As I mentioned in my last blog,
I was recently invited to participate in another meeting with the FDA at the
Brookings Institution. At the last
meeting (summarized in a previous
blog) Janet Woodcock made the extraordinary admission that the FDA would
have to reboot in order to get antibiotic development out of the cellar in the
US.
In this installment, I would like to explore a feasible way
forward for hospital acquired and ventilator associated pneumonia. The FDA guidance for these indications
currently suggests designs where the endpoint is mortality and where patients
are precluded from receiving prior antibiotics. These designs
have been discussed several times in this blog and are completely infeasible
and in many ways not at all real world (over 80% of ICU patients receive
antibiotics for e.g.).
Ventilator-associated pneumonia is a particularly
problematic indication because of great controversy around diagnostic accuracy
and the fact that the disease incidence is shrinking making these patients
difficult to find for enrollment in clinical trials. Adding to the difficulty are US rules suggesting that
nosocomial infections might not be reimbursed by Medicare that, in turn,
provide a disincentive to actually make an official diagnosis in the chart of
hospital-acquired pneumonia.
Many studies suggest that antibiotics for this indication
have a treatment effect of 40-60%. When looking at mortality, but more
importantly for our purposes, even when looking at clinical outcome via
pharmcometrics these numbers hold up. In spite of this, the FDA has
proposed a 10% non-inferiority margin within the microbiologically documented
population and the EMA proposes a 12.5% margin. The patient numbers required by
these margins, even when looking at clinical outcome as an endpoint, may be
difficult to achieve today. But the treatment effect numbers suggest that in a
clinical trial setting, we can think about non-inferiority margins of 15-20%
that still retain at least 50% of the treatment effect. This range of margin
would almost certainly bring these trials within the range of feasible patient
numbers.
So here is a proposed design with trial numbers that might
be required (note that for a combined HAP/VAP trial at least 30% of patients
would be required to have VAP).
Patient population.
Patients would be allowed up to 24 hours of prior antibiotic within 72 hours of enrollment in the trail (as per EMA). Patients who had failed prior therapy (predefined in protocol) would be allowed to enroll regardless of the time course of the prior failing antibiotic.
Endpoint - clinical response at test of cure.
Endpoint - clinical response at test of cure.
Analysis population – ITT and modified ITT (per protocol).
Non-inferiority margin – 17%. For a trial with a 60% treatment success rate, 90% power and 70% evaluability rate, 500 patients
would be required per trial or 1000 for two trials.
Option for a single trial – using support from a previous
trial in severely ill (PORT III-IV) trial in community acquired pneumonia, a
single trial in HAP/VAP would suffice for approval. For a 17% NI margin as
noted above but at 80% power, the trial population would be 750 patients total.
This proposal harmonizes, to a large extent, the FDA and EMA
proposed addendum. I have
increased the non-inferiority margin above that suggested by EMA to account for
both the very large treatment effect of antibiotics for this indication
including that seen for an endpoint of clinical outcome, and to allow for increased
feasibility at a time when the disease incidence and therefore patient
availability are decreasing.
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