Analytical characterization and pharmacological evaluation of the new psychoactive substance 4-fluoromethylphenidate (4F-MPH) and differentiation between the (±)-threo and (±)-erythro diastereomers

Misuse of (±)-threo-methylphenidate (methyl-2-phenyl-2-(piperidin-2-yl)acetate; Ritalin®; MPH) has long been acknowledged, but the appearance of MPH analogs in the form of ‘research chemicals’ has only emerged in more recent years. 4-Fluoromethylphenidate (4F-MPH) is one of these recent examples. This study presents the identification and analytical characterization of two powdered 4F-MPH products that were obtained from an online vendor in 2015. Interestingly, the products appeared to have originated from two distinct batches given that one product consisted of (±)-threo-4F-MPH isomers whereas the second sample consisted of a mixture of (±)-threo and (±)-erythro 4F-MPH. Monoamine transporter studies using rat brain synaptosomes revealed that the biological activity of the 4F-MPH mixture resided with the (±)-threo and not the (±)-erythro isomers based on higher potencies determined for blockage of dopamine uptake (IC₅₀ 4F-MPH_mixture = 66 nM vs. IC₅₀ (±)-threo = 61 nM vs. IC₅₀ (±)-erythro = 8,528 nM) and norepinephrine uptake (IC₅₀ 4F-MPH_mixture = 45 nM vs. (±)-threo = 31 nM vs. IC₅₀ (±)-erythro = 3,779 nM). In comparison, MPH was three times less potent than (±)-threo-4F-MPH at the dopamine transporter (IC₅₀ = 131 nM) and around 2.5 times less potent at the norepinephrine transporter (IC₅₀ = 83 nM). Both substances were catecholamine selective with IC₅₀ values of 8,805 nM and >10,000 nM for (±)-threo-4F-MPH and MPH at the serotonin transporter. These findings suggest that the psychostimulant properties of (±)-threo-4F-MPH might be more potent in humans than MPH.