Can Drug Transporters Change Brain THC Exposure? Evidence From Knockout Mice
Removing particular drug-efflux transporters increased brain THC exposure and altered a physiological response in mice. That observation offers a mechanism for variability, not a validated genetic test for human cannabis impairment.
What was removed?
Spiro and colleagues compared mice lacking P-glycoprotein or breast cancer resistance protein with wild-type animals. After THC administration, brain and blood concentrations were measured. A separate cohort was assessed for THC-induced hypothermia.
The study concerns engineered loss of transporter function. Such a model is not equivalent to every naturally occurring human variant or to taking a medicine that partially inhibits a transporter.
Brain and blood were distinct endpoints
Knockout mice retained higher brain THC concentrations, with delayed elimination particularly evident in one transporter-deficient group. They were also more sensitive to THC-induced hypothermia.
Concentration differences depended on collection time and compartment. A brain-to-blood relationship at one sampled time cannot become a fixed multiplier for every later sample. The physiological endpoint was temperature reduction, rather than a human cognitive or driving assessment.
From mechanism to an individual opinion
The work supports investigating transport processes alongside absorption, metabolism, and receptor response. It does not establish the size of any corresponding effect in humans, a transporter-based dosing rule, or an impairment threshold.
To attribute an unusual response to a genetic or medication interaction, identify the actual variant or interacting drug and supporting human evidence. A plausible mouse mechanism cannot independently establish why a particular person responded differently, nor prove that a given blood concentration represented a particular brain exposure.
The delay in brain elimination was particularly prominent in mice lacking Abcg2, the gene encoding breast cancer resistance protein. The transporter-specific difference makes the finding more precise than a general claim that all genetic changes increase THC effects. It also underscores why a human inference requires evidence about the particular transporter and functional alteration at issue.
Related reading: why a blood THC value does not directly establish impairment.
Primary research
Adena S. Spiro; Alexander Wong; Aurélie A. Boucher; Jonathon C. Arnold. Enhanced Brain Disposition and Effects of Δ9-Tetrahydrocannabinol in P-Glycoprotein and Breast Cancer Resistance Protein Knockout Mice. PLoS ONE (2012). DOI: 10.1371/journal.pone.0035937.
