Minimum important slowing of disease progression as determined by the ALS functional rating scale – a survey of patient expectations toward disease-modifying drugs in ALS

<i>Objective:</i> To define the minimum important slowing (MIS) of ALS progression that patients would expect from disease-modifying drug treatment in ALS. <i>Methods:</i> In a survey of ALS patients, the MIS in ALS progression (change in the ALS Functional Rating Scale-Revised, ALSFRS-R) was assessed by asking: "At what point of slowing of ALS, as determined by the ALSFRS-R, do you consider a drug to be important?" Data were collected during clinic visits or remotely via the ALS App. Participants were differentiated in the prognostic groups of slower (<0.5), intermediate (≥0.5 and ≤1.0), or faster (>1.0) ALS progression (ALSPR; ALSFRS-R/month). <i>Results:</i> Of 522 participants (ALS App, <i>n</i> = 397; clinic, <i>n</i> = 125), 395 (75.7%) completed the survey, while 127 (24.3%) selected the option "cannot estimate". The distribution of MIS was as follows: modest slowing of ALS progression (5% and 10% slowing, <i>n</i> = 146 patients, 36.9%), moderate slowing (20%, 30%, and 40% slowing, <i>n</i> = 135, 34.2%), and major slowing (≥50% slowing, <i>n</i> = 114, 28.9%). Median MIS was 20% (IQR 10-50%). Patients with faster ALSPR more frequently assessed a major slowing as the MIS (<i>n</i> = 18, 36.0%) compared to those with slower ALSPR (<i>n</i> = 54, 25.2%). <i>Conclusion:</i> A considerable number of participants viewed a modest slowing in ALS progression as the MIS, followed closely by preferences for moderate and major slowing. Expectations varied according to patients' individual ALS progression. These insights may inform the design of future clinical trials in ALS. Study limitations include potential selection and response biases, as well as the predominantly remote digital assessment.