Introduction. Fentanyl belongs to 4-anilidopiperidine class of synthetic opioid analgesics. It is characterized by high potency, rapid onset and short duration of action. A large number of fentanyl analogues have been synthesized so far, both to establish the structure-activity-relationship (SAR) and to find novel, clinically useful analgesic drugs. Objective. In this study, newly synthesized 3-alkyl fentanyl analogues were examined for analgesic activity and compared with fentanyl. Methods. Analgesic activity was assessed by tail-immersion test in rats. Results. The relative potency was: (±)cis-3-methyl fentanyl (8) gt (±)trans-3-methyl fentanyl (2) (±)cis−3−ethylfentanyl(1.5)gtfentanyl(1) (±)trans-3-ethyl fentanyl (0.9) gt (±)cis-3-butyl fentanyl (0.064) (±)trans−3−butylfentanyl(0.035)gt(±)cis−3−benzylfentanyl(0.008) (±)trans-3-benzyl fentanyl (0.0055). (±)Cis-3-iso-propyl fentanyl, and (±)cis-3-phenethyl fentanyl are inactive in doses up to 5 mg/kg. The duration of action (ED99) was: (±)cis-3-methyl fentanyl (90 min) gt (±)trans-3-methyl fentanyl (40 min) # (±)cis-3-ethyl fentanyl (60 min) (±)trans-3-ethyl fentanyl (40 min) # fentanyl (50 min) = (±)cis-3-butyl fentanyl (50 min) = (±)trans-3-butyl fentanyl (50 min) = (±)cis-3-benzyl fentanyl (50 min) = (±)trans-3-benzyl fentanyl (50 min). Symbols gt and lt denotes p lt 0.05. Conclusion. It is concluded that the analgesic potency of 3-alkyl fentanyl analogues is influenced by the steric factor (voluminosity of the group at the position 3 of the piperidine ring and the cis/trans isomerism). Otherwise, with the exception of 3-methyl fentanyl, the duration of action of 3-alkyl fentanyl analogues is not significantly affected by the stereochemistry.Uvod. Fentanil pripada grupi sintetskih opioidnih analgetika, 4-anilidopiperidina. Karakteriše ga visoka potentnost, brz početak i kratko trajanje dejstva. Do sada je sintetisan veliki broj analoga fentanila, kako u cilju određivanja odnosa strukture i farmakološke aktivnosti, tako i u cilju pronalaženja novog klinički korisnog analgetika. Cilj rada. U studiji su ispitivana analgetska dejstva novosintetisanih 3-alkil analoga fentanila i poređena sa fentanilom. Metod rada. Merenje analgetskog dejstva ispitivanih jedinjenja vršeno je uz pomoć testa potapanja repa pacova u toplu vodu. Rezultati. Relativna jačina jedinjenja iznosila je: (±)cis-3-metil fentanil (8) gt (±)trans-3-metil fentanil (2) (±)cis-3-etil fentanil (1,5) gt fentanil (1) (±)trans−3−etilfentanil(0,9)gt(±)cis−3−butilfentanil(0,064) (±)trans-3-butil fentanil (0,035) gt (±)cis-3-benzil fentanil (0,008) (±)trans-3-benzil fentanil (0,0055). (±)Cis-3-izopropil fentanil i (±)cis-3-fenetil fentanil nisu ispoljili dejstvo u dozama do 5 mg/kg. Dužina dejstva (ED99) iznosila je: (±)cis-3-metil fentanil (90 min) gt (±)trans-3-metil fentanil (40 min) # (±)cis-3-etil fentanil (60 min) (±)trans-3-etil fentanil (40 min) # fentanil (50 min) = (±)cis-3-butil fentanil (50 min) = (±)trans-3-butil fentanil (50 min) = (±)cis-3-benzil fentanil (50 min) = (±)trans-3-benzil fentanil (50 min). Oznake gt i lt označavaju P lt 0.05. Zaključak Zaključeno je da na analgetsku jačinu 3-alkil analoga fentanila utiče sterni faktor (voluminoznost grupe na položaju 3 piperidinskog prstena i cis/trans izomerizam). Inače, uz izuzetak 3-metil fentanila, stereohemija ne utiče značajno na dužinu dejstva 3-alkil analoga fentanila
