled trial in adults aged at least 65 years enrolled in New Zealand, Australia, and the USA at 54 sites. In the phase 2b trial, patients were aged 65-85 years, with asthma, type 2 diabetes, chronic obstructive pulmonary disease (COPD), congestive heart failure, were current smokers, or had an emergency room or hospitalisation for a respiratory tract infection (RTI) within the past 12 months. In the phase 3 trial, patients were aged at least 65 years, did not have COPD, and were not current smokers.
In the phase 2b trial, patients were randomly assigned to using a validated automated randomisation system to oral RTB101 5 mg, RTB101 10 mg once daily, or placebo in part 1 and RTB101 10 mg once daily, RTB101 10 mg twice daily, RTB101 10 mg plus everolimus once daily, or matching placebo in part 2. In the phase 3 trial, patients were randomly assigned to RTB101 10mg once daily or matching placebo. The phase 2b primary outcome was the incidence of laboratory-confirmed RTIs during 16 weeks of winter cold and influenza season and the phase 3 primary outcome was the incidence of clinically symptomatic respiratory illness defined as symptoms consistent with an RTI, irrespective of whether an infection was laboratory-confirmed.
The purpose of our trials was to investigate whether targeting ageing biology with mTOR inhibitors could improve immune function and decrease the incidence of RTIs in older adults at doses that were well tolerated. The mTOR inhibitor RTB101 10 mg once daily for 16 weeks was well tolerated in adults aged at least 65 years, increased expression of IFN-stimulated antiviral genes in peripheral blood, and decreased the incidence of laboratory-confirmed RTIs (the phase 2b primary endpoint), but not the incidence of clinically symptomatic respiratory illness defined as respiratory symptoms consistent with an RTI irrespective of whether an infection was laboratory confirmed (the phase 3 primary endpoint).
Several possible explanations exist for the divergent results of the phase 2b and phase 3 trials, including the change in primary endpoint and changes in the way respiratory symptoms were collected between the two trials. In the phase 2b trial, respiratory illness symptoms were collected during twice weekly telephone calls with patients and the primary endpoint required predefined symptomatic criteria to be met as well as laboratory confirmation of an infection. In the phase 3 trial, respiratory illness symptoms were collected in eDiaries that patients filled out each evening and the primary endpoint was based on symptoms alone without requiring laboratory confirmation of an infection. Multiple investigators in the phase 3 trial anecdotally noted that patients reported in their nightly eDiary respiratory illness symptoms such as cough or headache that were part of the prespecified diagnostic criteria for a clinically symptomatic respiratory illness even when the patient and the investigator did not think that the patient had an RTI.
Despite the negative phase 3 results, important lessons were learned from this clinical development programme that is the largest to date targeting ageing biology in humans. First, the results show that it is possible to target mechanisms underlying ageing biology safely with therapies such as mTOR inhibitors in older adults. Second, the results suggest that therapies that target ageing biology in older adults might ameliorate at least some aspects of ageing organ system dysfunction (such as deficient IFN-induced antiviral responses). Further refinement of clinical endpoints and more precise identification of responder patient populations will be important in future trials of therapies that intervene in ageing biology to improve immune function in older adults.