PET Studies of D-Methamphetamine Pharmacokinetics in Primates: Comparison with L-Methamphetamine and (2)-Cocaine

PET Studies of d-Methamphetamine Pharmacokinetics in Primates: Comparison with l-Methamphetamine and (2)-Cocaine

Joanna S. Fowler1–3, Carsten Kroll4, Richard Ferrieri1, David Alexoff1, Jean Logan1, Stephen L. Dewey1, Wynne Schiffer1, David Schlyer1, Pauline Carter1, Payton King1, Colleen Shea1, Youwen Xu1, Lisa Muench5, Helene Benveniste1,6, Paul Vaska1, and Nora D. Volkow5,7

1Brookhaven National Laboratory, Upton, New York; 2Mount Sinai School of Medicine, New York, New York; 3Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York; 4University of Mainz, Mainz, Germany; 5National Institute on Alcoholism and Alcohol Abuse, Rockville, Maryland; 6Department of Anesthesiology, Health Sciences Center, State University of New York at Stony Brook, Stony Brook, New York; and 7National Institute on Drug Abuse, Rockville, Maryland

l-methamphetamine and thus cannot account for the more intense The methamphetamine molecule has a chiral center and exists stimulant effects of d-methamphetamine. Lack of pharmacologic as 2 enantiomers, d-methamphetamine (the more active blockade by methamphetamine indicates that the PET image rep- enantiomer) and l-methamphetamine (the less active enantio- resents nonspecific binding, though the fact that methamphet- mer). d-Methamphetamine is associated with more intense stim- amine is both a transporter substrate and an inhibitor may also ulant effects and higher abuse liability. The objective of this study play a role. A comparison of 11C-d-methamphetamine and was to measure the pharmacokinetics of d-methamphetamine 11C-(2)-cocaine in the same animal showed that the slower clear- for comparison with both l-methamphetamine and (2)-cocaine ance of methamphetamine is likely to contribute to its previously in the baboon brain and peripheral organs and to assess the sat- reported longer-lasting stimulant effects relative to those of (2)- urability and pharmacologic specificity of binding. Methods: cocaine. High kidney uptake of d-methamphetamine or its labeled d- and l-methamphetamine and (2)-cocaine were labeled with metabolites may account for the reported renal toxicity of d-meth- 11C via alkylation of the norprecursors with 11C-methyl iodide amphetamine in humans. using literature methods. Six different baboons were studied in Key Words: PET; methamphetamine; 11C; brain; peripheral or- 11 PET sessions at which 2 radiotracer injections were adminis- gans tered 2–3 h apart to determine the distribution and kinetics J Nucl Med 2007; 48:1724–1732 11 of C-d-methamphetamine in brain and peripheral organs. Sat- DOI: 10.2967/jnumed.107.040279 urability and pharmacologic specificity were assessed using pretreatment with d-methamphetamine, methylphenidate, and tetrabenazine. 11C-d-Methamphetamine pharmacokinetics were compared with 11C-l-methamphetamine and 11C-(2)-cocaine in both brain and peripheral organs in the same animal. Results: 11C-d-andl-methamphetamine both showed high uptake and Methamphetamine is a highly addictive stimulant drug widespread distribution in the brain. Pharmacokinetics did not that is toxic to brain and peripheral organs (1). It is both a differ between enantiomers, and the cerebellum peaked earlier substrate and an inhibitor of monoamine transporters releas- and cleared more quickly than the striatum for both. 11C-d-Meth- ing dopamine and other neurotransmitters through the amphetamine distribution volume ratio was not substantially af- individual neurotransmitter transporters on the presynaptic fected by pretreatment with methamphetamine, methylphenidate, neurons and on intracellular vesicles (1). It has been shown to or tetrabenazine. Both enantiomers showed rapid, high uptake and clearance in the heart and lungs and slower uptake and be neurotoxic to laboratory animals at doses that are self- clearance in the liver and kidneys. A comparison of 11C-d-meth- administered in human abusers (2). Its chronic use also leads amphetamine and 11C-(2)-cocaine showed that 11C-d-metham- to structural abnormalities (3) and evidence of dopamine phetamine peaked later in the brain than did 11C-(2)-cocaine terminal damage in the brains of living human methamphet- and cleared more slowly. The 2 drugs showed similar behavior amine abusers (4,5). In addition to its addictive and toxic in all peripheral organs examined except the kidneys and pan- properties, methamphetamine abuse is also associated with 11 creas, which showed higher uptake for C-d-methamphetamine. risky sexual behavior that facilitates HIV infection (6). Conclusion: Brain pharmacokinetics did not differ between d-and The methamphetamine molecule has a chiral center and exists as 2 enantiomers, d-methamphetamine (S-(1)-N-a- Received Dec. 6, 2006; revision accepted Jul. 4, 2007. l R 2 For correspondence or reprints contact: Joanna S. Fowler, PhD, Medical dimethylphenethylamine) and -methamphetamine ( -( )- Department, Bldg. 555, Brookhaven National Laboratory, P.O. Box 5000, N-a-dimethylphenethylamine). The 2 enantiomers differ in Upton, NY 11973. E-mail: [email protected] their physiologic and pharmacologic potency, with the COPYRIGHT ª 2007 by the Society of Nuclear Medicine, Inc. d-enantiomer generally being associated with more potent

