Novel 3-O-pegylated carboxylate and 3-O-pegylated carbamate prodrugs of naltrexone for microneedle-enhanced transdermal delivery

Article abstract of DOI:10.1016/j.bmcl.2010.04.049

A small library of novel 3-O-pegylated carboxylate prodrugs (4a-4b) and 3-O-pegylated carbamate prodrugs (9a-9b) of naltrexone were synthesized. The goal behind the design of these prodrugs was to investigate their potential for microneedle-enhanced transdermal delivery. All the synthesized 3-O-pegylated carboxylate prodrugs (4a-4b) and 3-O-pegylated carbamate prodrugs (9a-9b) of naltrexone were found to have adequate stability in a transdermal formulation and improved apparent solubility compared to naltrexone. Viscosity effects were postulated to be responsible for the observed non-linearity in the flux-concentration profile of these prodrugs.

Full text of DOI:10.1016/j.bmcl.2010.04.049

Bioorganic & Medicinal Chemistry Letters 20 (2010) 3280–3283
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel 3-O-pegylated carboxylate and 3-O-pegylated carbamate prodrugs
of naltrexone for microneedle-enhanced transdermal delivery
Thirupathi Reddy Yerramreddy, Mikolaj Milewski, Narsimha Reddy Penthala, Audra L. Stinchcomb,
*
Peter A. Crooks
Department of Pharmaceutical Sciences, College of Pharmacy, University of Kentucky, Lexington, KY 40536, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A small library of novel 3-O-pegylated carboxylate prodrugs (4a–4b) and 3-O-pegylated carbamate pro-
drugs (9a–9b) of naltrexone were synthesized. The goal behind the design of these prodrugs was to inves-
tigate their potential for microneedle-enhanced transdermal delivery. All the synthesized 3-O-pegylated
carboxylate prodrugs (4a–4b) and 3-O-pegylated carbamate prodrugs (9a–9b) of naltrexone were found
to have adequate stability in a transdermal formulation and improved apparent solubility compared to
naltrexone. Viscosity effects were postulated to be responsible for the observed non-linearity in the
flux-concentration profile of these prodrugs.
Received 12 March 2010
Accepted 12 April 2010
Available online 18 April 2010
Keywords:
Naltrexone prodrugs
Alcohol dependence and opioid addiction
Microneedle-enhanced transdermal
delivery
Ó 2010 Elsevier Ltd. All rights reserved.
The opioid antagonist, naltrexone (Fig. 1, I, NTX) is used in the
treatment of opioid addiction and alcohol dependence.1,2 Naltrex-
one hydrochloride is available as a 50 mg oral tablet under the
trade name ReVia. Oral naltrexone therapy is associated with
numerous adverse gastrointestinal effects such as abdominal pain,
nausea and vomiting, and naltrexone undergoes extensive
first-pass metabolism leading to poor bioavailability (5–40%), thus
limiting its clinical utility.3,4 In addition, the major challenge in
naltrexone maintenance therapy has been the poor long-term pa-
tient compliance with therapy. Thus, naltrexone is the drug of
choice for only very highly motivated patients.1,5,6 To overcome
this problem, several depot injections of naltrexone have been clin-
ically investigated.5,7 However, this relatively invasive dosage form
requires the assistance of a health care professional. Hence, there is
a need to develop an alternative, efficient and less invasive naltrex-
one delivery system to overcome these disadvantages in naltrex-
one therapy. Recently, transdermal drug delivery has been
gaining greater popularity, and scientists have delivered numerous
lipophilic drug molecules successfully via passive patches utilizing
this route.8,9 Previous work from our laboratory has focused on 3-
O-alkyl carboxylate (Fig. 1, II) and 3-O-alkyl carbonate (Fig. 1, III)
prodrugs of naltrexone which were investigated as lipophilic trans-
dermal agents with improved physiochemical properties and in-
creased skin permeation profiles, with the goal of delivering a
therapeutic dose of naltrexone across human skin via a classical
transdermal patch.10–13
Very recently, microneedle-enhanced transdermal delivery of
drug molecules has been reported as a suitable drug delivery sys-
tem for highly water-soluble, hydrophilic molecules.14–17 Microm-
eter-size needles were shown to be long enough to penetrate the
skin, effectively breaching the stratum corneum barrier, but at
the same time being short enough to minimize pain sensation. In
general, this process results in up to several orders-of-magnitude
enhancement of transdermal drug transport.18,19
Previously, we have shown that although the creation of micro
channels in the skin after microneedle (MN) treatment did not
have a substantial effect on the delivery rates of the neutral
(free-base) form of naltrexone or 6-ß-naltrexol, an improvement
in the delivery of the salt form of these drugs was possible.20 In vi-
tro and in vivo animal studies proved the utility of the MN method
of enhancement showing approximately an order-of-magnitude
increase in the transdermal flux of highly water-soluble species
via MN delivery over that through classical transdermal skin patch
delivery.20,21 Subsequently, a first-in-human MN study demon-
strated that the combination of MN skin pretreatment and applica-
* Corresponding author. Tel.: +1 859 257 1718; fax: +1 859 257 7585.
