PeIA-5466 is potent and highly selective for α3β2 over α6/α3β2β3 nAChRs. Peptides based on PeIA-4769 and PeIA-5355 were synthesized with different residues in the 10th and 15th positions and a concentration–response analysis was performed for each. The IC50 values for inhibition of α3β2 and α6/α3β2β3 nAChRs for each peptide were compared to those of PeIA and the α3β2-α6/α3β2β3 selectivity ratio calculated. (A, B) PeIA-5106 and PeIA-5416 were synthesized to evaluate the effects of Nle10 and Ile15, respectively, on α3β2-α6/α3β2β3 nAChR selectivity. PeIA-5106 was 69-fold more potent than PeIA on α3β2 nAChRs, but only a minor increase (3fold) in α3β2-α6/α3β2β3 selectivity, over PeIA-4769, was found. PeIA-5416 with Ile15, was 220-fold selective for α3β2 nAChRs. An additional 70-fold enhancement of selectivity was obtained with PeIA-5466 (red curves). A minimum of 4 oocytes was used for each IC50 determination and the error bars indicate the SD; values are provided in Table 6.