1724 THE JOURNAL OF NUCLEAR MEDICINE • Vol. 48 • No. 10 • October 2007 physiologic and behavioral effects and higher abuse liability Mizugaki et al. reported differences in 11C-methamphet- (7). The d-enantiomer is also a more potent dopamine amine uptake with different anesthetics (12), and though the releaser. A microdialysis study in rats, comparing dopamine authors did not specify the enantiomeric form, earlier studies elevation by d- and l-methamphetamine, found that in this same group studied methamphetamine sensitization d-methamphetamine elevated dopamine by 650% at a dose using the active enantiomer, 11C-d-methamphetamine (13). of 2 mg/kg whereas l-methamphetamine elevated dopamine To our knowledge, direct comparison of the pharmacoki- by only 250% at a much larger dose of 12 mg/kg (8). netics of d- and l-methamphetamine in different regions of d-Methamphetamine is readily synthesized by reduction the brain and in the peripheral organs of baboons has not been of pseudoephedrine. It is the form that is the most commonly reported, nor has the saturability and pharmacologic speci- produced in clandestine laboratories and thus is the form that ficity of the d-enantiomer been assessed. Because high is the most commonly abused. In the past, methamphetamine uptake and rapid brain entry of drug is crucial in stimulant was commonly synthesized as the racemic mixture from reinforcement, and because peripheral organ toxicity is also a phenylacetone as one of the main chemical precursors, and concern, we set out, first, to determine whether brain uptake thus, human exposure to the l-enantiomer was of pharmaco- and kinetics are consistent with the more intense stimulant logic and toxicologic relevance (9). It is noteworthy that effects of d-methamphetamine relative to l-methamphet- d-methamphetamine is currently marketed as Desoxyn amine; second, to identify target organs for methamphet- (Abbott Laboratories) for the treatment of attention deficit amine and its labeled metabolites; third, to assess the brain disorder and exogenous obesity and that l-methamphetamine saturability and pharmacologic specificity of d-methamphet- is a component of the over-the-counter Vicks Vapor Inhaler amine; and fourth, to directly compare the pharmacokinetics (Procter & Gamble). of 11C-d-methamphetamine and 11C-(2)-cocaine in the brain 11C-d,l-Methamphetamine and its individual labeled and peripheral organs of the same animal. enantiomers have been synthesized by 11C-methylation of d,l-amphetamine and its individual enantiomers with MATERIALS AND METHODS 11C-methyl iodide (10). Biodistribution studies on mice showed that both enantiomers entered the mouse brain and Baboon Preparation heart and cleared according to a single exponential curve. All animal studies were reviewed and approved by the Brook- Brain uptake was decreased by treatment with reserpine, haven Institutional Animal Use and Care Committee. Six different which blocks the vesicular monoamine transporter (VMAT). baboons were studied in 11 PET sessions in which 2 radiotracers were administered 2 h apart. The baboons were anesthetized with a Several PET studies on monkeys and dogs have measured the 11 dose of ketamine (10 mg/kg); intubated and ventilated with a pharmacokinetics of C-labeled racemic methamphetamine mixture of isoflurane (1%–4%, Forane; Baxter Healthcare Corp.), and either d-orl-methamphetamine. For example, PET nitrous oxide (1,500 mL/min), and oxygen (800 mL/min); and then 11 studies with C-l-methamphetamine on rhesus monkeys catheterized for radiotracer injection and arterial sampling as showed no difference in uptake between different brain described previously (14). The baboon studies and details on drug regions, indicating that the binding is nonspecific (11). administration are summarized in Table 1.

