A study of the relationship between the structure and physicochemical parameters of a homologous series of oxprenolol esters at various pH values and temperatures

Article abstract of DOI:10.1021/js970112x

A number of β-adrenergic blockers, including timolol and propranolol, are administered in eyedrops for the treatment of glaucoma, but their therapeutic value is limited by a relatively high incidence of cardiovascular and respiratory, side effects. Because of poor ocular bioavailability, many ocular drugs are applied in high concentrations, which give rise to both ocular and systemic side effects. Methods to increase ocular bioavailability include (a) the development of drug delivery devices designed to release drugs at controlled rates, (b) the use of various vehicles that retard precorneal drug loss, and (c) the conversion of drugs to biologically reversible derivatives (prodrugs) with increased cornea penetration properties, from which the active drugs are released by enzymatic hydrolysis. A homologous series of aliphatic esters of oxprenolol were synthesized and investigated as potential prodrugs for ocular use. The stability of each O- acyl derivative was investigated in aqueous solutions over the pH range 2.2- 9.0 at 37°C. The observed rate constants (k(obs)), shelf-lives (t₉₀), lipophilicities, and Arrhenius parameters were determined for each ester. A study of the relationship between the structure and physicochemical parameters of the homologous series of oxprenolol esters at various pH values and temperatures was made.

Full text of DOI:10.1021/js970112x

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A Study of the Relationship between the Structure and Physicochemical
Parameters of a Homologous Series of Oxprenolol Esters at Various pH
Values and Temperatures
C. GERALDINE M. JORDAN
Received March 18, 1997, from the Department of Food Science, Faculty of Agriculture, University College, Belfield, Dublin 4, Ireland.
Final revised manuscript received June 18, 1997.
Accepted for publication June 19, 1997X.
synthesized and the kinetics of degradation of the prodrugs
Abstract
0 A number of â-adrenergic blockers, including timolol and
in aqueous solution were studied. Also, the relationship
between the structures and physicochemical parameters of the
prodrugs was determined.
propranolol, are administered in eyedrops for the treatment of glaucoma,
but their therapeutic value is limited by a relatively high incidence of
cardiovascular and respiratory side effects. Because of poor ocular
bioavailability, many ocular drugs are applied in high concentrations, which
give rise to both ocular and systemic side effects. Methods to increase
ocular bioavailability include (a) the development of drug delivery devices
designed to release drugs at controlled rates, (b) the use of various
vehicles that retard precorneal drug loss, and (c) the conversion of drugs
to biologically reversible derivatives (prodrugs) with increased corneal
penetration properties, from which the active drugs are released by
To further examine the basis of this instability and struc-
ture-activity relationships, hydrolytic studies of a homologous
series of aliphatic esters of the structurally related â-adren-
ergic blocker oxprenolol {1-[(1-methylethyl)amino-3-[2(-pro-
penyloxy)phenoxy]-2-propanol} are presented here. Oxpre-
nolol is an important â-adrenergic blocking agent of the
3-aryloxy-1-(alkylamino)-2-propanol type. The overall con-
formation of the side chain in oxprenolol and its orientation
relative to the aromatic ring is virtually identical to that in
propranolol hydrochloride. Because of thermal agitation, the
bonds of the allyl group are unusually long.16 Oxprenolol is
effective in reducing blood pressure, particularly systolic levels
in cats.17 The feasibility of substituting oxprenolol for adr-
energic blockers in the treatment of essential hypertension
with relatively minor side effects has been shown18 in a trial
comprising almost 900 patients. The configuration of the
biologically more active (-)-isomer is S.19
enzymatic hydrolysis.
oxprenolol were synthesized and investigated as potential prodrugs for
ocular use. The stability of each O-acyl derivative was investigated in
A homologous series of aliphatic esters of
aqueous solutions over the pH range 2.2−9.0 at 37 °C. The observed
rate constants (kobs), shelf-lives (t90), lipophilicities, and Arrhenius
parameters were determined for each ester. A study of the relationship
between the structure and physicochemical parameters of the homologous
series of oxprenolol esters at various pH values and temperatures was
made.
