Using Imaging to Assess Effects of THC on Brain Activity

Marijuana has become the most popular illicit drug in the western world (Substance Abuse and Mental Health Services Administration, 2004). Both acute and chronic exposure to cannabis is associated with changes in brain activity in frontal, limbic and also cerebellar regions, with generally increased brain activity following acute exposure and generally decreased activity during abstinence (Quickfall, 2006). Regions showing differences in brain activity involve the extended dopamine reward pathways and the frontocerebellar network.

Most brain imaging studies of acute marijuana exposure have demonstrated global cortical activity increases during administration of smoked cannabis or infused THC. Several techniques have been used to track alterations of the brain associated with marijuana intoxication (Mathew and Wilson, 1992, Levin, 2001 and Mathew et al., 1997). Using positron emission tomography (PET), studies have shown that marijuana intoxication is associated with global and rCBF increases that are most notable over the frontal regions and, in right-handed subjects, in the right hemisphere, (Mathew et al., 1997). Ratings of intoxication and euphoria correlated with the right hemispheric rCBF increase (Mathew et al., 1997). In another PET study, Volkow et al. (1991a) showed that increased cerebellar metabolism during intoxication, correlated with degree of intoxication and plasma THC levels (Volkow et al.,1991b). FMRI studies have also shown increases in brain activity when individuals are intoxicated. In two pharmacological fMRI studies, Bossong and colleagues investigated the acute effects of THC, administered with a Volcano vaporizer (6 mg), and found that activity during a memory task was significantly increased in a network of memory-related brain regions, particularly when workload increased.

An alternative technology to PET and fMRI that has not been used to assess the effects of THC on brain function is functional near-infrared spectroscopy (fNIRS). fNIRS is a neuroimaging technology for mapping the functioning human cortex that exploits the principles of near-infrared spectroscopy (NIRS). An fNIRS headset can be quickly fixed to the forehead and enables measurements of the prefrontal cortex (PFC) within minutes. NIRS is based on the fact that: 1) human tissues are relatively transparent to light in the NIR spectral window (650-1000 nm); 2) NIR light is either absorbed by pigmented compounds (chromophores) or scattered in tissues; 3) NIR light is able to penetrate human tissues (Delpy and Cope, 1997); and 4) the relatively high attenuation of NIR light in tissue is due to hemoglobin, the oxygen transport red blood cell protein, located in small vessels of the microcirculation, such as capillary, arteriolar and venular beds. NIRS is weakly sensitive to blood vessels >1 mm because they completely absorb the light. The absorption spectrum of hemoglobin depends on its level of oxygenation. NIRS is a non-invasive, safe technique that utilizes 1) laser diode and/or light emitting diode light sources spanning the optical window between 650 and 1000 nm, and 2) flexible fiber optics to carry the NIR light to (source) and from (detector) tissues. Fiber optics are suitable for any head position and posture. Adequate depth of NIR light penetration (almost one half of the source-detector distance) can be achieved using a source-detector distance around 3 cm. fNIRS has been used in several fields in which fMRI is impractical due to the constraints of the fMRI scanning environment. fNIRS can be utilized, for example, in infant and children developmental studies, in neurorehabilitation assessment, and in simultaneous brain activation studies on multiple subjects. In this study, we will use fNIRS to assess the effects of THC intoxication on hemodynamic response and connectivity in the prefrontal cortex.

Dronabinol (marinol) is an FDA approved, synthetic, orally active cannabinoid used for appetite stimulation that has been shown to be safe for multi dose use at doses up to 210 mg per day, although after dosing of 210 mg per day for 16 days, a withdrawal syndrome was described with abrupt discontinuation. Doses of 2.5-20 mg per day are used for appetite stimulation and antiemetic indications. Doses of 0.5 mg/kg body weight reliably produce intoxication (easy laughing, elation and subjective heightened awareness)

II. SPECIFIC AIMS Aim 1. Assess the effects of THC intoxication using dronabinol (synthetic THC) on the HbO signal during resting state and task-based activation in the prefrontal cortex (PFC) and resting state connectivity, as well as on neurocognitive task performance and correlations between these measurements and clinical signs of intoxication.

