Field of invention
The present invention relates to stable tris(hydroxymethyl) amino methane salts of labile
Angiotensin-Converting Enzyme (ACE) Inhibitors, process for preparation and
pharmaceutical formulations thereof.
Background of the invention
Cyclization of dipeptides to diketopiperazine impurity (Scheme A) is well documented in the
literature [J. Org. Chem. (1983) 48, 2295; J. Pharm. Sci. (1998) 87, 283; J. Chem Soc.
Perkin Trans.2 (1973), 13, 1845 - 1852; Science (1981), 213, 544 - 545; and the references
cited therein].

Angiotensin-Converting Enzyme (ACE) Inhibitors, a class of drug molecules effective in
treatment of various cardiovascular dysfunctions, are also dipeptides, and hence are prone
to get cyclized easily to diketopiperazine impurity. As a matter of fact, formation of
diketopiperazine impurity is a major stability issue of concern to the potent ACE inhibitors
like enalapril, moexipril, lisinorpil, perindopril, ramipril, quinapril, etc.
Quinapril hydrochloride (I) (CAS No. 82586-55-8) is chemically known as (3S)-2-[(2S)-2-
[[(1S)-1-(Ethoxycarbonyl)-3-phenylpropyl]amino]-1-oxopropyl]-1,2,3,4-tetrahydro-3-
isoquinoline carboxylic acid hydrochloride, also known as (S)-2-[(S)-N-[(S)^-carbox.y-3-
phenylpropyl] alanyl]-1,2,3,4 -tetrahydro-3-isoquinolinecarboxylic acid 1-ethyl ester
hydrochloride
2


Quinapril hydrochloride is commercially available in a pharmaceutical composition under the
brand names ACCUPRIL® (Parke-Davis); ACCUPRO® (Pfizer); ACEQUIN® (Recordati);
ACUITEL® (Pfizer); KOREC® (Sanofi-Aventis); QUINAZIL® (Malesci) and is covered under
US 5401766 and US 4344949.
Ramipril(ll), [CAS: 87333-19-5] (2S,3aS,6aS) -1- [(S) -2- [[(S) -1- (ethoxycarbonyl)-3-
phenylpropyl] -amino] propanoyl] octahydrocyclopenta[b]pyrrole-2-carboxylic acid,
represented by the formula (II) is another valuable angiotensin-converting enzyme (ACE)
inhibitor, a family of drugs used to treat high blood pressure and some types of heart failure.

Ramipril (II) is commercially available in a pharmaceutical composition under the brand
names ALTACE® (Aventis).
Ramipril of formula (II) is disclosed in US 5,061,722 (assigned to Hoechst AG) which
describes a process for the preparation of ramipril comprising condensation of benzyl
cis,endo-2-azabicyclo-[3.3.0]-octane-3-S-carboxylate hydrochloride with N-(1-S-carbethoxy-
3-phenyl-propyl)-S-alanine in presence of a coupling agent such as dicyclohexylcarbodiimide
in an organic solvent such as dimethylformamide. The diastereomeric mixture of (S,S,S,S,S)
and (R,R,R,S,S) isomers of ramipril benzyl ester thus obtained is separated at this stage by
silica gel chromatography using ethyl acetate/ petroleum ether as the eluting solvent. The
optically pure (S,S,S,S,S) benzyl ester is deprotected by hydrogenolysis or treatment with an
acid or base to give ramipril(ll).
US Patent No. 4,344, 949 (Hoefle et. al.) inter alia describes a process for the preparation of
quinapril hydrochloride comprising reaction of ethyl ester of (S,S)-a-[(l- carboxyethyl) amino]
phenylbutanoic acid with the benzyl or t-butyl ester of (S) -1, 2,3, 4-tetrahydro-3-isoquinoline
carboxylic acid in presence of 1-hydroxy benzotriazole, employing standard peptide bond
3

forming methods. The benzyl or t-butyl ester group of quinapril thus obtained is removed by
catalytic hydrogenation or by treatment with trifluoroacetic acid. Quinapril hydrochloride is
isolated either by solvent precipitation with diethyl ether or by lyophilisation of the aqueous
solution.
Other methods for preparation of quinapril hydrochloride are also disclosed by Bartonet et al.
in GB Patent No. 2,095, 252, by Patchett et. al. in EP Patent No. 0,065, 301.
However, all these reported methods for synthesis of quinapril suffer from a serious
drawback in that the product obtained by all the methods is invariably contaminated with
varying amounts of an impurity identified as the diketopiperazine impurity (DKP impurity) of
formula(lll), leading, most often to product not conforming to the pharmacopoeial
specifications.

The diketopiperazine impurity is formed either during removal of the carboxylic acid
protective group or it could be formed during drying of quinapril hydrochloride. The said
impurity once formed is difficult to remove by conventional separation techniques, including
fractional crystallization.
Ramipril is also prone to form the diketopiperazine impurity (IV) under heating and in solution
at various pH1.

The problem was somewhat tackled by forming the acid or base addition salts in case of
various ACE inhibitors like enalapril, moexipril, perindopril, trandolapril etc. The tools used to
stabilize other ACE inhibitors were found to be unsuccessful in case of quinapril as over a
period of time the quinapril or its salts start getting cyclized to diketopiperazine impurity or
hydrolyzed to diacid.
4

The product obtained by employing the process described in US Patent No. 4,344,949 for
quinapril hydrochloride results in an amorphous solid. Guo Y. et al. in Journal of
Pharmaceutical Sciences, (2000) 89, 128-143 describe stability and chemical degradation of
quinapril hydrochloride. Due to the amorphous nature quinapril hydrochloride it gets readily
converted to diketopiperazine impurity at 80°C. Studies on crystalline quinapril hydrochloride
acetonitrile complex shows initial evaproration of solvent at 60°C and further cyclization to
diketopiperazine impurity by loss of water and hydrogen chloride during heating of samples
of quinapril hydrochloride.
During conversion of quinapril hydrochloride to diketopiperazine impurity in solid stage
escape of hydrogen chloride is a rate limiting stage and which can be achieved by heating
the solid, where as in case of solution phase, formation of quinapril zwitterions is a rate
limiting stage, which can be achieved by increasing the pH of the solution2. The schematic
representation of various steps involved in formation of diketopiperazine impurity as
described in Li J. et al. (2002)2 is provided in scheme I.

