Efavirenz interference in urine screening immunoassays for tetrahydrocannabinol

Abstract

Background

It has been known for some time that the antiretroviral drug, efavirenz (EFV), cross-reacts in urine immunoassays for tetrahydrocannabinol (THC). Because published studies investigating this phenomenon are limited, cross-reactivity information for several immunoassays is lacking. Reports of possible false-positive THC results from clinicians conducting workplace testing prompted us to investigate cross-reactivity for assays frequently employed in our own setting. In light of the potentially deleterious consequences of misclassification, information about EFV cross-reactivity should be included in product information to facilitate interpretation of results and assay selection.

Methods

Random urine samples from 30 patients on EFV therapy were analysed for THC metabolites by two near-testing devices (THC One Step Marijuana and Rapid Response^® Drugs of Abuse Test Strips) and two automated immunoassays (Roche Diagnostics Cannabinoids II and Beckman Coulter SYNCHRON^® Systems THC2). THC confirmatory testing was performed by gas chromatography-mass spectrometry (GC-MS).

Results

GC-MS failed to detect THC metabolites in any of the samples, as did three of the four immunoassays. However, the Rapid Response^® test strips yielded positive results in 28 out of 30 samples, which could be reversed on re-testing after sample pretreatment with glucuronidase.

Conclusions

Our study supports previous findings that interference is attributable to a glucuronidated EFV metabolite. We postulate that cross-reactivity is influenced by the composition of immunogens used to elicit anti-THC antibodies. Since access to such information is restricted, contributions from scientists in the antibody industry may be enlightening.

Introduction

Therapy with efavirenz (EFV) is known to yield false-positive results for tetrahydrocannabinol (THC) by several urine screening immunoassays. This interference has been attributed to EFV 8-glucuronide (EFV-8-G), one of two major urinary metabolites. The other major urinary metabolite, 8-hydroxy-EFV, and the parent drug do not interfere. ¹ Given the widespread inclusion of EFV in first-line antiretroviral therapy, the use of THC assays that are subject to EFV interference for workplace testing, is problematic. Not only may employees face punitive measures based on false-positive results, but some may also feel obliged to disclose their human immunodeficiency virus status in order to refute a positive drug test. The aim of our study was to investigate EFV cross-reactivity for immunoassays commonly employed in the workplace and diagnostic laboratories in South Africa. These included two point-of-care tests: Rapid Response^® Drugs of Abuse Test Strip (BTNX Inc., Markham, ON, Canada), and THC One Step Marijuana Test Strip (ACON Laboratories, San Diego, CA, USA); and two automated immunoassays: COBAS Integra Cannabinoids II (Roche Diagnostics, Indianapolis, IN, USA), and SYNCHRON^® Systems THC2 (Beckman Coulter, Fullerton, CA, USA).

Methods and results

Ethical approval for the study was obtained from the institutional Research Ethics Committee. Random urine samples were collected from 30 adult patients attending the antiretroviral clinic at the Tshwane District Hospital in Pretoria. To increase the likelihood that urinary excretion of EFV metabolites would be adequate, only patients who had been taking 600 mg EFV daily for at least 14 days were selected. Samples were analysed for THC metabolites by the screening immunoassays and confirmed by gas chromatography-mass spectrometry (GC-MS). Using the Rapid Response^® assay, 28 out of 30 samples yielded positive results. All results obtained by the other immunoassays were negative. THC metabolite concentrations by GC-MS were undetectable in all samples, confirming false-positive results for the Rapid Response^® assay. On re-analysis by the Rapid Response^® assay after hydrolysis using β-glucuronidase, positive samples reverted to negative.