E-mail address: pcrooks@email.uky.edu (P.A. Crooks).
Figure 1. Structures of naltrexone (I), 3-O-alkyl carboxylate prodrugs of naltrexone
(II), and 3-O-carbonate prodrugs of naltrexone (III).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.04.049

T. R. Yerramreddy et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3280–3283
3281
tion of four naltrexone hydrochloride patches afforded drug plas-
ma levels in the lower end of the targeted therapeutic range.22
It is known that covalent attachment of a polyethylene glycol
(PEG) unit or polymer to a drug molecule can substantially increase
aqueous solubility of hydrophobic drugs and proteins.23 In addi-
tion, PEGylation is a useful tool in the field of pharmaceutics for
improving drug properties, such as enhancing stability, modifying
pharmacokinetics, shielding labile molecules from proteolytic en-
zymes, or eliminating protein immunogenicity.24,25 An elevated
aqueous solubility was postulated to contribute to a moderate in-
crease in flux observed for some derivatives.26–28 Based on previ-
ous reports from our laboratory,20 it could be anticipated that an
increase in aqueous solubility achieved through PEGylation of nal-
trexone would favorably affect flux through MN-treated skin.
The above observations encouraged us to design and synthesize
a small library of novel, highly polar, more water-soluble 3-O-
pegylated carboxylate (Fig. 2, 4a–4b) and 3-O-pegylated carbamate
(Fig. 2, 9a–9b) prodrugs of naltrexone. These analogs had terminal
hydroxy, and terminal methoxy groups attached to the polyethyl-
ene glycol (PEG) moiety and were considered as hydrophilic trans-
dermal agents with suitable physiochemical properties for delivery
via MN-enhanced transdermal delivery.
Scheme 1. Reagents and conditions: (a) NaOH-GEB (GEB; gel-entrapped base),
t-BuOH, 50–55 °C, 24 h; (b) TFA, rt, 12 h; (c) Pd/C, H2, THF, 24 h; (d) DCC, DMAP,
CHCl3, rt, 12 h.
The synthetic routes to the 3-O-pegylated carboxylate prodrugs
of naltrexone, that is, 3-O-[3-(2-(2-methoxy-ethoxy/2-hydroxy-
ethoxy)ethoxy)propanoyl]naltrexone (4a–4b) are illustrated in
Scheme 1.
t-Butyl
3-[2-(2-methoxyethoxy/2-benzyloxyethoxy)ethoxy]
propanoates (2a–2b) were prepared by Michael addition of 2-[2-
(methoxy/benzyloxy)ethoxy]ethanols (1a–1b) with t-butyl acry-
late in the presence of NaOH-GEB catalyst (GEB; gel-entrapped
base) in t-butyl alcohol at 50–55 °C.29 Deprotection of 2a–2b with
trifluoroacetic acid (TFA) at room temperature afforded 3-[2-(2-
methoxyethoxy/2-benzyloxyethoxy)ethoxy]propanoic acids (3a
and 3a1). Debenzylation of 3a1 with Pd–C/H2 in THF afforded 3-
[2-(2-hydroxyethoxy) ethoxy]propanoic acid (3b). Condensation
of 3a–3b with naltrexone using DCC in chloroform at room tem-
perature afforded 3-O-[3-(2-(2-methoxyethoxy/2-hydroxyeth-
oxy)ethoxy)propanoyl]naltrexones (4a–4b).
