Parametric Resonance May Explain Virologic Failure to HIV Treatment   Interruptions
  Pilot studies of structured treatment interruptions (STI) in HIV therapy have shown that patients can maintain low viral loads whilst benefiting from reduced treatment toxicity. However, a recent STI clinical trial reported a high degree of virologic failure. Here we present a novel hypothesis that could explain virologic failure to STI and provides new insights of great clinical relevance. We analyze a classic mathematical model of HIV within-host viral dynamics and find that nonlinear parametric resonance occurs when STI are added to the model; resonance is observed as virologic failure. We use the model to simulate clinical trial data and to calculate patient-specific resonant spectra. We gain two important insights. Firstly, within an STI trial, we determine that patients who begin with similar viral loads can be expected to show extremely different virologic responses as a result of resonance. Thus, high heterogeneity of patient response within a STI clinical trial is to be expected. Secondly and more importantly, we determine that virologic failure is not simply due to STI or patient characteristics; rather it is the result of a complex dynamic interaction between STI and patient viral dynamics. Hence, our analyses demonstrate that no universal regimen with periodic interruptions will be effective for all patients. On the basis of our results, we suggest that immunologic and virologic parameters should be used to design patient-specific STI regimens. 
Romulus Breban and Sally Blower