We report the synthesis and fibrillogenesis inhibiting activity of the new peptide derivatives 1–4, related to the pentapeptide Ac-LPFFD-NH2 (iAβ5p), proposed by Soto and co-workers and widely recognized as one of the most active β-sheet breaker agents. The Aβ25–35 fragment of the parent full-length Aβ1–42 was used as fibrillogenesis model. The activity of peptide derivatives 1–4 was tested in vitro by thioflavin T binding assay, far UV CD spectroscopy, and scanning electron microscopy. Their ability to hinder the toxic effect of Aβ25–35in vivo was studied by monitoring the viability of human SH-SY5Y neuroblastoma cells and the prevention of superoxide anion radical release from BV2 microglial cells. The results point to a favourable role in the fibrillogenesis inhibitory activity of the sulphonamide junction for compounds 1 and 2, containing an N,N-dimethyltaurine and a taurine amino-terminal moiety, respectively. Furthermore, compounds 1 and 2 show a significant protective effect on cell viability, rescuing the cells from the toxicity exerted by Aβ25–35 treatment.