To address the identified limitations presented in Table 2, CADTH assumed a 10-year treatment duration for voretigene neparvovec, used data from crossover patients in Study 301 to inform short-term transition probabilities, and updated the sponsor’s utility estimates with the values provided by clinical experts consulted by CADTH. The CADTH reanalysis of the sponsor’s economic model estimated that the incremental cost-effectiveness ratio (ICER) for voretigene neparvovec compared with best supportive care (BSC) was $200,477 per quality-adjusted life-year (QALY) gained. To achieve an ICER of $50,000 per QALY compared with BSC, the price of voretigene neparvovec would need to be reduced by more than 74%.
The submitted price of voretigene neparvovec is a key driver of overall costs and of the ICER within the model. While the cost of voretigene neparvovec is known and is incurred at the beginning of the model time horizon, the majority (96%) of the clinical benefit (QALYs gained) was estimated through extrapolation beyond the observed trial period (Study 301). The extrapolations were made based on several assumptions with high levels of untestable uncertainty. Estimates for treatment effectiveness and natural history were further associated with both significant parameter and structural uncertainties.
Since the expected duration of the treatment effect of voretigene neparvovec and the utility estimates were also key drivers in the model, CADTH conducted additional scenario analyses that highlighted a wide range of plausible ICER estimates across different durations of treatment effect. Since most of the benefit estimated in the model originates from the improvements in quality of life as opposed to increased life expectancy, the results are sensitive to the choice of utility weights. Previous studies in different clinical settings have shown that valuation of health states by proxies typically underestimates the utility weight in chronic disability health states compared with those elicited by the patients themselves. In such instances, this would overestimate differences in quality of life between voretigene neparvovec and BSC, which would result in a higher ICER for voretigene neparvovec. Together, these limitations indicate that the cost-effectiveness results should be cautiously interpreted.
Funding: CADTH receives funding from Canada’s federal, provincial, and territorial governments, with the exception of Quebec.
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