During the study period, SITC’s resistance task force published new recommendations for defining primary and secondary (acquired) resistance to anti-PD-1 therapy.46

*Non-progressors were defined as patients who received anti-PD-1 monotherapy treatment for ≥90 days and had PR or CR without subsequent progression during the entire study period.

†The original study criteria defined primary resistance as patients with a BOR of PD, or of CR, PR, or SD for <90 days before PD occurred. The SITC guidelines define primary resistance as patients with a BOR of PD, a BOR of CR, PR, or SD for <180 days before PD, and/or who progressed on treatment or within 12 weeks of stopping treatment.

‡The original study criteria defined secondary (acquired) resistance as patients with a BOR of CR, PR, or SD for ≥90 days before PD. The SITC guidelines define secondary (acquired) resistance as patients with CR, PR, or SD for ≥180 days who subsequently experienced PD while on treatment, within 12 weeks of stopping treatment, or after rechallenge. Secondary resistance under the original definition was further subdivided into ‘early’ (progression <1 year after BOR) or ‘late’ (progression ≥1 year after BOR) development of resistance.

§Data on whether patients progressed on treatment or within 12 weeks of stopping treatment were missing for two patients, who were therefore excluded from analyses reported by SITC definition.

BOR, best overall response; CR, complete response; PD-1, programmed cell death-1; PD, progressive disease; PR, partial response; SD, stable disease; SITC, Society for Immunotherapy of Cancer ; SITC, Society for Immunotherapy of Cancer.