Some of the efforts taken in past to stabilize quinapril or quinapril salts are discussed briefly
below:
US Patent No. 4,743,450 (Harris et al.) relates to pharmaceutical compositions containing
ACE inhibitors such as quinapril and their methods of manufacture. The patent teaches
5

stabilization of quinapril by making formulations containing a metal-containing stabilizer like
magnesium carbonate which prevents both cyclization and, discoloration and saccharide
supposedly prevents hydrolysis. However, the quantity of magnesium carbonate required to
stabilize quinapril hydrochloride is very high (around 5.8 to 16.5 parts by weight).
Furthermore, using this approach it is difficult to precisely control the exact final ingredients
in the composition.
To overcome the problems associated with the approach provided by Harris et al., Sherman
BC in US6531486 provided quinapril magnesium salt as a stable salt to be used in the
formulation. Although the magnesium salt is found to be stable as compared to quinapril
hydrochloride as such, as per the experimental data in our laboratory, however, it gets
cyclized on storage at higher temperature (see table IV).
US Patent Application No. 20040052835 (Klokkers K. et al.) and PCT application No.
2007065638 (Klokkers K. et al.) teach stabilization of ACE inhibitors by preparing salts of
their active metabolite (diacid) with strong acids or bases and formulating such salts in a
matrix-controlled trans dermal therapeutic system. However, before making such
composition there is a need to hydrolyze quinapril completely to it's diacid metabolite.
Lyophilization of quinapril hydrochloride in presence of citrate buffer system is described in
Li J., et al, Pharmaceutical Research (2002), 19, 20 - 26 as an approach to stabilize
quinapril hydrochloride. The approach works well at lower pH but as the pH increases the
concentration of zwitterions of quinapril increases [page 24 of Li J. et al. (2002)] which leads
to cyclization to diketopiperazine impurity.
Li J., et al., Journal of Pharmaceutical Sciences (2002), 91, 229 - 243 describes
complexation of quinapril hydrochloride with various carbohydrates like p-cyclodextrin,
hydroxypropyl p-cyclodextrin, Trehalose, Dextran etc. The authors found enhanced stability
of 1:1 complex of quinapril hydrochloride with p-cyclodextrin. Although the complex is more
stable than quinapril hydrochloride, there is a significant decrease in the purity of quinapril on
heating at 80°C for 30 hours (Figure 2 on page 234 of the said publication).
US Patent No. 4,761, 479 (Goel et. al.), Canadian Application No. 12,293, 705 Al (Llagostera
et. al), European patent No. 1572661 (Singh GP et al.) disclose a stabilization of quinapril
hydrochloride salt by forming crystalline solvates so that intermolecular interactions between
quinapril molecule and solvent molecule(s) would minimize the intramolecular interactions
which may lead to formation of diketopiperazine impurity and the crystalline structure would
6

minimize the mobility of molecules that is more in case of amorphous solids as described in
Guo Y. et al., Journal of Pharmaceutical Sciences, (2000) 89, 128-143. The problem related
to such an approach is weak binding of such solvents with quinapril molecule i.e. the product
get desolvated easily on heating and milling, and gets converted to amorphous quinapril
hydrochloride, hence imparting instability to the molecule, and also imposing regulatory
restrictions on organic volatile impurities in pharmaceutical products. This solvate approach
is essentially practiced as a synthetic aid eventually the product is dried to get quinapril
hydrochloride, but the stability problem remains and hence no worthwhile solution.
In case of ramipril, US5442008 (Fulberth et al.), US6790861 (Vivilecchia et al), US6509350
(Vivilecchia et al,), US6300362 (Vivilecchia et al,) disclose stabilization of the drug molecule
at formulation level by adding stabilizers like amino acid hydrochlorides. Whereas,
US6979462 (Spireas S.) discloses stable formulation by encapsulating the ACE inhibitors
like ramipril in hydrophobic barrier of fatty oils.
It is evident from the literature cited hereinbefore that:
1. Quinapril freebase is highly unstable, therefore it is immediately converted to
hydrochloride salt for manufacture of pharmaceutical formulation.
2. Attempts till now give some what stability but not fully satisfactory to quinapril
molecule at solid state, but in solution phase it is hopelessly unstable.
3. Attempts till now could make a stable ramipril formulation but stable ramipril is not yet
available being not only stable at elevated temperatures, but also in solution form at
various pH.
Hence, a need exists to develop a solid state labile ACE inhibitors viz. quinapril and ramipril,
etc. may be in the form of salt and / or a co-crystal with pharmaceutical^ well acceptable
compound, which would not only exhibit a solid state stability at elevated temperatures but
also a solution phase stability at varying pH.
As a result of our painstaking and innovative experimentation to develop a stable salt / co-
crystal of quinapril and ramipril with acids or bases, the present inventors found the salt of
quinapril free base with tris(hydroxymethyl) amino methane and ramipril with
tris(hydroxymethyl) amino methane. Tris(hydroxymethyl) amino methane is a
pharmaceutical^ well accepted excipient. It as surprisingly found that, the quinapril tris salt
and ramipril tris salt not only are very stable at 80°C for extended period of time but also
stable in solution phase at various pH.
7

Such unexpected stability arose, as would be described hereinbelow, due to formation of not
only the salt but also complex hydrogen bonding network with host and the guest i.e. a co-
crystal formation. Nature of such interactions is not only different qualitatively but also
quantitatively compared to a mechanical mixture, where there is no interaction between host
and guest, and also to formation of salt, where the interactions are only ionic between acid
and base.
It would not be out of the place for the readers to review the recent advancements in the field
of co-crystals.
Importance and applications of co-crystals in pharmaceutical industry are discussed by
Peddy Vishweshwar et al., "Pharmaceutical Co-crystals" J. Pharm. Sci. (2006) 95(3), 499 -
516. The studies on co-crystals were reported in Etter MC, and Adsmond DA "The use of co-
crystallization as a method of studying hydrogen bond preferences of 2-aminopyridine" J.
Chem. Soc, Chem. Commun. (1990) 589-591, Etter MC, et al., "Graph-set analysis of
hydrogen-bond patterns in organic crystals" Acta Crystallogr. Sect. B, Struct. Sci. (1990),
B46 256-262; Etter MC, et al., "Hydrogen bond directed co-crystallization and molecular
recognition properties of diarylureas" J. Am. Chem. Soc. (1990) 112 8415-8426, which are
incorporated herein by reference in their entireties. The following articles are also
incorporated herein by reference in their entireties: Carl HG and Hans PH,"On the inclusion
of solvent molecules in the crystal structures of organic compounds"; Acta Cryst. (2000),
B56, 625-534; SenthilKumar VS, Nangia A, et al., "Molecular Complexes of Some Mono-
and Dicarboxylic Acids with frans-l,4,-Dithiane-l,4-dioxide" Crystal Growth & Design (2002),
2(4), 325 - 328; and Desiraju G. R., "Crystal and co-crystal" Cryst..Eng. Comm. (2003), 5,
466-467. US patent application 20070059356 describes how effectively co-crystallization
enabled to convert the injectable API to conventional oral solid dosage form.
Objects of the invention
Thus an object of the present invention is to provide a novel stable solid state form of labile
ACE inhibitors with hardly any susceptibility for cyclization to form diketopiperazine impurity
even on drying at higher temperature and reduction if not, no susceptibility for hydrolysis to
diacid impurity, by forming a salt of the said ACE inhibitors with such a base that is having a
capability to bind with hydrogen bonding network.
Another object of the present invention is to provide a novel stable solid state form of
quinapril free base with no susceptibility for cyclization to form diketopiperazine impurity
even on drying at higher temperature and no susceptibility for hydrolysis to diacid impurity.
8

