FAP and neoplasm: We hypothesized that removal of the detrimental (4R)-fluoro substituent from the pyrrolidine ring of (2S)-4,4-difluoropyrrolidine-2-carbonitrile framework could yield a (4S)-mono-fluorinated version of FAPI-04 with superior FAP-inhibitory potency and, thus, demonstrate better FAP-mediated tumor uptake.