Nonenzymatic hydrolysis of cocaine via intramolecular acid catalysis

Article abstract of DOI:10.1002/(SICI)1522-2675(19990113)82:1<85::AID-HLCA85>3.0.CO;2-Q

The spontaneous hydrolysis of the methyl-ester group of cocaine (1) in vivo contributes to the metabolic clearance of the drug in man. Neighboring- group participation by the tropane N-atom of cocaine in this hydrolysis was suggested by the normal stability of the methyl-ester groups of pseudococaine and N-acylnorcocaine. For cocaine, the relative rate of methyl-ester to benzoyl-ester hydrolysis was ca. 10:1 at pH ≤ 7.4, and, although absolute rates increased with increasing pH, their ratio collapsed at pH > pK(a) (8.6). These data are consistent with intramolecular acid catalysis of alkaline hydrolysis of the cocaine methyl-ester group under physiologic conditions.

Full text of DOI:10.1002/(SICI)1522-2675(19990113)82:1<85::AID-HLCA85>3.0.CO;2-Q

Helvetica Chimica Acta ± Vol. 82 (1999)
85
Nonenzymatic Hydrolysis of Cocaine via Intramolecular Acid Catalysis
by Pan Li and Kang Zhao*
Department of Chemistry, New York University, New York, NY 10003, USA
and Shixian Deng and Donald W. Landry*
Department of Medicine, Columbia University, College of Physicians & Surgeons,
New York, NY 10032, USA
The spontaneous hydrolysis of the methyl-ester group of cocaine (1) in vivo contributes to the metabolic
clearance of the drug in man. Neighboring-group participation by the tropane N-atom of cocaine in this
hydrolysis was suggested by the normal stability of the methyl-ester groups of pseudococaine and N-
acylnorcocaine. For cocaine, the relative rate of methyl-ester to benzoyl-ester hydrolysis was ca. 10:1 at pH ꢀ
7.4, and, although absolute rates increased with increasing pH, their ratio collapsed at pH > pKa (8.6). These
data are consistent with intramolecular acid catalysis of alkaline hydrolysis of the cocaine methyl-ester group
under physiologic conditions.
The metabolic clearance of cocaine ([1R-(exo,exo)]-3-(benzoyloxy)-8-methyl-8-
azabicyclo[3.2.1]octane-2-carboxylic acid methyl ester; 1) in vivo is predominantly
achieved by the hydrolysis of its benzoyloxy and methyl-ester moieties (Scheme 1). The
benzoyl hydrolysis pathway (Path a), which converts cocaine to ecgonine methyl ester
(2) and benzoic acid (4), is the better characterized and is found to require the
mediation of hepatic or plasma esterases [1]. Methyl-ester hydrolysis, which yields O-
benzoylecgonine (3) and methanol (5) (Path b) [2], can be catalyzed by a human liver
carboxylesterase (hCE-1) [2a ± c], but is also believed to occur spontaneously under
physiological conditions (pH 7.4; 378) [2d ± e]. Some have attributed this putative
lability of the methyl-ester moiety of cocaine (1) to alkaline factors or other artifact of
sample handling [3]. Since the majority of commercially available methyl esters are
quite stable under neutral conditions, it is fundamentally interesting to confirm and
investigate the instability of the methyl-ester group of cocaine. Also, a particular
interest in this hydrolysis stems from our ongoing investigation of antibody-catalyzed
hydrolysis of cocaine at the benzoyloxy moiety as the basis for a new class of anti-
addiction therapeutic agents [4]. An understanding of the mechanism of cocaine
methyl-ester hydrolysis could guide the development of more effective transition-state
analogs for the conversion of cocaine (1) to the biologically inactive product 2 [5].
Finally, the design of transition-state analogs for the antibody-catalyzed hydrolysis of
the methyl-ester group of 1, a process that is intrinsically more difficult due to the small
size of the methyl-alcohol epitope, clearly requires an understanding of the mechanism
for spontaneous hydrolysis.
In this paper, investigations on the hydrolysis kinetics of the ester moieties of
cocaine (1) under physiologic conditions and at a variety of pH values are reported. The
close proximity of the methyl ester of cocaine to the tropane N-atom, and the