N-[2-(2-(2-Hydroxyethoxy/2-methoxyethoxy)ethoxy)ethyl]ph-
thalimides (6a–6b) were prepared by the reaction of 1-chloro-2-
[2-(2-hydroxyethoxy/2-methoxyethoxy)ethoxy]ethanes (5a–5b)
with potassium phthalimide in DMF under reflux conditions for
10 h. Deprotection of 6a–6b with hydrazine hydrate in refluxing
ethanol for 3 h afforded 2-[2-(2-hydroxy-ethoxy/2-methoxyeth-
oxy)ethoxy]ethanamines (7a–7b). 3-O-(p-Nitrophenyloxycarbon-
yl)naltrexone (8) was prepared by the reaction of p-nitrophenyl
chloroformate with naltrexone in the presence of triethylamine
in chloroform at room temperature for 2 h. Condensation of
7a–7b with 3-O-(p-nitrophenyloxycarbonyl)naltrexone (8) in the
presence of DMAP in chloroform at room temperature for 12 h
afforded 3-O-[2-(2-(2-hydroxyethoxy/2-methoxyethoxy)ethoxy)
ethylcarbam-ic]naltrexones (9a–9b). All the synthesized com-
pounds were characterized by 1H NMR and 13C NMR spectrometry
and MS (ESI) analysis.31
Scheme 2. Reagents and conditions: (a) DMF, reflux, 10 h; (b) ethanol, N2H4ÁH2O,
reflux, 3 h; (c) CHCl3, TEA, rt, 2 h; (d) DMAP, CHCl3, rt, 12 h.
The synthetic routes to the synthesis of 3-O-pegylated carba-
mate prodrugs of naltrexone, that is, 3-O-[2-(2-(2-hydroxyeth-
oxy/2-methoxyethoxy)ethoxy)ethylcarbamic]naltrexone (9a–9b),
are illustrated in Scheme 2.
The apparent solubilities of naltrexone and the four naltrexone
prodrugs were assessed at pH 5.0 in 0.3 M acetate buffer by equi-
librium of an excess quantity of drug/prodrug in 6 mL of buffer. At
pH 5.0, naltrexone and the naltrexone prodrugs are expected to be
fully ionized (highly soluble) and the buffer concentration of 0.3 M
allows maintenance of this pH throughout the duration of an
in vitro diffusion study. The solubility data is shown in Table 1.
Adequate stability is one of the crucial characteristics of a pro-
drug. In the case of the microneedle-targeted route of delivery, sta-
bility in aqueous buffers at skin and physiological pHs needs to be
established. The hydrolysis of both 3-O-pegylated carbamate pro-
drugs (9a–9b) and 3-O-pegylated carboxylate prodrugs (4a–4b)
Figure 2. Structures of 3-O-pegylated carboxylate (4a–4b), and 3-O-pegylated
carbamate (9a–9b) prodrugs of naltrexone.

3282
T. R. Yerramreddy et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3280–3283
Table 1
for the observed non-linear relationship between transdermal flux
values and prodrug concentration.30
Solubility of naltrexone prodrugs and naltrexone at pH 5.0 in 0.3 M acetate buffer
Analog
Solubility (in mM)
Relative solubilitya
Acknowledgment
4a
4b
9a
9b
629
952
1035
796
1.86
2.81
3.06
2.35
—
This work was supported by NIH Grant R01 DA13425.
NTX
338
References and notes
a
Relative solubility represents solubility of naltrexone prodrug divided by sol-
ubility of NTX.
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Hydrolysis rate constants of naltrexone prodrugs in acetate buffer at pH 5.0 and in
HEPES-buffered Hanks’ balanced salt solution at pH 7.4
Analog Acetate buffer solution
HEPES-buffered Hanks’ balanced salt
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9c
9d
0.00193
0.00184
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0.00111
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of naltrexone was investigated in acetate buffer at pH 5.0, and in
HEPES-buffered Hanks’ balanced salt solution at pH 7.4. The
hydrolysis of the labile prodrug moieties followed pseudo first-or-
der kinetics, and gave apparent hydrolysis rate constants. The
hydrolysis rate constant values (kpH 5.0 in acetate buffer; kpH 7.4
in HEPES-buffered Hanks’ balanced salt solution [hÀ1]) obtained
for the naltrexone prodrugs are illustrated in Table 2. The chemical
hydrolysis rates of the four prodrugs were sufficiently low to pro-
duce less than 10% hydrolysis in the donor compartment through-
out the 48 h duration of the diffusion study. Moreover, all four
prodrugs were hydrolyzed rapidly to naltrexone at physiological
pH (7.4) with the 3-O-carbamate naltrexone prodrug hydrolysis
rates being approximately six times faster than the corresponding
3-O-carboxylate naltrexone prodrug hydrolysis rates.