Aim 2. Investigate at which doses of dronabinol an effect on neurocognitive task performance or an effect on PFC activity can be observed.

Aim 3. Examine potential interaction following co-administration of THC with oral ethanol exposure in healthy volunteers.

Participants: 150 adults who use marijuana at least monthly (aged 18-55) will be recruited to participate in this Phase IIa study of dronabinol vs placebo. Subjects will be asked to refrain from using marijuana within 24 hours of the start of study procedures. Since THCCOOH, the main secondary metabolite of tetrahydrocannabinol (THC), can be detected in urine several weeks after last use, a negative urine toxicology screen for THC-COOH will not be required, participants will be admitted to the inpatient research unit of Massachusetts General Hospital the night prior to start of study procedures on each study day after screening, and participants who arive exhibiting overt signs of intoxication (e.g. congestion of the conjunctival blood vessels (red eyes), slowed speech response, giddiness) will not be tested the next morning.

Study Drug and Dosing: Participants will be given up to 50 mg of dronabinol, an FDA-approved synthetic form of THC that is used to treat loss of appetite that causes weight loss in people with AIDS. THC is the principle psychoactive drug in marijuana. The study physician, Dr. A. Eden Evins MD, as well as physicians on study staff at the Center for Addiction Medicine, will be responsible for administering the drug.

Dronabinol will be dosed to 50mg per study visit. The rationale for this dose is based on several factors. The study will be conducted in regular cannabis users who present at their first study visit with a positive urine screen for THC metabolites. Chronic cannabis use is associated with significant tachyphylaxis and tolerance to pharmacologic effects of THC and other cannabinoids. In healthy males who received 210 mg/day dronabinol for 16 days, tolerance developed to the CNS effects of dronabinol (including the euphoric 'high' effect) within 12 days. At doses of 5 mg/day for appetite stimulation in 139 patients, only 18% experienced ay CNS effect (feeling high, dizziness, confusion, somnolence). These CNS effects are the target symptoms we wish to assess in study participants. Because of tolerance, the rate of CNS effects is expected to be lower in our study population than in non-cannabis users who participated in dronabinol studies for anti-emetic and appetite stimulation indications.

Dronabinol was evaluated in 454 patients with cancer at doses up to 40 mg/day. At doses greater than 0.175 mg/kg approximately 58% reported feeling 'high'. The starting recommended antiemetic dose is 15-20mg/day, noting that dosage may be escalated during a chemotherapy cycle or at subsequent cycles based upon initial results.

For appetite stimulation, the recommended dose range is up to 20 mg/day, and at this dose approximately half of patients tested have reported CNS symptoms targeted in this study (feeling high, dizziness, confusion, somnolence).

Thus, we will dose at up to 50mg and will assess for CNS symptoms frequently during the study visit. The exact dronabinol dose will be clinically determined by a study physician. The dose will be designed to minimize adverse psychologic and physiologic effects of dronabinol and will be primarily based on the degree of tolerance expected (based on dose and frequency of cannabis use), self report of degree of intoxication experienced with each use, and any regular or prior negative psychological reactions to cannabis use or binges. This is an inexact determination as potency of cannabis used is almost never known. Other factors that will be considered when determining dose are height, weight, BMI, and baseline blood pressure. For example: Heavy daily cannabis users will generally receive 50 mg. Daily cannabis users who use 1/2-1 gram per day and who weigh over approximately 140 pounds will generally receive 50 mg, while those who use less than half a gram per day, use less than daily or who weigh less than 120 pounds will receive between 30 and 45 mg based on the physician's best judgment.

Urine Drug Screen: Before study procedures are initiated, we will perform a qualitative urine drug screen (Multi Drug 6 Panel Urine Test; Medimpex United Inc., Bensalem, PA) that will test for marijuana, amphetamines, methamphetamines, cocaine, and opiates. These rapid tests are simple, can be done at home, and are CLIA-waived. The test will be administered by a trained research coordinator.

Urine Pregnancy: Before study procedures are initiated, we will perform a qualitative urine pregnancy test for females. The test will be administered by a trained research coordinator.

Overview of Procedures Phase 2A. Investigate the effect of THC on fNIRS brain signature and its association with self-reported intoxication, laboratory measures of impairment, and the gold-standard behavioral field test of driving impairment used by law enforcement, the primary classifier.