TABLE 1 Summary of Baboon PET Studies

Study no. Baboon’s name Radiotracer Location Drug treatment

d vs. l BEJ284dy1,2 Spicey l (run 1); d (run 2) Brain None BEJ285dy1,2 Friendly d (run 1); l (run 2) Brain None BEJ307dy1,2 Daisy d (run 1); l (run 2) Brain None BEJ288dy1,2 Daisy d (run 1); l (run 2) Torso None BEJ290dy1,2 Chloe l (run 1); d (run 2) Torso None d vs. drug treatment BEJ280dy1,2 Daisy d (run 1); d (run 2) Brain None BEJ287dy1,2 Missy d (run 1); d (run 2) Brain Baseline; d-methamphetamine, 0.2 mg/kg intravenously, 5 min prior BEJ291dy1,2 Pearl d (run 1); d (run 2) Brain Baseline; methylphenidate, 0.5 mg/kg intravenously, 10 min prior BEJ329dy1,2 Spicey d (run 1); d (run 2) Brain Baseline; tetrabenazine; 4 mg/kg intravenously, 60 min prior d vs. 11C-(2)-cocaine BEJ303dy1,2 Friendly d (run 1); cocaine (run 2) Torso None BEJ298dy1,2 Missy Cocaine (run 1); d (run 2) Brain None

d 5 11C-d-methamphetamine; l 5 1C-l-methamphetamine.

PET STUDIES OF METHAMPHETAMINE ENANTIOMERS • Fowler et al. 1725 PET Studies Plasma Analysis for Fraction of 11C-Methamphetamine 11C-d- and l-methamphetamine were prepared from d- and Radioactivity in plasma samples from the baboons was measured l-amphetamine and 11C-methyl iodide according to the method in a calibrated well counter. Plasma, sampled at 7 different time of Inoue et al. (10). d-Amphetamine was obtained from K & K points, was analyzed manually by high-performance liquid chro- Laboratories, and l-amphetamine was obtained from Sigma- matography and automatically by solid-phase extraction using a Aldrich. Dynamic PET was performed on a Siemens HR1 high- laboratory robot (Zymark/Caliper Life Sciences) as previously resolution, whole-body PET scanner (4.5 · 4.5 · 4.8 mm at the described (17). High-performance liquid chromatography condi- center of the field of view) in 3-dimensional acquisition mode, tions were modified from the literature procedure to optimize the using 63 planes. For all scans (brain and body), a transmission separation of 4-hydroxymethamphetamine from methamphetamine scan was obtained with a 68Ge rotating rod source before radio- and its other potentially labeled metabolites and to verify that the tracer injection for each emission scan to correct for attenuation. solid-phase method separated the 4-hydroxy metabolite from meth- The specific activity of 11C-d-orl-methamphetamine ranged from amphetamine. Plasma samples were analyzed using a Spherisorb 18 to 37 GBq/mmol (0.5–1.0 Ci/mmol at the end of synthesis), and ODS1 5-mm column (Waters) (80:20 MeOH:0.1 M ammonium the dose injected ranged from 74 to 148 MBq (2–4 mCi). The formate with 1 mL of triethylamine solvent; 1.2 mL/min). Retention radiochemical purity was greater than 98%. Scanning was per- times were 4.5 and 7 min for 4-hydroxymethamphetamine and formed for 90 min with the following time frames (1 · 10 s, 12 · methamphetamine, respectively. High-performance liquid chroma- 5s,1· 20 s, 1 · 30 s, 8 · 60 s, 4 · 300 s, and 8 · 450 s). tography analysis was used only to validate the robot solid-phase Typically, 2 studies were performed 2 h apart on each scanning day extraction methodology. The total radioactivity concentration in to compare enantiomers, to assess the effect of pharmacologic plasma for each subject for each scan was corrected for the presence intervention, or to compare 11C-d-methamphetamine with 11C-(2)- of labeled metabolites determined by solid-phase extraction to cocaine. In the case of tetrabenazine pretreatment studies, there was obtain the input function that was used in distribution volume a 3-h interval between injections. estimation. 11C-(2)-Cocaine was synthesized from norcocaine (NIDA Research Technology Branch) according to the literature method Log D Determination (15). Radiochemical purity was greater than 98%, and specific A modification of a literature procedure was used (18). Briefly, an 11 11 activity was 18–92 GBq/mmol (0.2–2.5 Ci/mmol at the end of aliquot (50 mL) of C-methamphetamine or C-(2)-cocaine synthesis). Scanning was performed for 54 min with the following solution was added to a mixture of 1-octanol (2.5 mL) and phosphate- time frames (1 · 10 s, 12 · 5s,1· 20 s, 1 · 30 s, 4 · 60 s, 4 · 120 s, buffered saline (pH 7.4; 2.5 mL). The mixture was stirred in a vortex and 8 · 300 s). mixer at room temperature for 2 min and then centrifuged at 7,000 rpm for 2 min. An aliquot (0.1 mL) of the octanol layer and 1.0 mL of Image Analysis the buffer layer were sampled separately into 2 empty vials and Time frames were summed over the experimental period (90 counted. Two milliliters of the octanol layer were transferred into a min for 11C-d- and l-methamphetamine and 54 min for 11C-(2)- test tube containing 0.5 mL of fresh octanol and 2.5 mL of buffer, the cocaine), and planes were summed in groups of 2 for the purpose process of stirring and centrifuging was repeated, and the aliquots of region-of-interest (ROI) placement. ROIs were placed over the from each layer were extracted and counted until 6 measures of the striatum and the cerebellum and then projected onto the dynamic ratio of counts in the octanol to counts in the buffer were obtained. images to obtain time–activity curves. For 11C-d- and l-metham- Log D is the log (base 10) of the average of the ratios of the decay- phetamine, ROIs were also placed on the thalamus, frontal cortex, corrected counts in the octanol–buffer mixture. and temporal cortex. For both labeled compounds, a global ROI Measurement of Free Fraction of 11C-d- was placed over 3 central planes. Regions occurring bilaterally Methamphetamine in Plasma 11 were averaged. The C concentration in each ROI was divided by The radioactivity in an aliquot of 11C-d- and l-methamphetamine the injected dose to obtain the percentage injected dose (%ID)/ solution was measured in a well counter and added to 500 mLof 3 cm . A similar strategy was used for ROI placement for peripheral baboon plasma, and this was incubated for 10 min at room temper- organs, in which we chose regions on the heart, lungs, liver, ature. Aliquots (20–40 mL) of the incubated spike plasma were kidneys, pancreas, and spleen. For 3 baboon studies in which counted (unspun aliquot). Two hundred to 400 mL of the incubation 11 C-d-and l-methamphetamine were compared in the same animal, mixture were placed in the upper level of a Centrifree tube (Amicon 3 we compared the peak uptake (%ID)/cm , time to reach peak Inc.), and this was centrifuged at 2,000g for 10 min, during which uptake, clearance half-time from peak for the striatum and the time the temperature of the sample did not change. After centrifug- cerebellum, distribution volume ratio (DVR; striatum to cerebel- ing, the top portion of the Centrifree tube, containing the bound lum), and plasma integral (uncorrected and corrected for the portion, was removed and discarded, and precisely measured presence of labeled metabolites) using the paired t test, 2-tailed. aliquots (20–40 mL) of the liquid in the cup (unbound aliquot) were Clearance half-time from peak was determined by inspecting each counted. The free fraction is the ratio of the decay-corrected counts time–activity curve and determining the difference in time be- of the unbound aliquots to the decay-corrected counts of the unspun tween the time at peak and the time at which the radioactivity aliquots. concentration decreased by 50%. For the studies comparing baseline and drug administration (Table 1) in the same animal, RESULTS we compared time–activity curves and the striatum-to-cerebellum DVR using graphical analysis (Logan plot) (16). For studies com- d- and l-Methamphetamine Rapidly Distributes to paring 11C-d-methamphetamine and 11C-(2)-cocaine, we com- Subcortical and Cortical Brain Regions pared time–activity data for the brain and peripheral organs and Both 11C-d- and l-methamphetamines had high, rapid, DVRs for the brain. and widespread uptake in both subcortical and cortical

