XENOBIOTICA
899
strong carbon fluorine bond (C-F energy bond of 112 kcal/
mol) as compared to carbon-hydrogen bond (C-H energy
bond of 98 kcal/mol) and small van der Waals radius (1.47 Å)
(Jacobson et al. 2015). However, it is important to develop
fluorine labelled compounds that do not undergo defluorina-
tion in vivo as some fluorinated drugs in the past have been
found to undergo metabolism to toxicologically relevant spe-
cies such as methoxyflurane, widely used as an anaesthesia
in the 1960s, which was found to be associated with nephro-
toxicity due to extensive metabolism of methoxyfluorane
and high serum concentrations of inorganic fluoride (Park
and Kitteringham 1994). Moreover, rapid defluorination of
18F-labelled compounds makes interpretation of PET images
ambiguous due to rapid accumulation of fluoride ion in
bones and skull (Bonomi et al. 2018). Our in vitro metabolism
studies indicate that we do not observe metabolic defluori-
nation allowing us to further investigate our 18F analogues
for in vivo distribution. It should be noted however, that we
will need to interpret our PET results carefully as de-fluorin-
ation may occur via other mechanisms than those restricted
to hepatic microsomes, and metabolic N-dealkylation prod-
ucts will result in fluorine containing metabolites.
Funding
This work was supported by Natural Sciences and Engineering Research
Council of Canada; Sylvia Fedoruk Canadian Centre for Nuclear
Innovation; College of Pharmacy and Nutrition Graduate Scholarship;
Department of Chemistry Graduate Scholarship.
References
Agundez JA, Garcia-Martin E, Alonso-Navarro H, Jimenez-Jimenez FJ.
2013. Anti-Parkinson’s disease drugs and pharmacogenetic considera-
tions. Expert Opin Drug Metab Toxicol. 9 (7):859–874.
Bar Am O, Amit T, Youdim MB. 2004. Contrasting neuroprotective and
neurotoxic actions of respective metabolites of anti-Parkinson drugs
rasagiline and selegiline. Neurosci Lett. 355 (3):169–172.
Bar-Am O, Weinreb O, Amit T, Youdim MB. 2010. The neuroprotective
mechanism of 1-(R)-aminoindan, the major metabolite of the anti-par-
kinsonian drug rasagiline. J Neurochem. 112 (5):1131–1137.
Barreto GE, Iarkov A, Moran VE. 2014. Beneficial effects of nicotine, coti-
nine and its metabolites as potential agents for Parkinson’s disease.
Front Aging Neurosci. 6:340.
Benowitz NL, Hukkanen J, and Jacob P. 3rd, 2009. Nicotine chemistry,
metabolism, kinetics and biomarkers. Handb Exp Pharmacol. 192:
29–60.
Berthou F, Ratanasavanh D, Alix D, Carlhant D, Riche C, Guillouzo A.
1988. Caffeine and theophylline metabolism in newborn and adult
human hepatocytes; comparison with adult rat hepatocytes. Biochem
Pharmacol. 37(19):3691–3700.
Conclusion
Berthou F, Ratanasavanh D, Riche C, Picart D, Voirin T, Guillouzo A. 1989.
Comparison of caffeine metabolism by slices, microsomes and hep-
atocyte cultures from adult human liver. Xenobiotica. 19(4):401–417.
Bianco G, Abate S, Labella C, Cataldi TR. 2009. Identification and frag-
mentation pathways of caffeine metabolites in urine samples via
liquid chromatography with positive electrospray ionization coupled
to a hybrid quadrupole linear ion trap(LTQ) and Fourier transform ion
cyclotron resonance mass spectrometry and tandem mass spectrom-
etry. Rapid Commun Mass Spectrom. 23 (7):1065–1074.
Bier D, Hartmann R, Holschbach M. 2013. Collision-induced dissociation
studies of caffeine in positive electrospray ionisation mass spectrom-
etry using six deuterated isotopomers and one N1-ethylated homo-
logue. Rapid Commun Mass Spectrom. 27(8):885–895.
