Useful Markers of Consumption

While Δ⁹-THC has a relatively short half-life in serum, 11-COOH-Δ⁹-THC has a much longer half-life, ranging from days to weeks.⁸ This is due to the accumulation of 11-COOH-Δ⁹-THC in fatty tissues and its consequent delayed elimination. Accordingly, the detection of 11-COOH-Δ⁹-THC metabolites is a useful marker in forensic drug testing for Cannabis consumption for approximately one month after use.

HHC Metabolism

HHCs are semi-synthetic derivatives of THCs that are widely sought-after for both medicinal and recreational purposes and may be administered by vaping, ingestion, or combustion. HHCs are produced synthetically from CBD after the conversion to Δ⁸-THC (or Δ⁹-THC) followed by catalytic hydrogenation.⁹ This reaction forms a new stereocenter at carbon 9, which is not present in Δ⁸- or Δ⁹-THC, producing a diastereomeric mixture of HHCs comprised of 9(R)-HHC and 9(S)-HHC epimers (Figure 3).¹⁰ Most of the psychoactive effects of HHCs are attributed to 9(R)-HHC; the 9(S)-HHC epimer has minimal cannabimimetic activity.¹¹

Figure 3. The formation of 9(R)- and 9(S)-HHC epimers during HHC synthesis.

Similarities with Δ⁹-THC Metabolism

Overall, the metabolism and elimination kinetics of 9(R/S)-HHC is relatively unknown, though the latest research indicates that there are both similarities and differences compared to the metabolism of Δ⁹-THC. Like Δ⁹-THC, the formation and abundance of 9(R/S)-HHC metabolites is dependent on the method of consumption.¹²

Though detailed human metabolic studies are lacking, it is believed that 9(R/S)-HHC are metabolized at least in part in a manner analogous to Δ⁹-THC, leading to the formation of the 11-hydroxy metabolites 11-hydroxy-9(R)-HHC (11-OH-9(R)-HHC) and 11-hydroxy-9(S)-HHC (11-OH-9(S)-HHC; Figure 4).

Early studies on acylated 11-OH-HHC metabolites found psychoactivity in Rhesus monkeys. This could be a contributing factor to overall psychoactive effects of HHC, similar to reported activity of 11-OH-Δ⁹-THC following Δ⁹-THC consumption.¹³

Further oxidation of these intermediate metabolites produces the carboxylated metabolites 11-nor-9(R)-carboxy-HHC (11-nor-9(R)-COOH-HHC) and 11-nor-9(S)-carboxy-HHC (11-nor-9(S)-COOH-HHC), which have been detected in human biological samples after deglucuronidation via enzymatic hydrolysis.^10,12,14-16

Figure 4. The proposed metabolic pathway for the formation of 11-OH and 11-COOH-HHC metabolites.

Much like metabolism of Δ⁹-THC, hydroxylation of HHC may occur at several positions of the hydrogenated cyclohexyl ring or the alkyl side chain (Figure 5). Schrimer et al. were able to detect many side-chain hydroxylated HHC metabolites, though most of these metabolites could not be identified at the structural level.¹² They were, however, able to detect and identify 4'-OH-HHC in human biological samples following HHC consumption. Additionally, Lindbom et al. reported the positive identification of a 5'-OH-HHC metabolite in urine samples utilizing a synthesized standard, in addition to an unidentified side-chain hydroxylated HHC.¹⁷

Figure 5. Possible positions for putative oxidative HHC metabolites.