Yet another object of the present invention is to provide a novel stable solid state form of
ramipril with less or no susceptibility for cyclization to form diketopiperazine impurity even on
drying at higher temperature and reduction if not, no susceptibility for hydrolysis to diacid
impurity.
Further object of the present invention is to provide a novel tris(hydroxymethyl)amino
methane salt of quinapril with comparable solubility and dissolution rate with the reported
hydrochloride salt of quinapril.
Yet another object of the present invention is to provide a novel salt of quinapril and ramipril
that is stable during pharmaceutical unit operations.
A further object of the present invention is to provide a process for preparation of novel
crystalline tris(hydroxymethyl) amino methane salt of quinapril and ramipril.
Summary of invention
Thus in the present invention there is provided novel stable crystalline tris(hydroxymethyl)
amino methane salt of quinapril, wherein the stability is imparted not only by ionic bonds but
also by co-crystal like phenomenon i.e. intermolecular hydrogen bonding network.
Another aspect of the present invention is to provide a novel stable crystalline
tris(hydroxymethyl) amino methane salt of ramipril, wherein the stability is imparted not only
by ionic bonds but also by co-crystal like phenomenon.
Further aspect of the invention is novel crystalline tris(hydroxymethyl) amino methane salt of
quinapril free base having characteristic powder and single crystal X-ray diffraction pattern
as shown in figure 1 and with characteristic 20 values as given in Table II.
Yet another aspect of the present invention is novel crystalline tris(hydroxymethyl) amino
methane salt of ramipril having characteristic powder X-ray diffraction pattern as shown in
figure 26 and with characteristic 20 values as given in Table II.
Further aspect of the invention is an efficient process for preparation of tris(hydroxymethyl)
amino methane salt of quinapril by catalytic hydrogentation of quinapril benzylester in
presence of tris(hydroxymethylamino methane.
9

Yet another aspect of the invention is an efficient process for preparation of
tris(hydroxymethyl) amino methane salt of ramipril by treating ramipril with
tris(hydroxymethylamino methane.
Description of the invention:
Brief Description of Accompanying Figures:
Figure 1: Powder X-ray diffractogram of quinapril tris salt.
Figure 2: TGA thermogram of quinapril tris salt.
Figure 3: DSC thermogram of quinapril tris salt.
Figure 4: DSC thermogram of quinapril HCI salt.
Figure 5: TGA thermogram of quinapril HCI salt.
Figure 6: DSC thermogram of quinapril terbumine salt.
Figure 7: TGA of quinapril terbumine salt.
Figure 8: Powder X-ray diffractogram of quinapril terbumine salt.
Figure 9: Powder X-ray diffractogram of quinapril hydrochloride salt.
Figure 10: FTIR spectra of quinapril tris salt.
Figure 11: FTIR spectra of quinapril terbumine salt.
Figure 12: FTIR spectra of quinapril HCI salt.
Figure 13 to 17: Single crystal X-ray diffraction: hydrogen bonding network.
Figure 18 and 19: Single crystal X-ray diffraction: unit cell packing diagram.
Figure 20: TGA thermogram of ramipril tris salt.
Figure 21: DSC thermogram of ramipril tris salt.
Figure 22: Powder X-ray diffractogram of ramipril tris salt.
Figure 23: Powder X-ray diffractogram of ramipril terbumine salt.
Figure 24: Powder X-ray diffractogram of ramipril.
Figure 25: FTIR spectra of ramipril tris salt.
Figure 26: FTIR spectra of quinapril terbumine salt.
Figure 27: FTIR spectra of ramipril.
10

Tris(hydroxymethyl) amino methane salt (tris salt) and tert-butylamine salt (terbumine salt) of
quinapril was prepared as per the schematic representation provided hereinbelow (Scheme
II):

tris(hydroxymethyl) amino methane and tert-butyl amine salt of quinapril are obtained by
catalytic hydrogentation of quinapril benzylester in presence of tris(hydroxymethylamino
methane or te/Y-butylamine, filtration of catalyst, concentration of the clear solution to obtain
a solid residue and further purification of the product obtained
Thermogravimetric analysis (as shown in fig. 2) of quinapril tris salt shows no weight loss.
Differential scanning calorimetric analysis (Fig. 3) shows single endothermic peak at 156°C,
unlike quinapril hydrochloride showing first peak at 166°C of degradation of product to DKP
impurity and further peak at 254° i.e. melting point of DKP impurity.
Comparative thermal analysis data of quinapril tris salt and quinapril terbumine salt is
tabulated in Table I
Table I

Product DSC TGA
Q. HCI First peak of endotherm at
166°C and second at 254°C
(Figure 4) Weight loss of around
1%upto150°C. (Fig. 5)
Q. tris Single sharp peak of
endotherm at 156° (Figure 3) No weight loss (Fig. 2)
Q.
terbumine Single broad peak of
endotherm at 169.25° (Figure
6) No weight loss (Fig. 7)
11

The powder X-ray diffraction analysis and FTIR spectra of quinapril tris salt shows distinct
features. Comparative data of 26 values and FTIR absorption spectra of quinapril
hydrochloride, quinapril tris salt and quinapril terbumine salt is provided in Table II.