Bonomi RE, Laws M, Popov V, Kamal S, Potukutchi S, Shavrin A, Lu X,
Turkman N, Liu RS, Mangner T, et al. 2018. A Novel Substrate
Radiotracer for Molecular Imaging of SIRT2 Expression and Activity
with Positron Emission Tomography. Mol Imaging Biol. 20(4):594–604.
Brandange S, Lindblom L. 1979. Enzyme Aldehyde Oxidase Is an Iminium
Oxidase - Reaction with Nicotine Delta-1’(5’)Iminium Ion. Biochem
Biophys Res Commun. 91(3):991–996.
Campbell ME, Grant DM, Inaba T, Kalow W. 1987. Biotransformation of
caffeine, paraxanthine, theophylline, and theobromine by polycyclic
aromatic hydrocarbon-inducible cytochrome(s) P-450 in human liver
microsomes. Drug Metab Dispos. 15(2):237–249.
In this study, we evaluated the metabolic stability of C8-6-N,
C8-6-I, C8-6-C8, 19F-[C8-6-I], 19F-[C8-6-C8] and 19F-[C8-6-N] in
HLM, MLM, and RLM. Accurate mass measurement and tan-
dem mass spectrometry were used to identify and elucidate
the structure of the corresponding metabolites of the bifunc-
tional compounds in HLM, MLM, and RLM. The caffeine moi-
ety of the tested compounds C8-6-N, C8-6-C8, 19F-[C8-6-N]
and 19F-[C8-6-C8] was stable to in vitro Phase 1 metabolism,
whereas the nicotine and aminoindan moieties underwent
hydroxylation, presumably as a result of cytochrome P450
mediated metabolism. The caffeine moiety of C8-6-I and 19F-
[C8-6-I] underwent N3 and N1 demethylation similar to N3
and N1 demethylation of caffeine to paraxanthine and theo-
bromine. Hydroxylation of the alkyl fluoride moiety was
observed for 19F-[C8-6-C8] and 19F-[C8-6-I] but not for 19F-[C8-
6-N] and since [C8-6-N] was observed to be extensively
metabolized, this suggests a greater metabolic susceptibility
of the nicotine than the alkyl fluoride linker. Whether this
could affect the use of 19F-[C8-6-N] in our PET studies
remains to be seen. No defluorinated metabolites were
observed for any of our 19F-bifunctional compounds indicat-
ing that the fluorination of the caffeine moiety with propyl
fluoride is suitable for the synthesis of 18F-bifunctional com-
pounds for PET imaging and evaluation of biodistribution in
animal studies. Finally, the same metabolites were observed
for all compounds in HLM, MLM, and RLM, suggesting that
mouse and rat may be useful surrogates for future animal
studies of these bifunctional compounds.
Chau KY, Cooper JM, Schapira AH. 2010. Rasagiline protects against
alpha-synuclein induced sensitivity to oxidative stress in dopamin-
ergic cells. Neurochem Int. 57(5):525–529.
Chen K, Chen X. 2010. Design and development of molecular imaging
probes. Curr Top Med Chem. 10(12):1227–1236.
^
Chenevert R, Mohammadi-Ziarani G, Caron D, Dasser M. 1999.
Chemoenzymatic enantioselective synthesis of (-)-enterolactone. Can J
Chem. 77(2):223–226.
Davie CA. 2008. A review of Parkinson’s disease. Br Med Bull. 86:
109–127.
de Biase S, Merlino G, Lorenzut S, Valente M, Gigli GL. 2014. ADMET con-
siderations when prescribing novel therapeutics to treat restless legs
syndrome. Expert Opin Drug Metab Toxicol. 10(10):1365–1380.
Deftereos SN, Dodou E, Andronis C, Persidis A. 2012. From depression to
neurodegeneration and heart failure: re-examining the potential of
MAO inhibitors. Expert Rev Clin Pharmacol. 5(4):413–425.
Disclosure statement
No potential conflict of interest was reported by the author(s)