Materials and Methods
Ap p a r a tu ssThe homologous series of oxprenolol esters was
characterized by a variety of analytical techniques. The 1H NMR
spectra were recorded in CDCL3 solution with tetramethylsilane
(TMS) as internal standard at 80 MHz with a Bruker Specrospin
spectrometer. Mass spectra were obtained with a Kratos M5902
instrument. Spectra were run in an electron impact mode with an
ionization energy of 70 eV. The scan was taken over the range 720-
30 amu, with perfluorokerosene as the reference compound. Data
were processed with a computer system based on an Arcom Stebus
computer (80188 processor). The ultraviolet (UV) spectral data were
obtained with a Pye Unicam SP-8-100 double-beam spectrometer
equipped with a thermostatically controlled cell compartment using
1-cm quartz cells. The infrared (IR) spectral data were recorded with
a Pye Unicam SP-3-300 spectrometer with polystyrene as reference.
A differential scanning calorimetry (DSC) analysis of each compound
was carried out with a Perkin Elmer DSC-20 instrument and a
Thermal Analysis Data Station (TADS) for data collection, handling,
and presentation. Melting points that were determined from DSC
analysis compared favorably with those obtained with a Gallenkamp
melting point apparatus.
Introduction
Several â-adrenergic blocking agents, notably propranolol
and timolol, are used in glaucoma therapy. The corneal
penetration behavior of a number of â-blockers has been
investigated.1-3 Several of these compounds, including pro-
pranolol,4 have been shown to lower intraocular pressure. The
hydrophobic properties of these drugs are apparently the
primary determinants of their pharmacokinetic and corneal
penetration behavior. The development of prodrugs with
improved corneal absorption characteristics has been used
successfully to enhance the ocular bioavailability of a number
of drugs,5,6 including adrenaline and pilocarpine.
Considerable attention has been focused on the use of
bioreversible derivatives (prodrugs) to improve the delivery
characteristics of various drugs.7 A fundamental requisite for
the usefulness of the prodrug approach is the ready avail-
ability of chemical derivative types satisfying the prodrug
requirements, principally reconversion of the prodrug to the
parent drug in vivo. Esters are the best known prodrugs
because of the predominance of carboxylic and hydroxyl
substituents in drug molecules and the availability of enzymes
in living systems that are capable of hydrolyzing these
substituents.
High-performance liquid chromatography (HPLC) was carried out
with a system consisting of a Waters 501 HPLC pump, a variable-
wavelength UV detector attached to a Houston omniscribe recorder,
and a 20-µL Rheodyne loop injection valve. The column used (100 ×
4.6 mm) was packed with Spherisorb C-8 (5-µm particles). A pre-
column (50 × 4.6 mm) was similarly packed. A sample of 10 µL was
introduced with a Hamilton syringe.
In previous studies,8-10 esters of timolol were developed to
potentially diminish the systemic absorption of topically added
timolol by increased corneal absorption, and the cardiovas-
cular and respiratory side effects were thereby reduced.
However, these esters are unstable in aqueous solutions, so
a series of esters of both propranolol11-13 and timolol14,15 were
The pH value of each solution was determined with a Radiometer
M-26 pH meter fitted with a glass electrode (Radiometer G-202B) and
a calomel reference electrode (Radiometer K-401). Reference buffers
were Radiometer standard solutions (pH 4.00/22 °C, pH 6.97/22 °C,
and pH 8.86/22 °C). A Heto thermostat water bath with a Heto
contact thermometer attached was used in all experiments.
Potentiometric titrations were carried out with a Mettler DL 25
automatic titrator fitted with an interchangeable burette and rod
X Abstract published in Advance ACS Abstracts, August 1, 1997.