Table II:
Powder X-ray diffraction FTIR
Quinapril Quinapril Quinapril Quinapril tris Quinapril Quinapril
tris salt terbumine HCI salt salt terbumine salt HCI salt
(Fig. 1) salt (Fig. 8) (Figure 9) (29 (Fig. 10) (Fig. 11) (Figure 12)
(29 values) (29 values) values) (cm"1) (cm"1) (cm"1)
3.80, 7.57, 3.86, 7.62, 3324,3368, 3709, 3679, 3413,3027,
8.60, 9.33, 8.79, 9.66, Shows no 3085, 3033, 3420,3317, 2982, 2935,
9.46, 11.35, 11.39, 11.44, significant 2988, 2947, 3026, 2977, 2856, 1739,
14.11, 14.13, 14.19, peaks. 2926,2904, 2903, 2843, 1648, 1536,
14.38, 14.58, 15.18, 2858, 2606, 2745, 2634, 1496, 1449,
14.78, 16.00, 16.59, Amorphous 2551,2089, 2555, 2230, 1382, 1335,
15.16, 17.06, 17.55, powder. 1718, 1625, 1720, 1647, 1294, 1258,
16.17, 17.77, 18.09, 1570, 1537, 1565, 1497, 1207, 1140,
17.16, 18.40, 19.00, 1498, 1480, 1484, 1455, 1111, 1088,
17.59, 19.99,20.69, 1455, 1427, 1442, 1413, 1034, 1015,
17.74, 21.18,22.37, 1394, 1371, 1391, 1364, 985, 935,
18.23, 22.80, 23.72, 1339, 1286, 1337, 1283, 909, 855,
18.96, 24.74, 25.08, 1233, 1215, 1230, 1194, 749, 701,
20.14, 25.56, 26.67, 1196, 1180, 1150, 1112, 679, 633,
20.34, 28.36, 28.55. 1155, 1115, 1058, 1032, 595, 492,
20.68, 1083, 1060, 1016,983, 422
20.86, 1035,945, 942, 924,
22.49, 922,813,743, 852,817,
22.74, 654, 584, 508, 753, 744,
23.24, 441. 713,703,
23.81, 655,541,
24.56, 507, 466.
25.24,
26.17,
26.67,
26.93,
28.53
It is evident from the comparison of FTIR data for quinapril tris salt and quinapril terbumine
salt that the -OH stretch vibration (3709 cm'1) of carboxylic function in quinapril that is seen
in quinapril terbumine salt is absent in quinapril tris salt; also the carbonyl stretch vibration
seen at the wavelength of 1647 cm"1 in quinapril terbumine salt is shifted to 1625 cm"1 in
case of quinapril tris salt. Shifting of carbonyl stretch and absence of-OH stretch of carbonyl
function of quinapril in quinapril tris salt can be interpreted as involvement of these groups in
intermolecular hydrogen bonding.
Single crystal of quinapril tris salt developed by the inventors was taken up for single crystal
X-ray diffraction analysis. The single crystal X-ray analysis was carried out using SMART
12

APEX CCD diffractometer by full-matrix least-squares refinement on F2; goodness of fit on
F2 was 1.093 with a 99.7% completeness. Reflections were measured on diffractometer with
monochromatised Cu-Ka radiation. The structure was solved by direct method and the non-
hydrogen atoms refined anisotropically. All H atoms were refined isotropically. The data is
as shown below in Table III and in figures 13 to 19

Table III:
Crystal System Monoclinic
Space group P2i
Unit cell dimensions a = 10.277 (3) A
b = 6.197(17)A
c = 23.410 (7) A a = 90°
p = 91.24 (5)°
y = 90°
Cell volume lA90A~(7)X*
Cell measurement temperature 298 (2) K
R factor 0.1269
Single crystal X-ray data (Fig. 18 and 19) reveals four molecules of quinapril were
associated with four molecules of tris(hydroxymethyl) amino methane by complex hydrogen
bonding network In other words the material has got extra stability by complex hydrogen
binding, like co-crystals.
Features of the single crystal X-ray diffraction analysis can be summarized as follows (See
figures 13 to 19):
1. Quaternization of nitrogen atom of tris(hydroxymethyl) amino methane.
2. complex hydrogen bonding network between tris(hydroxymethyl) amino methane and
quinapril molecule wherein both the oxygen atoms of carbixylate ion and amide
carbonyl of quinapril bound with hydrogen bond donors -OH and -N+H3 on
tris(hydroxymethyl) amino methane molecule
As evident from single crystal X ray studies of quinapril tris salt, quinapril is not only bound
with tris(hydroxymethyl) amino methane by the way of ionic bond, but also bound with a
complex hydrogen bonding network involving large number of hydrogen bond donors and
acceptors (figure 13 to 19). This formation of complex hydrogen bonding network imparts
solid-state stability as evident from comparison of thermal stabilities quinapril tris salt and
quinapril terbumine salt (Table IV), as terf-butylamine do not posses large number of
hydrogen bond donors like that of tris (hydroxymethyl) amino methane.
13

Quinapril freebase is highly unstable, and gets degraded immediately to form a
diketopiperazine impurity and diacid impurity. To overcome thesephenomenon quinapril free
base was immediately converted to salt viz. hydrochloride. Although, formation of
hydrochloride salt provides superior stability to quinapril as compared to the free base, it
looses hydrogen chloride very easily on heating. The escape of hydrogen chloride from
quinapril hydrochloride leads to formation of neutral moiety and pursuant to scheme I, once
neutral moiety is formed it gets cyclized to diketopiperazine impurity immediately. Similarly in
solution phase, increase in pH causes conversion of quinapril hydrochloride to quinapril
zwitterions that lead to cyclization. In solution form quinapril hydrochloride is also susceptible
to under go hydrolysis to form diacid impurity, which is also not desirable.
In view of the above facts it is amazing that quinapril free base and tris co-crystal give such
unexpected stability to quinapril molecule.
It is evident from the single crystal X-ray diffraction and FTIR data, the interactions between
quinapril and tris(hydroxymethyl) amino methane occurs by two phenomena viz. (a)
formation of amine salt (b) complex intermolecular hydrogen bonding network. Therefore, it
imparts stability not only by the ionic bond but also through hydrogen bonds similar to co-
crystal formation, which was not possible in quinapril terbumine and quinapril hydrochloride
salts.
Thermal stability at 80°C of quinapril tris was compared with that of quinapril hydrochloride,
quinapril terbumine salt and quinapril magnesium salt (prepared by following the process
provided in US6531486). quinapril tris/ quinapril HCI/ quinapril Terbumine/ quinapril Mg (10 g
each) were kept at 80°C in a closed chamber for 72 hrs. Samples of each were withdrawn
after every 8 hrs and analyzed for impurity profile by HPLC. Impurity level in each sample is
provided in table - IV

Table - IV
Quinapril
Tris Quinapril HCI Quinapril
terbumine Quinapril Mg
Time,
hr Diaci
d3
(%) DKP
3
(%) Diaci
d
(%) DKP
(%) Diacid
(%) n DKP
(%) Diaci
d
(%) DKP
(%)
Initial 0.02 0.05 0.12 02 I ND 0.42 0.17 0.15
8 0.02 "birr1 0.22 1.42 ND 2.61 2.1 0.22
16 0.02 0.07 0.71 7.32 ND 19.24 4.35 0.29
24 0.02 0.05 0.74 7.83 ND 21.28 5.54 0.35
14