Discussion

The fact that deconjugation reverses false-positive THC results supports the hypothesis that interference is due to a glucuronidated EFV metabolite. In the two samples that tested negative from the outset, EFV-8-G concentrations may have been too low to cross-react significantly. Assuming that interference is due to structural similarity between EFV and cannabinoids, the findings of this study raise two interesting questions: (a) why are only some immunoassays affected; and (b) why is the interference particular to the glucuronidated metabolite? The answers may lie with assay-specific characteristics of the anti-THC antibodies. Comparisons of package insert data from 10 assays (Immunoassay package inserts: BTNX Inc. Rapid Response^® Drugs of Abuse Test Strip; ACON Laboratories THC One Step Marijuana Test Strip; Roche Diagnostics COBAS Integra 400/700/800 Cannabinoids II; Beckman Coulter SYNCHRON^® Systems THC2; Microgenics Corp. CEDIA^® Multi Level THC Assay; Immunalysis Corp. Cannabinoids (THCA/CTHC) Direct ELISA Kit; Dade Behring Inc. Syva^® Emit^® d.a.u.^® Cannabinoid 50 ng Assay; Abbott Laboratories AXSYM System Cannabinoids; Biosite Inc. Triage TOX Drug Screen; Alfa Scientific Designs Inc. Instant-View^® Marijuana (THC) Urine Cassette Test) for which EFV cross-reactivity information was available (four from this and six from a published study ¹ ) revealed that interference was independent of host species or whether antibodies were mono- or polyclonal (Table 1). Another determinant of antibody sensitivity and specificity in THC immunoassays is the position of carrier protein linkage. Being low-molecular-weight compounds, cannabinoid haptens (typically 11-nor-Δ⁹-THC-9-carboxylic acid) require linkage to carrier proteins to produce immunogens for antibody generation. ^2,3 Steric hindrance by carrier proteins results in the generation of antibodies directed towards exposed functional groups remote from the site of carrier protein linkage. Linkage to positions C1-, C2- or C5′  elicits antibodies with high selectivity for the cyclohexyl ring, which translates to lower sensitivity to C8- and C11-hydroxylated metabolites and their glucuronides (Figure 1). ³ Conversely, linkage to the cyclohexyl ring generates antibodies with greater cross-reactivity to C8-, C9- and C11-substituted moieties. ² This concept was taken one step further by Roche Diagnostic Systems with the development of a non-cannabinoid immunogen containing only the benzpyran core. ³ By eliminating the antigenic determinants of the cyclohexyl ring completely, antibodies with two to three times higher cross-reactivity to C8- and C11-hydroxylated metabolites than even the C9-linked immunogens were elicited. ³ Antibodies generated by C8- to C11-linked and benzpyran immunogens therefore offer superior detection for clinical purposes, because THC is extensively metabolized at these positions prior to urinary excretion. ² Inspection of the cross-reactivity data (albeit incomplete) for the 10 assays in Table 1 reveals that some assays, which are subject to EFV interference, exhibit comparatively lower sensitivities to C8- and C11-hydroxylated metabolites. Our postulate that antibodies used in these assays may have been elicited using immunogens with carrier protein linkage to C1-, C2- or C5′ could not be verified, because vendor and proprietary restrictions prevent access to detailed information about immunogen composition.

Figure 1

Δ⁹-11-nor-9-carboxytetrahydrocannabinol. Bold arrows indicate published linkage sites for carrier proteins to cannabinoid haptens (from ref. ² )

Table 1

Comparison of urine screening immunoassays for tetrahydrocannabinol metabolites

Immunoassay	SYNCHRON THC2	COBAS Integra Cannabinoids II	AXSYM Cannabinoids	One step Marijuana	Syva Emit d.a.u.	Rapid Response	CEDIA THC	THCA/CTHC Direct ELISA	Triage TOX	Instant-view THC
Supplier	Beckman Coulter	Roche	Abbott	ACON	Dade Behring	BTNX	Microgenics	Immunoanalysis	Biosite	Alfa Scientific
Method principle	EIA	MAI	FPIA	LFIC	EIA	LFIC	CEDIA	ELISA	FIA	LFIC
Anti-THC antibodies	Mouse monoclonal	Mouse monoclonal	Sheep polyclonal	Mouse monoclonal	Mouse monoclonal	Rabbit monoclonal	? monoclonal	Sheep polyclonal	Mouse monoclonal	Mouse?
EFV cross-reactivity	No	No	No	No	No	Yes	Yes	Yes	Yes	Yes
Cross-reactivity data
Cut-off (ng/mL)	20	50	13	50	50	50	50	50	50	50
11-nor-Δ8-THC-9 COOH	30	89	25	30		50	40	11	40	50
11-nor-Δ9-THC-9 COOH	20	50		50	50	50	50		50	50
11-nor-Δ9-THC-9 COOH-Glu	50	62							100
11-OH-Δ9-THC	50	115	25		90	>100,000	125	>1000	150	100
8-β-OH-Δ9-THC	50				65		1000
8-β-11-dihydroxy-Δ9-THC	30	58			50		5000	>1000
Cannabinol	100	2031	80	20,000		20,000	1000	>1000	2000

EIA, enzyme immunoassay; MAI, microparticle agglutination-inhibition; FPIA, fluorescence polarization immunoassay; LFIC, lateral flow immunochromatography; CEDIA, cloned enzyme donor immunoassay; ELISA, enzyme-linked immunosorbent assay; FIA, fluorescence immunoassay

Conclusions

Further elucidation of the mechanisms of EFV interference may require inputs from the antibody industry or the field of molecular modelling. Interestingly, media reports have in recent years described the emergence of EFV abuse in South Africa, whereby crushed tablets are smoked for their psychoactive effects. ⁴ Whether these effects are mediated by interaction of EFV or its metabolites with the endocannabinoid system is a question for future research into the psychopharmacology of EFV.

DECLARATIONS

Competing interests: None.

Funding: This research was funded by the University of Pretoria.

Ethical approval: The Faculty of Health Sciences Research Ethics Committee of the University of Pretoria approved this study (S110/2009).

Guarantor: NMO.

Contributorship: The study was conceived by JBL. NMO reviewed the literature and both authors contributed to protocol development. NMO was involved in gaining ethical approval and interpreted laboratory results. JBL supervised GC-MS analysis, performed deconjugation experiments and interpreted laboratory results. NMO wrote all drafts of the manuscripts. Both authors reviewed and edited the manuscript and approved the final version.

Acknowledgements: We are grateful to Bronwyn Adendorff for sample collection and processing, and collation of laboratory results. We thank Adriaan Marais for technical assistance with the GC-MS analysis. We thank Homemed Medical Diagnostics for donating THC urine test strips from ACON laboratories, and Roche Diagnostics and Ampath Laboratories for providing reagents and THC testing free of charge.