© 1997, American Chemical Society and
American Pharmaceutical Association
S0022-3549(97)00112-3 CCC: $14.00
Journal of Pharmaceutical Sciences / 1085
Vol. 86, No. 10, October 1997

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stirrer with a variable speed adjuster. Results were obtained in
tabular form with a GA 44 printer, and a dot matrix graphics printer
was used to plot the curve of the titration profile on a GA 14 recorder.
Ch em ica lssSamples of oxprenolol hydrochloride were obtained
from both Ciba-Geigy Ltd. and Sigma Chemical Company (U. K.).
The acid chlorides were obtained from Aldrich Chemical Company
(U. K.). All solvents used were either HPLC grade or distilled before
use. Solid reagents were analytical or reagent grade and were used
as supplied or were recrystallized before use.
In the cited method,23 the pKa and the apparent pKa [(pKa)app] are
obtained, respectively, from the results of potentiometric titrations
without and with n-octanol present. The log P value is calculated
from the difference between the pKa and (pKa)app values and the
volume of water and n-octanol.
For the titration of a salt of a weak base with a strong base in the
presence of n-octanol, the following equations have been derived for
the calculation of P and Papp
:
All solvents used in HPLC (i.e., acetonitrile, methanol, acetone,
and tetrahydrofuran) were HPLC grade. All buffer substances used
were of reagent or analytical grade. The ionic strength of each buffer
solution was adjusted to 0.5 by adding a specific quantity of analytical
grade potassium chloride. Commercial grade n-octanol was used in
the partitioning experiments.
Syn th esis of Oxp r en olol Ester ssThe homologous series of
aliphatic oxprenolol esters was synthesised according to a previously
described method.20
Sta bility Stu d ies in Solu tion sThe decomposition of the aliphatic
oxprenolol esters was studied in aqueous buffer solutions over the
pH range 2.2-9.0 at 37 ( 0.2 °C. Phosphate and citrate buffers were
used in the pH range 2.5-5.0, and borate buffers were used in the
pH range 8.0-12.0. The total buffer concentration was 0.05 M, and
a constant ionic strength (µ) of 0.5 was maintained for each buffer.
The rates of hydrolysis were measured by a reversed-phase HPLC
procedure capable of separating the esters from the parent com-
pounds. A mobile phase system consisting of acetonitrile:methanol:
0.02 M phosphate buffer of pH 4.5 (65:5.0:30, v/v) was used. All
solvents were deaerated in an ultrasonic bath for 15 min before use.
The phosphate buffer was filtered through a Millipore filter and was
deaerated. A flow rate of 1.0 mL min-1 achieved satisfactory results.
The column efluent was monitored at 275 nm.
Vw
- (pKa)app
P )
(1)
Vo[10pK
- 1]
a
Papp ) P(1 - R) ) P[10pK
+ 1]
(2)
(3)
- pH
a
- pKa
R ) [10pH
+ 1]
where P is the partition coefficient, Papp denotes the distribution
coefficient (apparent partition coefficient) at a specific pH, Vw is the
aqueous volume, and Vo represents the volume of n-octanol.
The value of log Papp, in terms of log P, is therefore given by the
following equation
log Papp ) log P - log (10pK
+ 1)
(4)
- pH
a
The apparent ionization constant (Ka)app is defined in terms of Ka
by 5:
Ka ) f-1(Ka)app
(5)
where f is a partition factor, which is given by eq 6:
VoP
f )
+ 1
(6)
(
)
Vw
Corresponding expressions may be derived for the titration of the
salt of a weak acid with a strong acid. The value of Ka is given by eq
7:
Ka ) f(Ka)app
(7)
where f is defined by eq 5. The partition coefficient is given by eq 8:
Vw
P )
(8)
- pKa
app
Vo[10(pK )
- 1]
a
and the apparent partition coefficient, Papp, is given by eq 9:
- pKa
Papp ) P [10pH
+ 1]
(9)
The retention times for the compounds were in the range of 2.65
min (O-acetyl) to 9.91 min (O-pivaloyl). In the case of the O-acetyl
derivative, a mobile phase consisting of acetonitrile:methanol:0.02 M
phosphate buffer of pH 4.5 (55:5.0:40, v/v) was used to ensure
separation. Hydrolysis was initiated by adding 7 mL of the stock
solution (20 mg% of each compound in methanol) to 3 mL of buffer
solution (pre-equilibrated at the appropriate temperature). At 20-
min intervals, 10-µL samples were chromatographed. Quantitation
of the compounds was carried out by measuring the peak heights in
relation to those of standard solutions that were chromatographed
under the same conditions.