32 0.02 0.05 0.94 10.7
1 ND 35.15 6.20 0.38
40 0.02 0.06 1.24 14.6
2 ND 36.2 7.29 0.43
48 0.02 0.06 T~57~" 19.8
1 ND 38.59 8.01 0.50
56 0.02 0.05 1.59 22.6
5 ND 39.81 ~877in 0.55
64 0.02 0.05 1.75 26.7 ND 60.15 9.57 0.58
72 0.02 0.05 2.28 26.8
1 ND 74.54 10.36 0.63
DKP: diketopiperazine impurity; Diacid: hydrolyzed diacid metabolite
It is evident from the data provided in table -IV, impurity levels in quinapril tris surprisingly
remains unchanged at 80°C even upto 72 hours, however impurity levels in quinapril
hydrochloride, quinapril terbumine and quinapril magnesium increased significantly.
Stability of quinapril tris salt was compared with quinapril hydrochloride at different pH
values. For solution phase stability study quinapril hydrochloride/quinapril tris (100 mg each)
were dissolved in 1ml each of water, 0.1N hydrochloride solution, 4.5 phosphate buffer
solution, 6.8 phosphate buffer solution 7.5 phosphate buffer solution in a separate sample
vials and kept for 30 minutes at 25°C. Then each sample was analyzed by HPLC to detect
impurity level.
The impurity level varies significantly at varying pH in case of quinapril hydrochloride,
however there is no significant change in the impurity level in case of quinapril tris salt (see
table -V)
Table -V

Sr.
no. Medium Quinapril
hydrochloride Quinapril tris

DKP (%) Diacid (%) DKP (%) Diacid
(%)
1. Initial 0.20 0.08 0.04 0.02
2. DM
Water 0.67 0.07 0.04 0.05
3. pH0.1 0.44 0.28 0.08 0.03
4. pH4.5 0.26 0.37 0.17 0.04
5. pH6.8 0.47 0.37 0.05 0.05
6. pH7.5 0.28 0.50 0.06 0.04
Solubility study of quinapril tris salt and quinapril hydrochloride in water, ethanol and
methanol reveals that quinapril tris salt has marginally superior solubility than that of
quinapril hydrochloride, (table -VI)
15

Table - VI
Solvent Quantity of solvent required to
dissolve 100 mg of

Quinapril
hydrochloride Quinapril Tris
Water 0.13 ml 0.1 ml
Methanol 1.4 ml 1.3 ml
Ethanol 2.2 ml 2 ml
Tris(hydroxymethyl) amino methane salt (tris salt) and tert-butylamine salt (terbumine salt) of
ramipril were prepared by treating ramipril with tris(hydroxymethyl) amino methane or tert-
butylamine in aqueous alcohol, evaporating the solvents and crystallizing the residue in
ester. Alternatively, tris(hydroxymethyl) amino methane and tert-butyl amine salt of ramipril
can be obtained by catalytic hydrogentation of ramipril benzylester in presence of
tris(hydroxymethylamino methane or tert-butylamine, filtration of catalyst, concentration of
the clear solution to obtain a solid residue and further purification of the product obtained
Thermogravimetric analysis (as shown in fig. 20) of ramipril tris salt shows no weight loss.
Differential scanning calorimetric analysis (Fig. 21) shows single endothermic peak at 78°C.
The powder X-ray diffraction analysis and FTIR spectra of ramipril tris salt shows distinct
features. Comparative data of 28 values and FTIR absorption spectra of ramipril, ramipril tris
salt and ramipril terbumine salt is provided in Table VII.
Table VII:

Powc er X-ray diffraction FTIR
Ramipril Ramipril Ramipril Ramipril tris Ramipril Ramipril
tris salt terbumine (Figure 24) salt terbumine salt (Figure 27)
(Fig. 22) salt (Fig. (29 values) (Fig. 25) (Fig. 26) (cm1)
(29 values) 23)
(29 values) (cm"1) (cm"1)
3.71,7.38, 7.34, 9.65, 7.56, 8.08, 3133,2097, 3315, 2964, 3280, 3067,
9.39, 10.45, 9.94, 10.67, 10.27, 1959, 1886, 2934,2844, 3026, 2993,
11.04, 10.97, 12.56, 1721, 1633, 2749, 2639, 2978, 2965,
11.21, 11.41, 12.74, 1558, 1417, 2558, 2233, 2935, 2865,
12.60, 12.70, 13.47, 1363, 1341, 2138,2024, 1743, 1703,
14.20, 14.44, 13.68, 1275,1231, 1723, 1637, 1652,1601,
14.71, 14.66, 14.07, 1150, 1122, 1570, 1498, 1584, 1498,
15.31, 15.34, 14.35, 1023,983, 1473, 1452, 1464, 1428,
15.57, 15.72, 15.08, 965,929,901, 1419, 1393, 1393, 1374,
16.46, 16.55, 15.71, 858,810,744, 1373, 1365, 1346,1321,
16.79, 16.16, 16.14, 704 1348, 1319, 1279, 1228,
17.61, 17.12, 16.97, 1283, 1271, 1186, 1164,
18.39, 17.71, 17.45, 1228, 1193, 1106, 1093,
18.71, 18.29, 17.89, 1145, 1112, 1064, 1040,
19.32, 19.29, 18.11, 1057, 1037, 1014, 932,
19.60, 19.74, 19.21, 968, 857, 745, 910,854,816,
16

20.17, 20.18, 20.05, 704, 478. 775,755,701,
20.60, 20.75, 20.78, 500
20.97, 21.58, 21.24,
21.46, 23.02, 22.69,
21.67, 23.80, 23.83,
22.12, 24.23, 24.33,
22.43, 24.93, 24.64,
22.96, 25.67, 25.11,
23.25, 26.45, 25.67,
23.79, 26.98, 25.93,
23.98, 28.06, 30.78 26.76,
24.68, 27.50,
25.15, 27.86,28.31
25.23,
25.85,
26.42,
26.81,
It is evident from the comparison of FTIR data for ramipril tris salt ramipril terbumine and
ramipril the carbonyl stretch vibration seen at the wavelength of 1652 cm"1 in ramipril and
1637 cm"1 in ramipril terbumine is shifted to 1633 cm"1 in case of ramipril tris salt. Shifting of
carbonyl stretch of ramipril in ramipril tris salt can be interpreted as involvement of this
groups in intermolecular hydrogen bonding.
Thermal stability of ramipril tris was compared with that of ramipril and ramipril terbumine
salt at 80°C. ramipril tris/ ramipril trebumine/ ramipril (10 g each) were kept at 80°C in a
closed chamber for 72 hrs. Samples of each were withdrawn after every 8 hrs and analyzed
for impurity profile by HPLC. Impurity level in each sample is provided in table - VIII

