The medical use of cannabinoids (THC and CBD) is mostly described as a well-tolerated therapeutic option [18,19,20,21,22,23,24,25]. Nevertheless, some adverse drug effects have been described in connection with the use of THC and nabiximols, including seizures [6,7], although for this particular case, no causal relationship to the cannabinoids administered has been proven so far. Especially in case of such serious effects, the appropriate use of the cannabinoid preparation should be reviewed with high priority. Therefore, a common method for therapeutic drug monitoring might be useful. In this work, we addressed the question of whether the administration of nabiximols can be monitored by immunological methods on urine samples for THC, because these methods are very common in clinical toxicology, so such therapeutic drug monitoring could be widely performed.

Because THC-COOH has an elimination half-life of approximately 3–11 days with frequent abuse of THC [26], a very consistent urinary elimination is expected, so the measurement of urine concentrations may also be useful in assessing steady-state plasma concentrations. Therefore, a THC-COOH-selective method is expected to be valuable in assessing the steady-state cannabinoid concentrations under nabiximols therapy. As our data show, the selective measurement of THC-COOH by GC/MS correlated with patient-reported nabiximols doses. Moreover, the correlation of the values obtained by the immunological method was in the same range, although the method was not selective for THC-COOH. THC, 11-nor-Δ⁸-THC-COOH, 11-OH-THC, 8β-OH-THC, and 8β,11-diOH-THC were also found to react positively in the immunoassay. Moreover, a significantly lower concentration of cannabinoids was measured by the immunological method after hydrolysis compared to the condition before. This suggests that besides the described metabolites, hydrolysable metabolites were already measured with this immunological system under the conditions of non-hydrolysis, which could explain the divergent findings obtained with both methods. Therefore, in patients with different metabolism patterns, weak correlations of doses and measured concentrations should be expected. On the other hand, the correlation of different doses for an individual patient should be meaningful.

Patients´ statements about administered doses of their medications are always questionable. Some patients want to avoid the side effects of a drug, or, for example, in the case of nabiximols, want to avoid conflicts with driving ability. Others expect better effectiveness when a recommended dose is increased. Therefore, monitoring of drug levels may be useful. This is often mandatory if the physician is considering a change in drug dosing regimen. Significant correlations for administered doses and plasma levels are usual under steady state conditions. Because THC and CBD are eliminated with short half-lives [10,11], steady-state conditions cannot be achieved for these substances; the same is true for 11-OH-THC, for the same reason. Concentrations of these substances in urine samples are also short-lived; higher concentrations can be expected only with a recently applied dose. In our study, a period of at least 2 h was demanded between the last application of nabiximols and the collection of blood and urine samples from the patients. Accordingly, no measurable and, in particular, no higher concentrations of THC, CBD, or 11-OH-THC were expected in the plasma samples, whereby a higher concentration might indicate more frequent use by the patients than reported.

A well-known criterion for assessing the interval since last smoking of marijuana is the THC-COOH/THC ratio [15,16]. Because oromucosal applications might have similar pharmacokinetic properties, this THC-COOH/THC ratio might also indicate the time interval since the last nabiximols application. This would require a reliable measurement of low THC concentrations in a range below 1 ng/mL (as expected approximately >2 h after application of nabiximols). Such measurements may be possible especially in the scientific field, but this does not seem to be practicable for routine laboratory use due to the complex analytical procedure. Considering this THC-COOH/THC ratio for an interval of at least 2 h between application and sample collection, a ratio greater than approximately 2–5 should be expected. Assuming no altered resorption kinetics in MS patients (e.g., due to a damaged oral mucosa in later stages of MS), an incorrect time interval might be assumed only in three (patient numbers 9, 15, 16) of our patients (Table 2).

The ratio of cannabinoids including hydrolysis products is also used to allow an assessment of the time period between cannabinoid administration and sample collection [17]. As in the study by Abraham et al. [17], a high efficiency of enzymatic hydrolysis was found for 11-OH-THC and a poor one for THC-COOH in our samples. Moreover, in agreement with Bergamaschi et al. [14], a high hydrolysis efficiency of CBD-derived metabolites was confirmed. Unfortunately, many of the concentrations measured in our samples were below the LOQ, so no reliable ratios could be calculated at this point.

THC-COOH has a longer half-life than the other tested substances and could therefore be a suitable candidate for monitoring steady-state cannabinoid levels under nabiximols therapy. In the work of Karschner et al. [10], two differently dosed applications of nabiximols were compared with respect to concentration–time courses for CBD, THC, and 11-OH-THC. Low concentrations were measured in plasma samples, with a concentration maximum approximately 3 h after application. Concentrations decreased with half-lives of about 3 h for all three compounds. In their work, as in ours, higher concentrations of THC were found compared with CBD, whereas 11-OH-THC was in the range of THC concentrations. Very similar results were published by Stott et al. [11], although in their work, the maximum concentrations were reached at about 1 h after nabiximols application (in their control groups). Such a very short time to maximal concentrations for THC was also described by Indorato et al. [13], where, in their study, the mean concentration of THC after application of one nabiximols puff was approximately 0.5 ng/mL.

Finally, we have no reason to believe that our patients’ statements about their dosing regimens are incorrect. Therefore, the rather weak correlation of the nabiximols doses with the measured cannabinoid concentrations could be due to differences in the metabolism of the patients, possibly resulting from different concomitant medications. In conclusion, simple immunological cannabinoid measurements in urine samples could provide an estimate of the nabiximols dose. It is expected that meaningful results could be obtained via this approach, especially in longitudinal follow-up of individual patients.