The value of log Papp, in terms of log P, is given by the following
equation:
- pKa
log Papp ) log P - log (10pH
+ 1)
(10)
Results and Discussion
Kin etics of Degr a d a tion of Oxp r en ololsThe degrada-
tion of all the aliphatic oxprenolol esters was studied in
aqueous solution at 37 °C over the pH range 2.2-9.0. The
decomposition of the esters displayed strict first-order kinetics
for several half-lives at constant pH and temperature. Typical
first-order plots for the degradation of O-acetyloxprenolol are
Pseudo-first-order rate constants for the hydrolysis were deter-
mined from the slopes of linear plots of the logarithm of residual
oxprenolol ester versus time.
Deter m in a tion of P a r tition Coefficien tssThe apparent parti-
tion (distribution) coefficients (Papp) of the oxprenolol esters were
determined in the n-octanol:buffer system (pH 7.40) at 22 °C by
potentiometric titration with a multiparametric curve-fitting tech-
nique.21,22 The method involves the potentiometric titration of the
compound both in water and in a rapidly stirred mixture of water
and n-octanol. The method is rapid and accurate for compounds with
ionization constant (pKa) values between 4 and 10. Distribution
coefficients calculated over a range of pH values may be presented
graphically as distribution profiles. Subtraction of the titration curve
of solvent alone from that of the compound in the solvent allows the
calculation of pKa values.
shown in Figure 1. Pseudo-first-order rate constants (kobs
)
were determined from plots of log (100[A]/[Ao]) versus time,
where [A]o and [A] are the concentrations at time t ) 0 and t
) t, respectively. The value of the rate constant is equal to -
2.3026 (slope). The values of the observed pseudo-first-order
rate constant (kobs) at each value are given in Table 1.
The rates of degradation for the straight-chain aliphatic
derivatives decrease with increasing chain length, the most
stable compound being the O-pivaloyl derivative and the least
1086 / Journal of Pharmaceutical Sciences
Vol. 86, No. 10, October 1997

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Figure 1
s
First-order plots for the degradation of O-acetyloxprenolol over the pH
9.0 at 37 C.
range 2.2
−
°
Table 1
Degradation of Oxprenolol Esters in Aqueous Solution at Different pH
Values and at 37 C (µ ) 0.5)
sObserved Pseudo-First-Order Rate Constants (kobs) for the
°
1
kobs
pH 2.2 pH 3.0 pH 4.0 pH 5.0 pH 6.2 pH 7.4 pH 8.0 pH 9.0
(× )
103) (min-
Ester
O-Acetyl
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
6.7
3.8
21.4
18.1
15.6
12.7
s
75.4
66.2
36.1
32.8
0.3
161.8 160.9
O-Propionyl
O-Butyryl
O-Valeryl
O-Pivaloyl
86.3
43.8
40.6
0.4
91.5
48.0
40.9
0.4
s
s
s
a No degradation.
