I'll try to answer
1. N-desethyl isotonitazene: It is a derivative of isotonitazene, which acts as an opioid agonist.
Comparison to Fentanyl:In vitro pharmacological data show that N-desethyl isotonitazene is an active opioid agonist and is approximately 20x more potent than fentanyl
2. Etonitazepyne: is a benzimidazole derivative with potent opioid effect.
Comparison to Fentanyl: In data provided by the US Drug Enforcement Administration, etonitazepyne showed much greater agonism to mu-opioid receptors than fentanyl (about 31 times) and morphine (about 42 times). Its agonism to delta-opioid receptors was similar to that of fentanyl and morphine but lower to kappa-opioid receptors. The overall effectiveness of etonitazepine in vivo is 20 times higher than that of fentanyl in humans.
It described euphoria; drowsiness and wakefulness; temporary relief of pain, stress or low mood; itchiness; severe nausea and/or vomiting; severe sweating or fever; slow and/or difficulty in breathing; blue lips or fingertips; cold, clammy skin; tiny pupils; unresponsiveness and/or loss of consciousness. These reports were not verified, and no further details.
3. Protonitazene: is a benzimidazole derivative with potent opioid effects
Comparison to Fentanyl: Scientists calculated the potency (EC50) and efficacy (Emax) of protonitazene relative to those of fentanyl and hydromorphone. Protonitazene was highly active in mu-opioid receptors activation, with a potency and efficacy slightly greater than those of fentanyl (107%) and significantly greater than those of hydromorphone (174%).
Protonitazene was analytically confirmed in nine fatal poisonings or deaths in the USA. Although data on humans are limited, a review of original research on benzimidazole opioids noted that, when administered intravenously, all the drugs caused respiratory depression, with a narrow therapeutic ratio between analgesia and respiratory depression
4. Metonitazene: is an analgesic compound related to etonitazene. It has been shown to have approximately 100 times the potency of morphine by central routes of administration, but if used orally it has been shown to have approximately 10 times the potency of morphine.
Comparison to Fentanyl: Scientists calculated the potency (EC50) and efficacy (Emax), of metonitazene, relative to Fentanyl and Hydromorphone (HM). In mu-opioid receptor activation, metonitazene was highly active, with potency and efficacy slightly greater than fentanyl (113-121%) and significantly greater than hydromorphone (184-340%).
Separately, I would like to note that the pharmacological profiles of all substances make people think that they can be used in the presence of tolerance to achieve the desired effect. However, combintations of fentanyl or classical opioids with these synthetic substances with opioid properties often led to fatal outcomes. Be careful