1726 THE JOURNAL OF NUCLEAR MEDICINE • Vol. 48 • No. 10 • October 2007 brain regions, consistent with its measured log D (20.38 6 and lungs (Fig. 3). In contrast, both enantiomers showed high 0.01; range, 20.35 to 20.40) (19) and a large (80%) free uptake and relatively slow clearance of 11C from the kidneys fraction in plasma for both enantiomers. A coregistered and the liver. The rank-order half-time for clearance from brain image of 11C-d-methamphetamine (summed frames peak uptake for both enantiomers was lung ..heart . over 90 min) is shown in Figure 1. Time–activity curves are spleen ..kidneys ..liver. shown in Figure 2 for one of the baboons to whom both enantiomers were administered in the same scanning ses- d-Methamphetamine Accumulation and Clearance Are sion. Figure 1 shows that 11C was present throughout the Not Affected by Methamphetamine, Methylphenidate, brain, and Figure 2 shows that the peak times and clearance or Tetrabenazine Pretreatment rates for most brain regions were similar except for the To assess whether d-methamphetamine binding in vivo in cerebellum, which peaked earlier and cleared more rapidly baboon brain is saturable and specific for the dopamine and for both enantiomers (Fig. 2; Table 2). There was essen- norepinephrine transporters or VMAT, we performed serial tially no difference in the PET measurements between studies on the same baboon at baseline and after pretreatment the d- and l-enantiomers in peak uptake, time to peak, or with d-methamphetamine (0.2 mg/kg, 5 min prior), with DVR (Table 2). The plasma clearance of 11C showed a methylphenidate (which binds to the dopamine and norepi- trend toward being slower (trend P , 0.08) for the d- than nephrine transporters, 0.5 mg/kg, 10 min prior (21)), and with the l-enantiomer (as reflected by the integral at 60 min, Table tetrabenazine (which binds to the VMAT, 4 mg/kg, 60 min 2). However, when corrected for the fraction of total prior). We also assessed binding reproducibility in 1 animal radioactivity as 11C-methamphetamine (which was signif- with no drug treatment. There was no change in either the icantly lower for 11C-d-methamphetamine than for 11C-l- time–activity curves (data not shown) or the DVR (Table 4) methamphetamine [Table 3]), there was no significant for the striatum and cerebellum with either d-methamphet- difference between the plasma input for the d- and amine or methylphenidate pretreatment. Though tetrabena- l-enantiomers (49,876 6 1,221 vs. 50,208 6 1,887 Bq/ zine pretreatment reduced 11C uptake in both the striatum and mL · min, respectively). the cerebellum (data not shown), the plasma input of radiotracer was also lower and the striatum-to-cerebellum DVR d- and l-Methamphetamine Show Similar Distribution was not substantially changed (Table 4). We noted that the and Kinetics in Peripheral Organs, with Highest administered dose of tetrabenazine was previously found to Accumulation in Kidneys and Liver elevate synaptic dopamine in the baboon as shown by a Both 11C-d-and l-methamphetamines showed rapid uptake change in 11C-raclopride binding (22). and clearance from the heart, lungs, and spleen, with 11C being retained slightly longer in the spleen than in the heart d-Methamphetamine and (2)-Cocaine Differ in Brain Distribution and Kinetics in Brain and in Peripheral Organs The brain distribution, kinetics, and clearance rates were very different between 11C-d-methamphetamine and 11C- (2)-cocaine when compared in the same animal. In contrast to d-methamphetamine, (2)-cocaine was localized almost exclusively in the striatum, as shown in Figures 1 (coregistered images for 11C-d-methamphetamine and 11C-(2)- cocaine) and 4 (time–activity curves in the striatum and cerebellum). The striatum-to-cerebellum DVR was 1.7 for 11C-(2)-cocaine and 1.22 for 11C-d-methamphetamine for the baboon in whom both tracers were injected. (2)-Cocaine peaked earlier than d-methamphetamine in the striatum (3.5 vs. 8.0 min) and in the cerebellum (1.75 vs. 5.5 min) and also cleared more rapidly from the striatum (18- vs. 56-min half-time from peak) and the cerebellum (9- vs. 51-min half-time from peak). The total brain uptake at the time of peak uptake was 4.3% and 5.0% for d-methamphetamine and (2)-cocaine, respectively. In peripheral organs, the major difference between the 2 FIGURE 1. Summed brain images for 11C-d-methamphet- stimulant drugs was the high accumulation and slow clear- amine (top row, from 0–90 min) and 11C-l-(2)-cocaine (bottom 11 11 ance from the kidneys seen after injection of C-d-meth- row, from 0–54 min) in same animal. C distribution is 11 widespread over cortical and subcortical brain regions for amphetamine but not after C-(2)-cocaine (Fig. 5). Also, 11 11C-d-methamphetamine but is highly localized in striatum for C-d-methamphetamine had higher uptake in the pancreas 11C-(2)-cocaine. Images are coregistered to MRI atlas (20). than did 11C-(2)-cocaine. On the other hand, the heart and