Figure 2
)
s
pH
−
°C.
rate profiles for the degradation of aliphatic oxprenolol esters (
µ
0.5) at 37
Table 2 Predicted Values of the Shelf-Life (t90) for Various Oxprenolol
s
Esters in Aqueous Solution at Different pH Values and at 37
0.5)
°C (µ )
t90 (min)
Ester
pH 2.2 pH 3.0 pH 4.0 pH 5.0 pH 6.2 pH 7.4 pH 8.0 pH 9.0
O-Acetyl
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
15.7
27.7
s
s
s
4.9
5.8
6.8
8.3
1.4
1.6
2.9
3.2
0.7
1.2
2.4
2.6
0.7
1.2
2.2
2.6
O-Propionyl
O-Butyryl
O-Valeryl
O-Pivaloyl
s
309.9 257.0 266.7
a No degradation.
stable compound being the O-acetyl derivative. Their shelf-
lives (t90) of degradation (times for 10% degradation) decrease
as the temperature increases. These values are determined
with eq 11 and are listed in Table 2:
Scheme 1
ln(1.11)
kobs
not determined in this pH range. Oxprenolol esters are
generally more stable than the corresponding propranolol
because of the overriding steric effects (shielding) of the ortho
substituent on the benzene ring.
t90
)
(11)
The results show that the shelf-lives at 37 °C are greatly
dependent on the ester structure; the shelf-lives increase as
the chain length of the ester increases (Table 2). At pH 5.0
(37 °C), most esters studied do not degrade, indicating a very
high stability. The shelf-lives at this temperature are of the
order of minutes at pH values >5.0. The influence of pH on
the rate of hydrolysis of the esters at 37 °C is shown in Figure
2.
The shape of the pH-rate profiles indicates that (a) the free
base and the protonated forms of the esters undergo hydrolysis
at different rates and (b) the hydrolysis can be described in
terms of specific base-catalyzed reactions involving both
species as well as a specific acid-catalyzed reaction involving
the protonated ester (Scheme 1). All esters studied are quite
stable at pH values <5.0. Consequently, rate constants were
The reactivity of the esters is a function of steric and polar
factors. Because the polar effects of the acyl groups in the
aliphatic esters are similar, the observed differences in
reactivity in neutral and alkaline solutions may be ascribed
to differences in the steric properties. It was shown24 that
the rates of acid-catalyzed esterification are solely a function
of steric effects. The relationship between the steric substitu-
ent parameter, ν,24-26 and the logarithm of the shelf-life was
investigated. The literature values of this parameter are
listed in Table 3. One would expect a linear relationship for
a plot of log t90 versus ν for the aliphatic oxprenolol esters at
pH 7.4 and 37 °C and that is what was obtained (Figure 3).
As demonstrated by HPLC, the disappearance of almost all
the esters studied was accompanied by the progressive ap-
Journal of Pharmaceutical Sciences / 1087
Vol. 86, No. 10, October 1997

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Table 3
s
Literature Values for the Steric Substituent Constant (
ν) of
Table 4
sIonization Constants (pKa) and Apparent Ionization Constants
Various Alkyl Groups
(pKa)app of Oxprenolol and Its Aliphatic Esters at 22 °C
Compound
R2
ν2
Compound
Oxprenolol
pKa
(pKa)app
7.18
0.52a
0.56a
0.68a
0.68a
1.24a
9.48; 9.50;a 9.60;b 9.32c
O-Acetyl
-CH3
-CH2CH3
-(CH2)2CH3
-(CH2)3CH3
-C(CH3)3
O-Propionyl
O-Butyryl
O-Valeryl
O-Pivaloyl
O-Acetyl
8.70
5.06
O-Propionyl
8.58
4.99
O-Butyryl
O-Valeryl
O-Pivaloyl
8.06
7.83
8.07
4.29
4.00
4.71
a Data from ref 24.
a Reference 32. b Potentiometric titration (20 C); ref 33. c Potentiometric titration
°
C); ref 3.