Differences with Δ⁹-THC Metabolism
Stereoselective Metabolism of HHC Epimers

The two epimers, 9(R)-HHC and 9(S)-HHC, have different metabolic profiles, which could be attributed, in part, to stereoselective metabolism and elimination.¹⁶

Di Trana et al. investigated the metabolic fate of a 50:50 mixture of 9(R)- and 9(S)-HHC after inhalation (the identity of the mixture was confirmed prior to starting the study).¹⁶ The authors found that 9(R)-HHC was present in higher concentrations in the blood. However, 9(S)-HHC was present in higher concentrations in the urine. These differences in elimination could explain in part the reported higher potency of the 9(R)-HHC. Indeed, it appears the metabolic enzymes are stereoselective for the two epimers. 9(S)-HHC may be preferentially hydroxylated at carbon 8,  whereas carbon 11 is preferred for 9(R)-HHC (Figure 5).^16,17 However, both epimers were hydroxylated at carbon 8, forming the metabolites 8(R)-hydroxy-9(R)-HHC (detectable in blood and urine) and 8(S)-hydroxy-9(S)-HHC (detectable only in urine).  

8-OH Metabolites of HHC Epimers

Hydroxylation at carbon 8 may play a larger role in HHC metabolism than Δ⁹-THC metabolism.^12,16 There are four possible stereoisomers for 8-hydroxy-HHC: 8(R)-OH-9(R)-HHC, 8(S)-OH-9(S)-HHC, 8(S)-OH-9(R)-HHC, and 8(R)-OH-9(S)-HHC.¹⁵ Cayman offers analytical standards that provide coverage of these metabolites (Figure 6).

Figure 6. 8-OH metabolites of 9(R)- and 9(S)-HHC epimers. Stereochemistry at the C9 position of the 8-hydroxy compounds is opposite of the parent HHC compounds due to nomenclature priorities, as described by IUPAC naming conventions.

Some studies have found that the 8(R)-OH-9(R)-HHC and 8(S)-OH-9(S)-HHC metabolites are abundant after HHC consumption in humans, suggesting them as the main metabolite and as a useful marker for HHC consumption.¹⁶ However, other studies have noted them as minor metabolites or as undetectable.^12,15 Whether 8-OH-HHC metabolites have any value as markers of HHC consumption is a matter of broad and current interest.

Summary of HHC Metabolism

In summary, the metabolism of 9(R/S)-HHC proceeds somewhat similarly to Δ⁹-THC metabolism, though with some distinct differences, especially regarding the stereoselective metabolism of the 9(R)- and 9(S)-epimers and the formation of 8-OH-HHC metabolites (Figure 7). However, further research is needed to fully characterize the metabolic pathways and identify the full range of HHC metabolites.

Figure 7. An overview of the current understanding of HHC metabolism.

Tentative Markers of Consumption

There is still much work to be done on characterizing the metabolism of HHCs. While HHC metabolites have been detected following consumption, there is little consensus on the profile of detectable HHC metabolites, their relative abundance, ideal biological matrix, or pharmacokinetics. Table 1 summarizes the HHC metabolites that have been identified in human biological samples.^10,12,14-16

Table 1. HHC metabolites that have been detected in human biological samples.

A good marker for consumption is one that is abundant, specific to the target parent compound, and universally detectable across all biological samples. The detection and identification of HHC metabolites is further complicated by the observation that the HHC metabolites 11-OH-HHC and 11-COOH-HHC have also been observed as Δ⁹-THC metabolites.¹⁰ This precludes the use of these HHC metabolites as specific markers of HHC consumption, as these metabolites are shared between HHCs and Δ⁹-THC.

Summary

Although there are many similarities between the metabolism of Δ⁹-THC and 9(R/S)-HHC, there are also distinct differences, summarized in Table 2 below.

Table 2. A comparative summary of Δ⁹-THC and 9(R/S)-HHC metabolism

Future Directions

Experimental studies are just beginning to uncover the metabolism of HHCs. Given the early stage of this field, there is considerable uncertainty on HHC metabolism and best analytical approaches. The ability of analytical methods to detect HHC metabolites appear to be dependent on various factors, including the method of ingestion, biological matrix, and analytical method used. As of now, there is no single method known to us that can comprehensively detect all HHC metabolites across these different factors and no single metabolite that can be used as a marker of HHC consumption.

Available from Cayman

Cayman is the leading provider of research tools for HHCs and other semi-synthetic cannabinoids. We keep abreast of emerging semi-synthetic cannabinoids and respond quickly to provide reference standards as the research evolves.

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