PET STUDIES OF METHAMPHETAMINE ENANTIOMERS • Fowler et al. 1727 FIGURE 2. Time–activity curves for cortical and subcortical brain regions for 11C-d-methamphetamine (A) and 11C-l- methamphetamine (B) in same baboon (BEJ285dy1 and BEJ285dy2 refer to study number). Brain kinetics are similar for different brain regions. lungs showed similarly rapid clearance—and the liver, slow d-Methamphetamine is also widely distributed in both accumulation and clearance—of 11C for both 11C-d-meth- subcortical and cortical brain regions as can be seen in the amphetamine and 11C-(2)-cocaine. time–activity curves in Figure 2 and in the image in Figure 1. Widespread distribution to different brain regions and slow clearance may play a role in the neurotoxicity of methamphetamine because of its access to different populations of DISCUSSION neurons, where it has the potential to release transmitters Striatal uptake of methamphetamine, its interactions with from vesicular stores. the plasma dopamine transporter and VMAT, and the Despite the fact that d-methamphetamine induces a sig- consequent large vesicular release of dopamine are believed niﬁcantly higher release of striatal dopamine in rats (8) and is to be responsible for the intense reinforcing effects of the a more powerful stimulant in humans (7), we did not see any drug. Here, we have shown that d-methamphetamine up- difference in pharmacokinetics between 11C-d- and l-meth- take into the baboon brain is relatively rapid, with peak amphetamine. Thus, factors other than a difference in brain uptake in the striatum occurring approximately 7 min after pharmacokinetics must account for the more intense stimu- injection. For drugs of abuse, rapid brain uptake is associ- lant effects of d-methamphetamine. ated with reinforcement because the rate at which drugs of Though methamphetamine is known to interact with the abuse increase dopamine modulates their reinforcing ef- dopamine transporter and VMAT, releasing large amounts fects. The increases in dopamine must occur quickly to be of dopamine (8,24), 11C-methamphetamine uptake was not perceived as reinforcing (23). blocked by either methamphetamine itself or methylphenidate