The performance of 4a in in vitro diffusion experiments
employing a transdermal microneedle formulation has been re-
ported elsewhere.30 Briefly, full thickness Yucatan minipig skin
(1.7 mm thickness, diffusion area 0.95 cm2) was utilized, and
pierced with solid metal, 750 mm-long microneedles before
mounting the skin in a PermeGear flow-through diffusion cell sys-
tem at 32 °C. Substantial non-linearity in the flux-concentration
profile of 4a was observed, ultimately resulting in decreased trans-
port rates. It was observed that the viscosity of the donor solution
of 4a increased exponentially as a function of prodrug concentra-
tion. It has been suggested that changes in the viscosity properties
of the donor solution may have a detrimental effect on delivery
rates of drugs through MN-treated skin.32 Thus, it is postulated
that the non-linearity of the flux of 4a with increasing concentra-
tion is caused by elevated viscosity of the donor solution. The other
prodrugs reported herein, that is, 4b, 9a and 9b, showed similar
behavior to 4a (data not published).
27. Thomas, J. D.; Majumdar, S.; Sloan, K. B. Molecules 2009, 14, 4231.
28. N’Da, D. D.; Breytenbach, J. C. J. Pharm. Pharmacol. 2009, 61, 721.
29. Chaphekar, S. S.; Samant, S. D. Appl. Catal. A: General 2003, 242, 11.
30. Mikolaj, M.; Thirupathi Reddy, Y.; Priyanka, G.; Crooks, P. A.; Stinchcomb, A. L. J.
Controlled Release 2010, in press.
31. Analytical data and yields of the all the synthesized compounds: (2a): 1H NMR
(CDCl3): d 1.43 (s, 9H), 2.48 (t, 2H), 3.57 (s, 3H), 3.56 (t, 2H), 3.61–3.65 (6H),
3.69 (t, 2H); 13C NMR (CDCl3): d 28.5, 33.5, 59.1, 66.2, 70.1, 70.2, 70.4, 71.5,
81.9, 172.9; MS (ESI) m/z: 249 (MH+); Yield: 92%: (2b): 1H NMR (CDCl3): d 1.39
(s, 9H), 2.43 (t, 2H), 3.55–3.73 (m, 10H), 4.72 (s, 2H), 7.32–7.43 (m, 5H); 13C
NMR (CDCl3): d 28.6, 33.3, 66.7, 70.0, 70.1, 70.3, 72.8, 82.0, 127.3, 127.4, 128.6,
137.5, 173.2; MS (ESI) m/z: 325 (MH+); Yield: 97%: (3a): 1H NMR (CDCl3): d
2.61 (t, 2H), 3.38 (s, 3H), 3.59 (t, 2H), 3.63–3.66 (m, 6H), 3.76 (t, 2H), 10.82 (s,
1H); 13C NMR (CDCl3): d 34.6, 59.5, 66.2, 70.1, 70.2, 70.4, 70.6, 177.2; MS (ESI)
m/z: 193 (MH+); Yield: 94%: (3a1): 1H NMR (CDCl3): d 2.58 (t, 2H), 3.56–3.76
(m, 10H), 4.54 (s, 2H), 7.24–7.33 (m, 5H), 11.18 (s, 1H); 13C NMR (CDCl3): d
33.9, 66.5, 70.0, 70.2, 70.4, 73.1, 127.5, 127.6, 128.5, 137.3, 177.3; MS (ESI) m/z:
269 (MH+); Yield: 95%: (3b): 1H NMR (CDCl3): d 2.56 (t, 2H), 3.56–3.62 (m, 6H),
3.63–3.76 (m, 4H), 7.12 (br s, 2H); 13C NMR (CDCl3): d 34.6, 61.3, 66.4, 70.1,
70.3, 70.4, 177.3; MS (ESI) m/z: 179 (MH+); Yield: 98%: (4a): 1H NMR (CDCl3): d
0.15–0.18 (m, 2H), 0.55–0.58 (m, 2H), 0.87 (m, 1H), 1.57–1.67 (m, 3H), 1.86 (m,
1H), 2.13 (m, 1H), 2.28–2.42 (m, 4H), 2.87 (t, 2H), 2.96–3.20 (m, 2H), 3.38 (s,
3H), 3.55–3.66 (m, 8H), 3.87, (t, 2H), 4.68 (s, 1H), 6.67 (d, J = 8.4 Hz, 1H), 6.82
(d, J = 8.4 Hz, 1H); 13C NMR (CDCl3): d 3.98, 4.22, 9.44, 23.07, 30.71, 31.05,
31.26, 34.92, 34.96, 36.12, 43.51, 50.66, 59.15, 59.28, 61.82, 69.73, 63.75, 66.42,
66.45, 69.06, 70.05, 70.25, 70.33, 70.34, 70.43, 70.53, 71.62, 72.53, 90.52,
119.33, 122.84, 129.98, 130.21, 132.24, 147.46, 169.01, 207.63; MS (ESI) m/z:
516 (MH+); Yield: 86%: (4b): 1H NMR (CDCl3): d 0.12–0.16 (m, 2H), 0.51–0.57
(m, 2H), 0.88 (m, 1H), 1.55–1.66 (m, 3H), 1.85 (m, 1H), 2.16 (m, 1H), 2.25–2.40
(m, 4H), 2.85 (t, 2H), 2.99–3.21 (m, 2H), 3.58–3.67 (m, 8H), 3.72, (t, 2H), 4.67 (s,
1H), 6.65 (d, J = 8.1 Hz, 1H), 6.83 (d, J = 8.1 Hz, 1H); 13C NMR (CDCl3): d 3.93,
4.19, 9.43, 23.04, 30.72, 31.03, 31.25, 34.90, 34.98, 36.13, 43.53, 50.65, 59.17,
61.70, 61.81, 69.72, 63.77, 66.40, 66.47, 69.05, 70.03, 70.24, 70.32, 70.35, 70.45,
In conclusion, 3-O-pegylated carboxylate prodrugs of naltrexone
(4a–4b) and 3-O-pegylated carbamate prodrugs of naltrexone (9a–
9b) have been synthesized and fully characterized (1H NMR, 13C
NMR, and mass spectroscopy).31 These prodrugs had higher aqueous
solubilities and showed an approximately 2–3-fold enhancement in
their aqueous solubilities over naltrexone. In stability studies, these
prodrugs of naltrexone were hydrolyzed rapidly at physiological pH.
The 3-O-pegylated carbamate prodrugs had hydrolysis rates that
were approximately six times faster than the hydrolysis rates of 3-
O-pegylated carboxylate prodrugs in pH 7.4 HEPES-buffered Hanks’
solution. Viscosity effects may be a confounding factor responsible

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3283
70.51, 72.52, 90.54, 119.31, 122.82, 129.99, 130.23, 132.25, 147.48, 169.03,
207.61; MS (ESI) m/z: 502 (MH+); Yield: 85%: (6a): 1H NMR (CDCl3): d 2.20 (s,
1H), 3.43 (t, 2H), 3.50–3.57 (m, 6H), 3.65 (t, 2H), 3.82 (t, 2H), 7.60–7.64 (m, 2H),
7.74–7.78 (m, 2H); 13C NMR (CDCl3): d 38.3, 61.1, 65.5, 70.3, 70.5, 70.6, 123.5,