Table - VIM
Time (hr) DKP4 (%)
Ramipril DKP (%)
Ramipril Tris DKP (%)
Ramipril TBA
0 0.59 0.02 1 065
•8 14.02 0.06 1.81
16 27.25 0.11 5.90
24 32.49 0.11 8.52
32 62.92 0.22 31.76
40 68.47 ^32 ' 38.26
48 77.47 0.50 43.89
56 80.46 0.73 49.22
64 95.17 1.06 55.95
72 98.71 1.45 80.28
DKP: diketopi perazine impur ity.
It is evident from the data provided in table -VIII the rate of degradation of ramipril tris to
diketopiperazine impurity is surprisingly slow at 80°C when compared to ramipril and ramipril
terbumine salt. After 72 hours at 80°C ramipril and ramipril terbumine salt gets almost
17

completely degraded to diketopiperazine impurily, however only 1.45% diketopiperazine
impurity is formed in case of ramipril tris salt.
Stability of ramipril tris salt was compared with ramipril at different pH as follows: ramipril tris
salt and ramipril were added to 0.03 molar ammonium phosphate buffer adjusted pH 3.0, 5.0
and 8.0 with phosphoric acid. The mixtures at different pH were stirred for 60 minutes at
90°C and the samples were tested for relative impurities. The results are summarized in
table -IX)

Table -IX
Sr.
no. Medium ramipril ramipril tris

DKP (%) Diacid (%) DKP (%) Diacid
(%)
1. Initial 0.02 nill 0.02 nill
3. pH3 48.23 3.33 1.73 4.41
4. pH5 47.83 3.71 1.36 2.93
5. pH8 74.02 1.47 0.26 1.33
It is evident from the data summarized in table -IX the degradation of ramipril tris salt to
diketopiperazine impurity is surprisingly lower in solutions at various pH at 90°C when
compared to ramipril. At pH 3 and 5 ramipril gets cyclized to around 48 % and at pH 8 it gets
degraded to around 74%, whereas ramipril tris salt gets cyclized only to around 1.73 % at pH
3, 1.36 % at pH 5 and 0.26% at pH 8 under the stated conditions.
Ramipril tris salt has approximately two folds more solubility in water, 100 mg of ramipril tris
salt require 0.2 ml water to get completely dissolved, as compared to ramipril, 100 mg of
which required 0.5 ml of water.
Accordingly, it can be inferred from table IV, V, VIII and IX that the level of the drug will be
available intact for the activity in the body for a longer period of time and hence, would
certainly have better biological efficacy, when formulated and ingested in the form of tris salt.
Interestingly, quinapril tris salt and ramipril tris salt were levorotatory, i.e. for quinapril tris salt
it is -24.34° (c=1 in water at 20°C for sodium line) and for ramipril tris salt it is -14.82° (c=1
in water at 20°C for sodium line), however the specific optical rotations for quinapril
hydrochloride and ramipril as reported in the Merck Index (13th Edition) are +14.5° (c=1.2 in
ethanol at 23°C for sodium line) and +33.2° (c=1 in 0.1 N ethanolic HCI at 24°C for sodium
line) i.e. dextrorotatory. It would be abundantly clear from such phenomenon to a person
skilled in the art that these products are not mere physical mixtures.
18

Suitable pharmaceutical formulations may conveniently be presented containing
predetermined amount of quinapril tris salt or ramipril tris salt for treatment of cardiovascular
ailment.
For comparing quinapril hydrochloride and ramipril were prepared by practicing the
processes provided in the US patents 4,344, 949 and 5,061,722.
1H NMR spectra were recorded on Bruker 400MHz, powder X-ray diffraction recorded on
PANalytical model PN3040/60X'Part Pro, FTIR spectra were recorded on Perkin-Elmer
model spectrum 100, Thermal analyses were done on Mettler Toledo DSC 821 e.
The present invention is illustrated in more detail by referring to the following Examples,
which are not to be construed as limiting the scope of the invention.
Example 1. Preparation of (S,S,S)2-{2-[(1-ethoxycarbonyl-3-phenylpropyl)amino]-1-
oxopropyl]-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid tris(hydroxymethyl) amino
methane salt:
25 gm (0.0388 moles) of (S,S,S)-2-[2-{(1-ethoxycarbonyl)-3-phenylpropyl)amino]-1-
oxopropyl]-1,2,3- ,4-tetrahydro-3-isoquinoline carboxylic acid benzyl ester maleate salt was
dissolved in a mixture of 125 ml water and 125 ml dichloromethane. The pH of the solution
was adjusted 7.5 to 8.5 using aqueous ammonia. Reaction mixture was stirred for 30
minutes, the organic phase separated and washed with 50 ml water. The organic phase was
evaporated under reduced pressure below 40°C; the free base of quinapril benzyl ester was
obtained.
The above residue of quinapril benzyl ester was dissolved in 250 ml ethanol. To this
solution, 2.5 gm of 10% Pd/C, 4.7 gm (0.0388moles) tris (hydroxymethyl) amino methane
and 60 ml water were added. The reaction mass was subjected to catalytic hydrogenation at
40-60 psi pressure at 20-30° C for 2 hours. After completion of reaction, the reaction mass
was filtered and the filtrate evaporated to dryness. To the residue 100ml acetonotrile was
added and stirred for 30 minutes. The solid was filtered and dried at 40-45°C for 10 hours.
Dry weight of quinapril tris (hydroxymethyl) amino methane salt: 22 gm
IR Spectra [KBr] (cm1): 3324, 3368, 3085, 3033, 2988, 2947, 2926, 2904, 2858, 2606, 2551,
2089, 1718, 1625, 1570, 1537, 1498, 1480, 1455, 1427, 1394, 1371, 1339, 1286, 1233,
19