TABLE 2 Comparison of Data for Striatum and Cerebellum

Parameter 11C-d-methamphetamine 11C-l-methamphetamine

% dose/cm3 (peak) Striatum 0.033 6 0.008 0.036 6 0.003 Cerebellum 0.034 6 0.01 0.034 6 0.004 Peak time (min) Striatum 7.2 6 2.3 7.5 6 2.6 Cerebellum 4.5 6 4.5 4.5 6 1.5 Clearance half-time from peak (min) Striatum 80.3 6 18.6 98.5 6 22.3 Cerebellum 59.3 6 18.3 67.5 6 12.3 DVR (Striatum:cerebellum) 1.22 6 0.04 1.22 6 0.027 Plasma integral for total 11C at 60 min* 2,411 6 399 (1,348 6 33) 1,833 6 203 (1,351 6 51)

*There was a trend toward greater 11C in plasma at 60 min for d-enantiomer (P , 0.08). Data in parentheses are fraction of 11Cas methamphetamine (mean 6 SD, n 5 3).

1728 THE JOURNAL OF NUCLEAR MEDICINE • Vol. 48 • No. 10 • October 2007 TABLE 3 if differences in their pharmacokinetics could account for Comparison of Fraction of Total 11C in Plasma That Is differences in their behavioral and toxic effects. The peak Present as Parent Radiotracer at Different Times After striatal uptake was faster for (2)-cocaine than for metham- Injection phetamine (3.5 vs. 7 min, respectively), and the clearance Time 11C-d-methamphetamine 11C-l-methamphetamine rates were also much faster for (2)-cocaine than for (min) (n 5 9) (n 5 3) P d-methamphetamine (Fig. 4) (15). This result is consistent with differences in the self-reported temporal course of a 2936 2.4 95 6 2NS 5836 5896 2.3 0.06 ‘‘high’’ between these 2 drugs. Whereas for methamphet- 10 69.9 6 6.8 82.3 6 3.5 0.01 amine the peak behavioral effects occur at about 15 min, with 20 53.4 6 8.5 72 6 6.2 0.006 a half-time of about 45 min (27), for (2)-cocaine peak effects 30 44.4 6 8.5 67 6 9.6 0.003 occur at about 4 min, with a half-time of about 20 min (23). 60 32.3 6 7.7 58.7 6 13.05 0.002 The peak striatal uptake for (2)-cocaine was also higher 90 26.9 6 6.4 49 6 17 0.01 than the striatal uptake for d-methamphetamine, as may be accounted for by the higher log D for cocaine relative to that NS 5 not statistically significant. of d-methamphetamine (1.31 6 0.03 vs. 20.38 6 0.01) (Fig. 4). Prior studies have shown that 11C-(2)cocaine binds to the dopamine transporter (28), as demonstrated by blockade of (a stimulant drug that binds to the dopamine and norepineph- striatal uptake by drugs that block the dopamine transporter, rine transporters (25)) or by tetrabenazine (a drug that blocks contrasting with uptake of methamphetamine, which cannot VMAT). This finding indicates that the PET image is domi- be blocked and thus represents nonspecific binding. nated by nonspecific binding. We note that many labeled drugs Because the PET image provides no direct information and other labeled compounds that have pharmacologic spec- on the chemical form of the 11C in the brain, there is a ificity for a molecular target do not show specific binding to question of whether we are observing in the brain the that target when they are imaged because they also bind to kinetics of the drug itself or of a combination of the labeled other sites (nonspecific and nontarget) and this other binding drug and its labeled metabolites. 11C-d- and l-methamphet- overwhelms the specific signal. 11C-Nicotine is another ex- amine are labeled on the N-methyl group. Major metabolic ample. There is no doubt that nicotine binds to nicotine pathways for methamphetamine are N-demethylation to receptors in the brain, as can be demonstrated by its pharma- produce amphetamine, which would not be labeled, and 4- cologic activity and by observing the ability of unlabeled hydroxylation to produce 4-hydroxymethamphetamine nicotine to displace a radiotracer with specificity for the (29), which would retain the label. Labeled methanol and nicotinic acetylcholine receptor (26). However, 11C-nicotine carbon dioxide would be produced from N-dealkylation and cannot be blocked by unlabeled nicotine because it binds to are expected to contribute little to the brain 11C. An exami- many other nonspecific sites and the signal is lost in the noise. nation of the fraction of 11C that is present in the form of Similarly, 11C-methamphetamine binding cannot be blocked parent radiotracer in the brain is not possible in these baboon by unlabeled methamphetamine because it also binds to studies; however, from an analysis of labeled methamphet- nonspecific sites that dominate the image. amine in baboon plasma during the PET study, more than We compared d-methamphetamine and (2)-cocaine in the 50% of the parent compound remains for the d-enantiomer same animal in both the brain and the peripheral organs to see and more than 70% remains for the l-enantiomer at 20 min