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Helvetica Chimica Acta ± Vol. 82 (1999)
Scheme 1. Cocaine Hydrolysis
possibility of a proximity effect [6], led us to also study several related compounds in
which either the substituents on the tropane N-atom were varied or the stereochemical
relationship of the N-atom and the ester group was altered.
Sodium phosphate buffer (100 mm) with a specific pH value was mixed with cocaine
(1) and its homologs 6 ± 8, respectively. Aliquots were analyzed by high-performance
liquid chromatography (HPLC) immediately, and at intervals after incubation at 378.
HPLC peak integrals were measured to give the concentrations of substrate and
products and, the rate constant ratios (kmet/kben) of methyl-ester and benzoyloxy-group
hydrolysis were then obtained. A linear relationship for ln[Cs] vs. time indicated
pseudo-first order kinetics for the hydrolysis of cocaine and its homologs (data not
shown). Since kall can be approximated as the sum of kmet and kben for the initial
hydrolysis, the individual kmet and kben can be then calculated as shown in the Table.
Table. Rate Constants of Ester Hydrolysis for Cocaine (1) and Homologs 6 ± 8 at pH 7.4 and 378
Compound
kall [h 1]a)
t1/2 [h]
kmet [h 1]b)
kben [h 1]c)
kmet/kben
1
6
7
8
0.0562
0.0176
0.00457
0.00108
12.33
39.29
151.6
641.8
0.0509
0.0146
0.0216
0.00036
0.00536
0.00306
0.00241
0.00072
9.5
4.8
0.9
0.5
a) Rate constant of overall hydrolysis. b) Rate constant of methyl-ester hydrolysis. c) Rate constant of
benzoyloxy hydrolysis.
Cocaine (1) hydrolyzed significantly faster at pH 7.4 than its homologs 7 and 8, and
the difference was due primarily to the relatively high rate constant of the methyl-ester
hydrolysis (kmet) (Table). Norcocaine (6), another tropane in which the N-atom and
methoxycarboxyl group are proximate, also displayed a ratio of kmet/kben greater than 1.
In contrast, pseudococaine (7), which possesses an equatorial methoxycarboxyl group,

Helvetica Chimica Acta ± Vol. 82 (1999)
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and N-acetylnorcocaine (8) which possesses a nonbasic N-atom gave comparable kmet
/
kben ratios. These data are consistent with an intramolecular participation of the tropane
N-atom, as an amine or ammonium salt, in the methyl-ester hydrolysis of cocaine (1).
Unlike the hydrolytic behavior of trans- and cis-aryl 2-aminocyclohexanecarboxylates
which showed little steric influence on the rates of hydrolysis [6e], the methyl-ester
group of pseudococaine (7) was not approachable by the tropane N-atom due to a
geometric constraint.
The rate constants for hydrolysis of the methyl-ester and benzoyloxy groups of
cocaine (1) were determined (Fig.) and increased as pH was increased from 6.9 to 10.4,
as expected for alkaline hydrolysis. At any given pH, the ratio kmet/kben exceeded 1, but
the magnitude of this ratio declined toward 1 at pH values greater than the pKa of 1
(8.6), suggesting that the ammonium salt of the tropane participated in the step
determining kmet. In contrast, for pseudococaine (7), the magnitude of kmet equaled that
of kben throughout the neutral to alkaline pH range for these reactions, and the
proportionally increasing rates of hydrolysis with increasing pH are consistent with
simple alkaline hydrolysis of these esters.
Figure. a) pH Dependence of kmet/kben for cocaine (1) (d) and pseudococaine (7) (s). b) pH Dependence of
kmet (m) and kben (&) for cocaine (1) and kmet (n) and kben (u) for pseudococaine (7). I ˆ 0.1 m (NaCl).
Thus, the methyl-ester group of cocaine (1) is, indeed, much more labile than that of
the benzoyloxy group under neutral conditions. The hydrolysis of the methyl-ester
group of 1 can be assumed to involve intramolecular acid catalysis as shown in
Scheme 2. This intramolecular catalysis would explain the relative lability of the
methyl-ester group of cocaine (1) and norcocaine (6) compared to their benzoyloxy
groups. The absence of such catalysis was also implied in the hydrolysis of the methyl-
ester groups of pseudococaine (7) and N-acetylnorcocaine (8). This analysis is useful
for the design of a new class of analogs to mimic the hypothesized transition state 10 for
cocaine hydrolysis. We are now seeking to determine whether the monoclonal

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Helvetica Chimica Acta ± Vol. 82 (1999)
Scheme 2. A Rationale for the Rapid Hydrolysis of the Methyl-Ester Group of Cocaine (1) under Physiologic
Conditions
antibodies elicited by such analogs are catalytic [4]1) and can promote the substrate-
assisted degradation of cocaine.
Support of the Office of National Drug Control Policy (to D.W.L.) and the NSF Faculty Early Career
Development Program (to K.Z.) are gratefully acknowledged. P.L. acknowledges the New York University
Deanꢀs Dissertation Fellowship.
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1
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For selected examples of monoclonal-antibody-catalyzed hydrolysis of esters, see [7].

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Received August 21, 1998