(35
°
Table 5s
Second-Order Rate Constants (kOH) for the Hydroxide
Ion-Catalyzed Hydrolysis of the Protonated Species (Values are Quoted
at two pH values: 6.2 and 7.4)
1
kOH
(×
10-5) (M-1 min-
)
Compound
pH 6.2
pH 7.4
O-Acetyl
6.34
7.95
4.68
3.87
s
1.48
O-Propionyl
O-Butyryl
O-Valeryl
O-Pivaloyl
1.33
0.81
0.86
0.0008
rate constant for the protonated ester form, and kOH and k1
OH
denote the second-order rate constants for the apparent
hydroxide ion-catalyzed hydrolysis of the protonated and
unprotonated species, respectively.
The processes described by eq 12 may be represented
schematically (Scheme 1). In this scheme, the R2CO group
corresponds to the R group defined previously; for example,
R2 ) CH3 for the O-acetyl ester.
The pKa values of the esters (Table 4) were lower than that
of the parent compound (V), indicating that they are less basic
than oxprenolol. The difference may be ascribed to the greater
polar effect of the ester moiety relative to a hydroxyl group.
The electron-withdrawing effect of the ester moiety decreases
the basicity of the oxprenolol esters relative to the parent
oxprenolol.
In their studies of propranolol esters, Bundgaard and co-
workers11 concluded that the possible spontaneous (water-
catalyzed) reaction of the protonated species (described by the
second term on the right-hand side of eq 12) is insignificant
to the overall reaction. This result is in contrast to the
findings for the timolol esters.14 Previous studies on both
propranolol11 and timolol14 esters have indicated that the
values of kOH are much greater than those of kH. These values
are of the order 107-108 greater than kH. Because values of
kobs at pH values <5.00 have not been determined, the data
as presented would not provide reliable values of kH. Thus,
the first term (on the right-hand side) in eq 12 cannot be
isolated. For example, in the case of O-acetyloxprenolol, the
value of the degree of ionization [aH/(aH + Ka] at pH 5.00 is
∼0.9998. The difference between this value and unity is large
by comparison with the ratio kH/kOH. This term would not be
the predominant term under these conditions, because the
value of kOH aOH would not be negligible by comparison with
kH aH. Only at very low pH values can the value of kH be
assigned unambiguously.
Figure 3
for aliphatic oxprenolol esters (the
groups; Table 3.
s
Plot of log t90 (at pH 7.4 and 37
°
C) versus the steric parameter (
ν)
ν values refer to the alkyl moiety in the acyl
pearance of free oxprenolol, and a very small trace of a third
product was also observed in all esters except the O-pivaloyl
derivative. This unknown product was observed at higher pH
values (pH > 6), was very stable, and had a short retention
time.
The shape of the pH-rate profiles can be described in terms
of specific acid- and base-catalyzed reactions of the protonated
species along with a specific base-catalyzed reaction of the free
base form of the esters according to eq 12:
aH
aH
kobs ) kHaH
+ koaH
+
(
)
(
)
aH + Ka
aH + Ka
aH
Ka
k
OHaOH
+ kOHaOH
(12)
(
)
(
)
aH + Ka
aH + Ka
The values of kOH, listed in Table 5, were estimated at the
pH values 6.2 and 7.4. These values are much more reliable
because the third term in eq 12 under these conditions is the
predominant term; thus:
where aH and aOH refer to the hydrogen ion and hydroxide
ion activities, respectively, aH/(aH + Ka) and Ka/(aH + Ka)
represent the fractions of total ester in the protonated and
free base forms, respectively, and Ka is the ionization constant
of the protonated NH group in the esters. Thus, the degree
of ionization R may be identified with the term aH/(aH + Ka),
and (1 - R) is equal to Ka/(aH + Ka). The rate constant ko
refers to the spontaneous or water-catalyzed hydrolysis of the
protonated form of the ester, kH is the specific acid-catalyzed
aH
kobs = kOHaOH
(13)
(
)
aH + Ka
At pH 6.2, the value of kOH determined with eq 13 is 6.39
1088 / Journal of Pharmaceutical Sciences
Vol. 86, No. 10, October 1997