FIGURE 3. Comparison of 11C-d-methamphetamine (A) and 11C-l-methamphetamine (B) distribution in peripheral organs in baboon (BEJ288dy1 and BEJ288dy2 refer to study number). 11C distribution is similar for the 2 enantiomers.

PET STUDIES OF METHAMPHETAMINE ENANTIOMERS • Fowler et al. 1729 TABLE 4 the same animal (Fig. 5). 11C-(2)-Cocaine also had a rapid, Comparison of DVRs at Baseline and After Drug Treatment high uptake and rapid clearance from the heart. This same for 11C-d-Methamphetamine pattern has been reported in the human heart (34). Though the DVR DVR (after residence time of (2)-cocaine in the heart is short, prolonged Drug (baseline) treatment) % Change inhibition of the norepinephrine transporter is shown (as assessed by 6-18F-fluoronorepinephrine), demonstrating Control 1.21 1.28 15 that the effects of (2)-cocaine on cardiac neurotransmitter d-Methamphetamine 1.19 1.16 22 Methylphenidate 1.28 1.19 27 activity can persist long after the drug has cleared (35). Tetrabenazine 1.18 1.24 15 Further studies are required to determine whether d-methamphetamine has a long-lasting pharmacodynamic effect on norepinephrine transporter in the heart. We note that methamphetamine could also affect the heart through central after intravenous injection (Table 3). Therefore, the major mechanisms. fraction of 11C that is delivered to the brain soon after Methamphetamine has also been reported to produce injection is in the form of the parent compound. Thus, it is kidney damage and peroxidative injury in laboratory rats. highly likely that it is mostly the parent compound that In both an acute and a chronic model of methamphetamine enters the brain soon after injection, when the powerful rein- administration, immunohistochemical markers, renal func- forcing effects of the drug are crucial. This likelihood is tion markers, and blood ion concentrations indicated cel- supported by the identification of 11C-methamphetamine in lular damage consistent with pathologic changes seen in mouse brain after the injection of 11C-methamphetamine autopsy samples from methamphetamine abusers (36,37). (10). For (2)-cocaine, all the 11C that is in the brain is in As can be seen in Figure 3, the kidney would have the the form of (2)-cocaine because norcocaine, the only (2)- highest exposure of all the organs to methamphetamine and cocaine metabolite that can cross the blood–brain barrier, its labeled metabolites, possibly accounting for the previ- would not be labeled (30). ously reported pathologic changes noted in autopsy sam- In addition to its effects on the brain, methamphetamine ples from methamphetamine abusers. toxicity to other organs has been documented. Toxicity is In this study, we also showed a higher accumulation of generally associated with oxidative damage, which is asso- 11C-methamphetamine in the pancreas, compared with that ciated with high levels of catecholamines induced by the of 11C-(2)-cocaine (0.036%/cm3 at peak vs. 0.025%/cm3 at interaction of methamphetaminewith neurotransmitter trans- peak, Fig. 5). This accumulation could reflect nonspecific porters, as well as with other factors such as inflammation binding or binding to VMAT2 in the pancreas. The accu- (31). In the heart, acute methamphetamine use produces mulation is relevant because toxicity to the pancreas has cardiac lesions whereas chronic use is associated with car- been documented both in preclinical studies and in autopsy diomyopathy (32,33). We have found that although 11C-d- cases of individuals dying from a methamphetamine over- methamphetamine has a high initial distribution to the heart dose (38). Because VMAT2 in the pancreas is located in (0.059%/cm3 at 0.21 min after injection), its residence time in b-cells of the islets of Langerhans, which are responsible the heart is short: a half-time of approximately 0.5 min from for the synthesis of insulin (39), this may be the mechanism peak. This pattern was similar to that of 11C-(2)-cocaine in underlying methamphetamine-induced insulin release (40).