1215, 1196, 1180, 1155, 1115, 1083, 1060, 1035, 945, 922, 813, 743, 654, 584, 508, 441.
(Figure 10).
1H NMR (DMSO-D6 + CDCI3): 5 7.23 (m, 9H), 5.12 (m, 1H), 4.86-4.60 (m, 3H), 4.07 (m, 2H),
3.65 (m, 1H), 3.34 (s, 6H), 3.15 (m, 2H), 2.48-2.87 (m, 2H), 1.83 (m, 2H), 1.02-1.2 (m, 6H)
Thermogravimetric analysis (fig. 2) shows no weight loss. Differential scanning calorimetric
analysis (Fig. 3) shows single endothermic peak at 156°C.
The Specific Optical Rotation for Quinapril Tris is: -24.34 degrees (C = 1 in Water at 20°C
for sodium line)
X-ray powder diffraction analysis (Fig. 1) shows peaks at about 3.80, 7.57, 8.60, 9.33, 9.46,
11.35, 14.11, 14.38, 14.78, 15.16, 16.17, 17.16, 17.59, 17.74, 18.23, 18.96, 20.14, 20.34,
20.68, 20.86, 22.49, 22.74, 23.24, 23.81, 24.56, 25.24, 26.17, 26.67, 26.93, 28.53.
± 0.2 °26.
Single crystal X-ray diffraction analysis of the product reveals a Monoclinic crystal system
with a space group P2-,, wherein the molecules are bound with ionic as well as complex
intermolecular hydrogen bonding network. The unit cell dimensions are a = 10.277 (3) A, b =
6.197 (17) A, c = 23.410 (7) A; a = 90°, p = 91.24 (5)°, y = 90° (See table III and figures 13 to
19).
Example 2. Preparation of (S,S,S)2-{2-[(1-ethoxycarbonyl-3-phenylpropyl)amino]-1-
oxopropyl]-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid tertiary butyl amine salt:
25 gm (0.0388 moles) of (S,S,S)-2-[2-{(1-ethoxycarbonyl)-3-phenylpropyl)amino]-1-
oxopropyl]-1,2,3- ,4-tetrahydro-3-isoquinoline carboxylic acid benzyl ester maleate salt was
dissolved in a mixture of 125 ml water and 125 ml dichloromethane. The pH of the solution
was adjusted 7.5 to 8.5 using aqueous ammonia. Reaction mixture was stirred for 30
minutes, the organic phase separated and washed with 50 ml water. The organic phase was
evaporated under reduced pressure below 40°C; the free base of quinapril benzyl ester was
obtained.
The above residue of quinapril benzyl ester was dissolved in 250 ml ethanol. To this
solution, 2.5 gm of 10% Pd/C and 4.25 gm (0.0582moles) tertiary butyl amine were added.
20

The reaction mass was subjected to catalytic hydrogenation at 40-60 psi pressure at 20-30°
C for 2 hours. After completion of reaction, the reaction mass was filtered at 50°C and the
filtrate evaporated to dryness. To the residue 100ml acetonotrile was added and stirred for
30 minutes. The solid was filtered and dried at 40-45°C for 10 hours.
Dry weight of quinapril tertiary butyl amine salt: 20 gm
IR Spectra [KBr] (cm"1): 3709, 3679, 3420, 3317, 3026, 2977, 2903, 2843, 2745, 2634, 2555,
2230, 1720, 1647, 1565, 1497, 1484, 1455, 1442, 1413, 1391, 1364, 1337, 1283, 1230,
1194,1150,1112,1058,1032,1016,983, 942,924, 852,817, 753,744, 713,703,
655,541, 507, 466. (Figure 11).
1H NMR (CDCI3): 5 7.23 (m, 9H), 5.12 (m, 1H), 4.86-4.60 (m, 3H), 4.11 (m, 2H), 3.77 (m,
1H), 3.3 (m, 2H), 2.65 (m, 2H), 1.1.98 (m, 2H), 1.15-1.26 (m, 6H), 0.96 (s, 9H)
DSC shows single broad peak of endotherm at 169.25° (Figure 6), TGA reveals no weight
loss (Fig. 7).
X-ray powder diffraction analysis (Fig. 8) shows peaks at about 3.86, 7.62, 8.79, 9.66, 11.39,
11.44, 14.13, 14.19, 14.58, 15.18, 16.00, 16.59, 17.06, 17.55, 17.77, 18.09, 18.40, 19.00,
19.99, 20.69, 21.18, 22.37, 22.80, 23.72, 24.74, 25.08, 25.56, 26.67, 28.36, 28.55.
± 0.2 °28.
Example 3. Preparation of ramipril-tris salt
Ramipril (5gm) was dissolved in ethanol (30ml) at 35-40°C. Tris (hydroxymethyl) amino
methane (1.45gm) was separately dissolved in water(1.5ml) at ambient temperature. Both
the solutions were mixed & stirred for 30 min at 25°C. The reaction mixture was
concentrated under reduced pressure at 30 - 35°C to get oily residue. Ethyl acetate (40 ml)
was added to the residue and the mixture was heated to 35 - 40°C to get clear solution. The
solution was then cooled to 25°C to obtain crystals of ramipril tris salt. The product was
filtered and dried at 40°C for 8 hours under reduced pressure. Dry wt: 4.9gm(75%)
IR Spectra [KBr] (cm'1): 3133, 2097, 1959, 1886, 1721, 1633, 1558, 1417, 1363, 1341, 1275,
1231, 1150, 1122, 1023, 983, 965, 929, 901,858, 810, 744, 704. (Figure 25).
1H NMR (MeOH-D): 8 7.16-7.23 (m, 5H), 4.17-4.31 (m, 4H), 3.71-3.74 (s, 6H), 3.52-3.58 (m,
1H), 2.61-2.69 (m, 3H), 1.76-2.02 (m, 8H), 1.45-1.54 (m, 2H), 1.18-1.32 (m, 6H).
21

DSC shows single sharp peak of endotherm at 78°C (Fig 21), TGA reveals no weight loss
(Fig. 20).
The Specific Optical Rotation for Ramipril Tris is : -14.82 degrees (C = 1 in Water at 20°C for
sodium line)
X-ray powder diffraction analysis (Fig. 22) shows peaks at about 3.71, 7.38, 9.39, 10.45,
11.04, 11.21, 12.60, 14.20, 14.71, 15.31, 15.57, 16.46, 16.79, 17.61, 18.39, 18.71, 19.32,
19.60, 20.17, 20.60, 20.97, 21.46, 21.67, 22.12, 22.43, 22.96, 23.25, 23.79, 23.98, 24.68,
25.15, 25.23, 25.85, 26.42, 26.81 ± 0.2 °29.
Example 4. Preparation of ramipril-tris salt
Ramipril benzyl ester 25g was dissolved in 250 ml ethanol. To this solution, 2.5 gm of 10%
Pd/C, 5.97gm (0.05moles) tris (hydroxymethyl) amino methane and 70 ml water were added.
The reaction mass was subjected to catalytic hydrogenation at 40-60 psi pressure at 20-30°
C for 2 hours. After completion of reaction, the reaction mass was filtered and the filtrate
evaporated to dryness. To the residue 100ml ethyl acetate was added heat reaction mixture
to 35-40°C and stirred for 30 minutes at 25°C. The solid was filtered and dried at 40-45°C
under reduced pressure for 10 hours. Dry weight of ramipril tris (hydroxymethyl) amino
methane salt: 16gm
The product obtained is identical to that of the one obtained in example 3.
Example 5: Preparation of ramipril terbumine salt.
Ramipril (5gm) was dissolved in ethylacetate (100ml) at 35-40°C. To the clear solution tert-
butylamine (1.38 ml) was added and the resultant mixture was stirred for 15-20 min at
ambient temperature. The solid obtained was Filtered, washed with 30 ml ethyl acetate and
dried at 40°C fro eight hours under reduced pressure. Dry wt: 4.65(79%)
IR Spectra [KBr] (cm'1): 3315, 2964, 2934, 2844, 2749, 2639, 2558, 2233, 2138, 2024, 1723,
1637, 1570, 1498, 1473, 1452, 1419, 1393, 1373, 1365, 1348, 1319, 1283, 1271, 1228,
1193, 1145, 1112, 1057, 1037, 968, 857, 745, 704, 478. (Figure 26).
1H NMR (MeOH-D): 8 7.16-7.25 (m, 5H), 4.13-4.47 (m, 4H), 3.50-3.71 (m, 2H), 2.61-2.74 (m,
4H), 1.75-2.17 (m.7H), 1.45 (m, 3H), 1.35 (s, 12H), 1.17-1.32 (m, 2H).
22