Subjects will be scheduled for 2 study sessions. In a double-blind, placebo-controlled, random order cross-over study of single dose THC (dronabinol) at physician determined doses of 20-50mg, participants will receive THC capsules or identical placebo. The MGH research pharmacy will generate a blinded randomization code for order of dosing and will dispense blinded drug in the dose ordered and identical placebo for use on separate study days. In order for a complete washout of effects, and to minimize the effect of repeat testing for the neuropsychology tasks, study visits for each subject will be conducted at least 1 week apart. On each study day, participants will be admitted to the MGH inpatient research unit, and will have a light standardized breakfast after overnight fast and sleep. They will be asked not to deviate from normal behaviors (e.g. if they drink caffeinated beverages or smoke cigarettes in the mornings, they will be asked to do so on the study day so that no symptoms of caffeine or nicotine withdrawal will appear during the scanning period. On each study day, participants will complete tests of motor function/spatial navigation and fNIRS data collection before study drug is administered. cognitive tasks will be repeated approximately 90 minutes after study drug administration, fNIRS will be repeated at approximately 120 minutes post dose, and the field sobriety test will be conducted immediately following the second fNIRS assessment. Blood pressure, heart rate and ratings of subjective drug effects will be monitored at approximately 15-minute intervals throughout the study. A third fNIRS assessment will be conducted when intoxication has resolved, at approximately 220 minutes post dose.

Participants: 150 healthy adults, aged 18-55, who use cannabis at least weekly, will be enrolled. Participants must test positive for cannabis at the screening visit and have no serious unstable medical illness to participate (See Participant Selection Section).

Phase 2B. Examine potential interaction following co-administration of THC with oral ethanol exposure in healthy volunteers.

[Design adapted from recently published alcohol interaction studies (Modi et al., 2007; Trevisan et al., 2008).]

Phase 2B is a randomized, double-blind, placebo-controlled, 2 by 2 crossover study of effect of dronabinol, ethanol, and combined dronabinol and ethanol on brain activation and connectivity as measured by fNIRS. Participants will be assigned randomly for order to receive single doses of each of the following study medications on 4 separate study visits conducted at least one week apart:

- Placebo THC + Ethanol

- THC + Placebo Ethanol

- THC + Ethanol

- Placebo THC + Placebo Ethanol

Participants will be admitted in the evening, have an overnight fast, be provided a standard breakfast of approximately 400 calories (e.g. juice, cereal, bagel), approximately 45 minutes before administering study medication.

Ethanol will be dosedby weight to obtain a breath alcohol concentration (BrAC) of approximately 0.05, consistent with BrAC following 1-2 standard drinks, using established methods of dosing based on previously published procedures (e.g. King 2005, Alcohol Clin Exp Res, Vol 29, No 4, 2005: pp 547-552).

The required volume of 95% ethanol will be calculated from the nomogram published by Watson (1989), and diluted to a final concentration of 20% by volume with diet soda. The nomogram, based on total body water, estimated for each subject using standard equations based on gender, age, height, and weight (Watson et al., 1980) will be used to prepare the ethanol for dosing by study staff who are not otherwise involved in study procedures.

In an attempt to standardize the drinking time to 8 minutes, the total dose will be split into 4 equal aliquots and placed in cups with lids. The subject will be given a cup every 2 minutes with instructions to sip the drink using a straw.

• Placebo Ethanol drink will be made as an equal volume of diet soda placed in identical cups with lids and straws. 0.25 ml of 95% ethanol will be floated on the top of the soda in each cup in an attempt to blind the participant and study staff who are involved with assessments / data acquisition.

The alcohol administration procedure will take place approximately 45 min before the start of the second fNIRS session.

Breathalyzer measurements will be taken approximately every 5-10 minutes after the alcohol administration procedure, until BrACs have returned to <0.01. Participants will be required to stay in the laboratory until BrACs have returned to <0.01. A member of our study staff will then (1) arrange cab service, and (2) escort the participant to the cab at the end of the study visit. Alternatively, participants may arrange travel home with a family member or friend. If they choose to do this, a study staff member will ensure that someone is there to bring them home prior to departure.