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synthesis of a vast number of structurally diverse lipophilic deriva-
tives with marked antiepileptic activity [6–13]. It has been found
that substances crossing the BBB through many pathways (Trans-
membrane diffusion, Carrier mediated-transport and transcytosis).
The use of transporters or precursors is one of the common path-
way that is used to breech the BBB and among these trasporters
L-type amino acid transporters (LAT1) have perhaps been most
common (▶Fig. 1) [13].
this can actually enlighten the path to researchers for the develop-
ment of novel therapeutics that can utilize LAT1 as a drug delivery
platform (▶Fig. 2).
A number of literatures are available which support the design
of current nipecotic acid prodrug [13, 17–24].
On the basis of above concepts, we have arrived on the conclu-
sion for the design of nipecotic acid-L-serine-conjugated amino
acid (▶Fig. 3)
The most common amino acid with highest affinity for LAT1 is
the phenylalanine, apart from this LAT1 also transports over 10
other large neutral amino acids [14] and to a lesser extent small
neutral amino acids [15].
Experimental
Chemistry
Basic pharmacophoric requirement for LAT1 affinity
All the chemicals and solvents used for the synthesis were of ana-
lytical grade. Melting points were determined using open capillary
tubes on a Stuart Melting Point apparatus (SMP10) and were un-
corrected. The reaction progress was monitored by performing thin
layer chromatography (TLC) on a precoated Merck silica gel 60F254
aluminum sheets (Merck, Germany). The visualization of TLC was
done using UV cabinet (254 nm) or iodine vapors. IR spectroscopy
was performed using FT-IR spectrophotometer Shimadzu 8400S;
oily products were analyzed in the form of films, and solid com-
pounds were analyzed as KBr pellets. The results of FT-IR spectros-
copy were recorded as % Transmittance vs. Wavenumber (cm-1).
1H-NMR spectra (500 MHz) were recorded using Bruker Advance
spectrophotometer using TMS as an internal standard. Elemental
analysis of the synthesized derivatives (C,H,N,O) were performed
using Exeter CE-440 Elemental Analyzer and the results obtained
were within±0.4% of the theoretical values. Mass spectra were ob-
tained on a Hewlett Packard model GCD-1800A Electron Impact
mass spectrometer at 70 eV ionizing beam and using direct inser-
tion probe.
In ▶Fig. 1, the model of the binding site of the cerebrovascular
LAT1 transporter has been shown (▶Fig. 1). It is already proven that
for a substrate to have affinity for LAT1, it must contain a) an un-
substituted, free carboxyl group, b) an unsubstituted α-primary
amino group, c) either a H or CH3 on the α-carbon and d) a neutral,
uncharged side chain with hydrophobic bulk.
Design consideration
LAT-1 is important because it transports several prescription drugs,
such as the antiparkinsonian drug L-dopa and the anticonvulsant
gabapentin, across the BBB, thereby enabling their pharmacologic
effects [10, 11]. This function at the BBB has made LAT-1 a target
for drug delivery by modifying CNS-impermeable drugs such that
they become LAT-1 substrates and have enhanced BBB penetration
[12–15].
A vast number of structurally diverse lipophilic derivatives of
nipecotic acid with marked antiepileptic activity like SKF 89976A
[6, 8,16], NO-328 (now marketed as tiagabine) [10], Cl966 [11, 12]
and N-(mono)- or N-(diaryl methoxy) alkyl derivatives [13] have
been characterized as potent in vitro GABA uptake inhibitors along
with demonstration of their antiepileptic activity in several in vivo
rodent models (▶Fig. 2).
Reaction Scheme
Synthesis of nipecotic acid Prodrug
N-Boc-Nipecotic Acid (Piperidine-1,3-dicarboxylic acid
1-tert-butyl ester) (2)
It was well said that the identification of novel LAT1 substrates
eg. gabapentine, paroxetine, clomipramine, leucine and duloxetine
may focus on the mechanism through which drugs enter the brain,
Synthesis of N-Boc nipecotic acid was according to the literature
[19,25] (▶Fig. 4). Nipecotic acid (1) 1.0 gm was dissolved in dioxane
(10.7mL) containing NaOH solution (1 N, 9.6ml) and stirred for 5 h,
with the addition of di-tert-butyl-dicarbonate (1.9 g). The solvent was
evaporated, and the resulting aqueous mixture was separated with
ethyl acetate (15 mL). Then, the pH of the resulting solution was
brought down to 2.0 by the use of 1 N HCl solution with vigorous stir-
ring. The layers were separated, and the aqueous layer was extracted
with EtOAc (3×10mL). The combined EtOAc extracts were dried over
Na2SO4, filtered, and concentrated. Yield: 1.27 g, 71% as white solid:
IR (KBr, vmax cm-1): 3450-3400 (-OH Str.), 2890 (C-H Str.), 1730 (C=O
Str.), 1242 (C-O Str.), 1091 (C-O Str.), 935 (-OH Bend). 1H NMR (500
MHz, CDCl3) δ ppm: δ 10.63 (s, 1H, -OH-carboxylic acid); 4.10–3.57
(m, 2H, 2-H), 3.34–3.10 (m, 2H, 6-H), 2.50–2.30(m, 1H, 3-H), 1.73–
1.65 (m, 2H, 4-H), 1.50–1.45(m, 2H, 5-H), 1.30–1.20 (m, 9H, -Boc).
MS (m/z): 230.11 (M+1); Anal. calc. for C11H19NO4, C, 57.62; H, 8.35;
N, 6.11; O, 27.91; Found: C, 57.02; H, 8.01; N, 6.00; O, 27.32.
▶Fig. 1 Basic Pharmacophore.
Dhanawat M et al. L-Serine-Nipecotic Acid-prodrug… Drug Res 2021; 71: 94–103 | © 2020. Thieme. All rights reserved.
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Dhanawat, Meenakshi
