670 M.A. Huestis
THC in the low- and high-dose cigarettes, respectively. The small percentage of the
total dose found in the urine as THCCOOH is not surprising considering the many
factors that influence THCCOOH excretion after smoking. Prior to harvesting,
cannabis plant material contains little active THC. When smoked, THC carboxylic
acids spontaneously decarboxylate to produce THC with nearly complete conver-
sion upon heating. Pyrolysis of THC during smoking destroys additional drug.
Drug availability is further reduced by loss of drug in the side-stream smoke and
drug remaining in the unsmoked cigarette butt. These factors contribute to high
variability in drug delivery by the smoked route. It is estimated that the systemic
availability of smoked THC is approximately 8% to 24% and that bioavailability
depends strongly upon the experience of the cannabis user (Lindgren et al. 1981;
Ohlsson et al. 1980; Perez-Reyes et al. 1981). THC bioavailability is reduced due
to the combined effect of these factors; the actual available dose is much lower
than the amount of THC and THC precursor present in the cigarette. Most of the
THC dose is excreted in the feces (30%–65%), rather than in the urine (20%) (Wall
and Perez-Reyes 1981; Wall et al. 1983). Another factor affecting the low amount
of recovered dose is measurement of a single metabolite. Numerous cannabinoid
metabolites are produced in humans as a result of THC metabolism, most of which
arenotmeasuredorincludedinthepercentage-of-dose-excretedcalculationswhen
utilizing GC/MS.
2.4.3
Cannabinoid Glucuronide Conjugates
Specimen preparation for cannabinoid testing frequently includes a hydrolysis step
to free cannabinoids from their glucuronide conjugates. Most GC/MS confirma-
tion procedures in urine measure total THCCOOH following either an enzymatic
hydrolysis withβ-glucuronidase, or more commonly, an alkaline hydrolysis with
sodium hydroxide. Alkaline hydrolysis appears to efficiently hydrolyze the ester
THCCOOH glucuronide linkage.
2.4.4
Urinary Biomarkers of Recent Cannabis Use
Significantly higher concentrations of THC and 11-OH-THC in urine are observed
whenEscherichia coliβ-glucuronidase is employed in the hydrolysis method com-
pared to eitherHelix pomatiaβ-glucuronidase or base (Kemp et al. 1995a,b).
THC and 11-OH-THC are primarily excreted in urine as glucuronide conjugates
that are resistant to cleavage by alkaline hydrolysis and by enzymatic hydrolysis
procedures employing some types ofβ-glucuronidase. Kemp et al. demonstrated
thatβ-glucuronidase fromE. coliwas needed to hydrolyze the ether glucuronide
linkages of the active cannabinoid analytes. Mean THC concentration in urine
specimens from seven subjects collected after each had smoked a single 3.58%