X-ray powder diffraction analysis (Fig. 23) shows peaks at about 7.34, 9.65, 9.94, 10.67,
10.97, 11.41, 12.70, 14.44, 14.66, 15.34, 15.72, 16.55, 16.16, 17.12, 17.71, 18.29, 19.29,
19.74, 20.18, 20.75, 21.58, 23.02, 23.80, 24.23, 24.93, 25.67, 26.45, 26.98, 28.06, 30.78 ±
0.2 °2e.
23

We claim:
1. Quinapril tris salt having distinct X-ray diffraction peaks at 29 of 3.80, 7.57, 8.60,
9.33, 9.46, 11.35, 14.11, 14.38, 14.78, 15.16, 16.17, 17.16, 17.59, 17.74, 18.23,
18.96, 20.14, 20.34, 20.68, 20.86, 22.49, 22.74, 23.24, 23.81, 24.56, 25.24,
26.17, 26.67, 26.93, 28.53± 0.2°.
2. Quinapril tris salt as claimed in claim 1 having peak of endotherm at about 156°C
in its differential scanning calorimetry profile.
3. Quinapril tris salt as claimed in claim 1 having unit cell dimensions of
a = 10.277 (3) A a = 90°
b = 6.197 (17) A p = 91.24 (5)°
c = 23.410 (7) A 7 = 9o°
4. Ramipril tris salt having distinct X-ray diffraction peaks at 20 of 3.71, 7.38, 9.39,
10.45, 11.04, 11.21, 12.60, 14.20, 14.71, 15.31, 15.57, 16.46, 16.79, 17.61,
18.39, 18.71, 19.32, 19.60, 20.17, 20.60, 20.97, 21.46, 21.67, 22.12, 22.43,
22.96, 23.25, 23.79, 23.98, 24.68, 25.15, 25.23, 25.85, 26.42, 26.81 ± 0.2°.
5. Ramipril tris slat as claimed in claim 4 having peak of endotherm at about 78°C in
its differential scanning calorimetry profile.
6. The process for preparation of tris salt of labile ACE inhibitor comprising catalytic
hydrogenation of benzyl ester of labile ACE inhibitor in presence of tris
(hydroxymethyl) amino methane in an organic solvent.
7. The process as claimed in claim 6 wherein the organic solvent is selected from
the group of C1 to C5 aliphatic alcohol, water or mixtures thereof.
8. The process as claimed in claim 7 wherein the organic solvent is mixture of
ethanol and water.
9. The process as claimed in claim 6 wherein the catalyst is 10% palladium on
charcoal.
10. The process as claimed in claim 6 wherein the product is isolated by evaporation
of solvent.
24

11. The process as claimed in claim 10 wherein the product isolation was affected by
addition of acetonitrile or ethyl acetate.
12. The process as claimed in claim 6 wherein the labile ACE inhibitor is quinapril.

13. The process as claimed in claim 6 wherein the labile ACE inhibitor is ramipril.
14. The process for preparation of ramipril tris salt comprising treating a mixture of
ramipril and an organic solvent with a solution of tris (hydroxymethyl) amino
methane in water, isolation of the product by removal of solvents and
crystallization in ester.
15. The process as claimed in claim 14 wherein the organic solvent is selected from
C1 to C5 alcohols or the mixture thereof.
16. The process as claimed in claim 15 wherein the organic solvent is ethanol.
17. The process as claimed in claim 14 wherein the ester is ethyl acetate.
18. Method of stabilizing a labile ACE inhibitor comprising preparation of its salt with
a base having capability to form a complex hydrogen-bonding network with the
labile ACE inhibitor.
19. Method of stabilizing a labile ACE inhibitor as claimed in claim 18 wherein the
base is tris (hydroxymethyl) amino methane.
20. Use of therapeutically effective amount of quinapril tris salt for the manufacture of
medicament for treatment of cardiovascular dysfunctions.
21. Use of therapeutically effective amount of ramipril tris salt for the manufacture of
medicament for treatment of cardiovascular dysfunctions.
Dated this 10th day of March 2008

25

Quinapril tris salt having distinct X-ray diffraction peaks at 20 of 3.80, 7.57, 8.60, 9.33,
9.46, 11.35, 14.11, 14.38, 14.78, 15.16, 16.17, 17.16, 17.59, 17.74, 18.23, 18.96, 20.14,
20.34, 20.68, 20.86, 22.49, 22.74, 23.24, 23.81, 24.56, 25.24, 26.17, 26.67, 26.93,
28.53± 0.2°. Ramipril tris salt having distinct X-ray diffraction peaks at 2 of 3.71, 7.38,
9.39, 10.45, 11.04, 11.21, 12.60, 14.20, 14.71, 15.31, 15.57, 16.46, 16.79, 17.61, 18.39,
18.71, 19.32, 19.60, 20.17, 20.60, 20.97, 21.46, 21.67, 22.12, 22.43, 22.96, 23.25,
23.79, 23.98, 24.68, 25.15, 25.23, 25.85, 26.42, 26.81 ± 0.2°. A process for preparation
of tris salt of labile ACE inhibitor comprising catalytic hydrogenation of benzyl ester of
labile ACE inhibitor in presence of tris (hydroxymethyl) amino methane in an organic
solvent. A process for preparation of ramipril tris salt comprising treating a mixture of
ramipril and an organic solvent with a solution of tris (hydroxymethyl) amino methane in
water, isolation of the product by removal of solvents and crystallization in ester. Method
of stabilizing a labile ACE inhibitor comprising preparation of its salt with a base having
capability to form a complex hydrogen-bonding network with the labile ACE inhibitor.