FIGURE 4. Time–activity curves for 11C-d-methamphetamine (A) and 11C-(2)-cocaine (B) in cerebellum and striatum in same baboon (BEJ298dy1 and BEJ298dy2 refer to study number). Clearance of 11C-d-methamphetamine is lower and slower than that of 11C-(2)-cocaine.

1730 THE JOURNAL OF NUCLEAR MEDICINE • Vol. 48 • No. 10 • October 2007 FIGURE 5. Comparison of distribution and clearance of 11C-d-methamphetamine (A) and 11C-(2)-cocaine (B) in peripheral organs in same baboon (BEJ303dy1 and BEJ303dy2 refer to study number). Both compounds show similar distribution and kinetics in heart, lungs, and liver, but d-methamphetamine has much greater uptake and slower clearance in kidneys and higher uptake in pancreas than does 11C-(2)-cocaine.

An objective of this study was to ask whether the clearance of methamphetamine is likely to contribute to its pharmacokinetics of 11C-methamphetamine at a tracer dose previously reported (27) longer-lasting stimulant effects would be similar to the pharmacokinetics of a typical be- relative to those of (2)-cocaine. d-Methamphetamine and haviorally active dose of 0.25–0.5 mg/kg by smoking or (2)-cocaine showed similar behavior in all peripheral intravenous injection. Though we do not know the effects of organs examined except the kidneys and pancreas, which chronic administration on methamphetamine pharmacoki- showed higher uptake for 11C-d-methamphetamine, possi- netics, we showed in this study that a single administration of bly accounting for the renal toxicity and pancreatic lesions 0.2 mg/kg in an anesthetized baboon did not change the reported in methamphetamine abusers. These studies set pharmacokinetics of 11C-d-methamphetamine. Nonetheless, the stage for future studies using 11C-d-methamphetamine a prior study on mice showed that chronic methamphetamine as a tool to measure d-methamphetamine pharmacokinetics administration significantly elevated the uptake of 11C-meth- in humans. amphetamine and that uptake normalized after drug with- drawal (41). This finding can be examined in future studies on ACKNOWLEDGMENTS human methamphetamine abusers. We also note that prior studies on monkeys found an influence of different types of This research was carried out at Brookhaven National anesthesia on methamphetamine kinetics (12); thus, future Laboratory under contract DE-AC02-98CH10886 with the studies on humans should assess methamphetamine kinetics U. S. Department of Energy and supported by its Office of in the awakened state. Biological and Environmental Research and by NIH K05DA020001 and also in part by Deutscher Akademischer Austauschdienst (DAAD), Bonn, for a student fellowship for CONCLUSION Carsten Kroll. We are grateful to Donald Warner and Michael Comparative PET studies showed no differences in brain Schueller for advice and assistance in different aspects of 11 pharmacokinetics between C-d- and l-methamphetamine, these studies. suggesting that pharmacokinetics are unlikely to account for the different pharmacodynamic effects of the 2 enantiomers reported in humans and animals. Both enantiomers REFERENCES also showed rapid, high uptake and clearance in the heart, 1. Barr AM, Panenka WJ, MacEqan W, et al. The need for speed: an update on lungs, and kidneys and slower uptake and clearance in the MAMP addiction. J Psychiatry Neurosci. 2006;31:301–313. 2. Zhu JP, Xu W, Angulo JA. MAMP-induced cell death: selective vulnerability in liver and kidneys, possibly accounting for some of the toxic neuronal subpopulations of the striatum in mice. Neuroscience. 2006;140:607– effects of methamphetamine in peripheral organs such as 622. the heart and kidneys. Lack of pharmacologic blockade by 3. Thompson PM, Hayashi KM, Simon KL, et al. Structural abnormalities in the brains of human subjects who use MAMP. J Neurosci. 2004;24:6028–6036. methamphetamine indicates that the PET image represents 4. Volkow ND, Chang L, Wang G-J, et al. Association of dopamine transporter nonspecific binding, though the fact that methamphetamine reduction with psychomotor impairment in MAMP abusers. Am J Psychiatry. is both a transporter substrate and an inhibitor may also 2001;158:377–382. 11 5. Fitzmaurice PS, Tong J, Yazdanpanah M, Liu PP, Kalasinsky KS, Kish SJ. play a role. A comparison of C-d-methamphetamine and Levels of 4-hydroxynonenal and malondialdehyde are increased in brain of 11C-(2)-cocaine in the same animal showed that the slower human chronic users of MAMP. J Pharmacol Exp Ther. 2006;319:703–709.

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