123.7, 132.1, 132.3, 167.6; MS (ESI) m/z: 280 (MH+); Yield: 69.7%: (6b): 1H
NMR (CDCl3): d 3.34 (s, 3H), 3.41 (t, 2H), 3.52–3.59 (m, 6H), 3.63 (t, 2H), 3.81 (t,
2H), 7.59–7.62 (m, 2H), 7.73–7.76 (m, 2H); 13C NMR (CDCl3): d 38.2, 59.5, 65.2,
70.2, 70.4, 70.5, 71.6, 123.6, 123.7, 132.0, 132.1, 167.8; MS (ESI) m/z: 294
(MH+); Yield: 78%: (7a): 1H NMR (CDCl3): d 2.61 (br s, 3H), 2.72 (t, 2H), 3.37–
3.58 (m, 10H); 13C NMR (CDCl3): d 41.4, 61.5, 70.0, 70.2, 70.4, 72.6; MS (ESI) m/
z: 150 (MH+); Yield: 76%: (7b): 1H NMR (CDCl3): d 1.69 (br s, 2H), 2.73 (t, 2H),
3.26 (s, 3H), 3.38–55 (m, 10H); 13C NMR (CDCl3): d 41.7, 59.5, 70.1, 70.3, 70.5,
71.3, 72.8; MS (ESI) m/z: 164 (MH+); Yield: 83%: (8): 1H NMR (CDCl3): d 0.11–
0.17 (m, 2H), 0.53–0.59 (m, 2H), 0.90 (m, 1H), 1.58–1.64 (m, 3H), 1.88 (m, 1H),
2.18 (m, 1H), 2.36–2.44 (m, 4H), 2.60 (m, 2H), 3.0–3.19 (m, 2H), 4.78 (s, 1H),
6.73 (d, J = 8.1 Hz, 1H), 7.00 (d, J = 8.4 Hz, 1H), 7.53 (dd, J = 8.6 Hz, 2H), 8.29 (dd,
J = 9.0, 1.5 Hz, 2H); 13C NMR (CDCl3): d 4.08, 4.39, 9.43, 23.18, 30.72, 31.45,
34.91, 36.19, 44.2, 50.68, 59.23, 61.83, 63.75, 72.58, 90.58, 119.38, 122.59,
129.89, 130.33, 132.35, 147.45, 148.78, 207.33; MS (ESI) m/z: 507 (MH+); Yield:
85%: (9a): 1H NMR (CDCl3): d 0.14–0.18 (m, 2H), 0.53–0.59 (m, 2H), 0.88 (m,
1H), 1.57–1.66 (m, 3H), 1.87 (m, 1H), 2.10 (m, 1H), 2.28–2.47 (m, 4H), 2.68 (m,
2H), 3.02–3.23 (m, 2H), 3.45–3.48 (m, 2H), 3.62–3.68 (m, 10H), 4.72 (s, 1H),
6.05 (br s, NH), 6.57 (d, J = 8.4 Hz, 1H), 6.94 (d, J = 8.4 Hz, 1H); 13C NMR (CDCl3):
d 4.05, 4.37, 9.42, 23.21, 30.72, 31.53, 36.30, 38.89, 41.34, 43.53, 50.63, 59.29,
61.84, 61.99, 69.06, 69.98, 70.21, 70.45, 70.51, 72.71, 90.71, 119.43, 122.89,
128.83, 130.13, 132.28, 147.38, 154.02, 207.98; MS (ESI) m/z: 517 (MH+); Yield:
73%: (9b): 1H NMR (CDCl3): d 0.10–0.11 (m, 2H), 0.49–0.52 (m, 2H), 0.86 (m,
1H), 1.54–1.65 (m, 3H), 1.86 (m, 1H), 2.11 (m, 1H), 2.26–2.46 (m, 4H), 2.69 (m,
2H), 3.01–3.22 (m, 2H), 3.35 (s, 3H), 3.38–3.41 (m, 2H), 3.54–3.62 (m, 10H),
4.66 (s, 1H), 5.95 (br s, NH), 6.62 (d, J = 8.1 Hz, 1H), 6.86 (d, J = 8.1 Hz, 1H); 13C
NMR (CDCl3): d 4.02, 4.35, 9.40, 23.23, 30.73, 31.52, 36.31, 38.87, 40.92, 43.51,
50.62, 59.27, 59.34, 61.97, 69.065 69.97, 70.22, 70.43, 70.53, 71.61, 72.72,
90.73, 119.42, 122.87, 128.81, 130.11, 132.26, 147.37, 153.43, 207.96; MS (ESI)
m/z: 531 (MH+); Yield: 78%.
32. Mikolaj, M.; Stinchcomb, A. L. Pharm. Res. 2010, in press.