Download An investigation into salbutamol sulphate ion pair interactions and

An investigation into salbutamol sulphate ion pair interactions and their influence on
transport across a hydrophilic membrane
A. Patel1,2, C.P. Page1, M.B. Brown3,4, S.A. Jones2
1The
Sackler Institute of Pulmonary Pharmacology, Institute of Pharmaceutical Science, King’s College London, 150 Stamford Street, London, SE1 9NH, UK
2Drug Delivery Research Group, Institute of Pharmaceutical Science, King’s College London, 150 Stamford Street, London, SE1 9NH, UK.
3MedPharm Ltd, 50 Occam Road, Surrey Research Park, Guildford, Surrey, GU2 7AB, UK
4School of Pharmacy, University of Hertfordshire, College Lane, Hatfield, Herts, AL10 9AB, UK
Introduction and Aims
Methods
Fourier transform infra red (FT-IR) spectroscopy:
Pharmaceutical salts are important as converting an
acidic or basic drug into a salt has the ability to
modify the physicochemical properties of the parent
molecule (Serajuddin, 2007).
If the salt associates in solution it can reduce the
overall charge of the parent molecule, hence
modifying the solution state properties.
All spectras were obtained with a universal Omni-Cell system (SpecAc, UK) for
the analysis of liquids equipped with CaF2 windows (Mylar spacer at 0.1 mm).
A resolution of 4 cm-1 was employed for the scans captured over the 550 – 4000
cm-1 range.
Test solutions of salbutamol base & salbutamol in the presence of sulphate were
prepared in deuterated water & deuterated methanol (80:20) at pH 6.5.
Figure 1. Schematic representation of a charged ion (A+)
& a neutral ion-pair [A+B-] absorption across the
membrane.
Salbutamol transport:
For an ion-pair to form however an excess amount of counter ion is required. This can be supplied in
the formulation. The aim of this work was to form a salbutamol sulphate ion-pair and to characterise
its transport properties.
Transport of salbutamol across regenerated cellulose membrane (RCM,
Medicell International, London, U.K) was assessed using individually calibrated
Franz cells (MedPharm Ltd, UK) at 37°C over a period of four hours.
The objectives of this work were:
To determine the affinity constant between salbutamol and sulphate in solution using Fourier
transform infra red (FTIR) spectroscopy.
The donor solution consisted of salbutamol (0.00178 M) & salbutamol in the
presence of increasing sulphate concentration (0.00000673 - 0.0673 M) made
up to volume in de - ionised water (pH 7.4).
To assess how salbutamol transported across a model membrane when presented as an ionpair.
Salbutamol content was determined using HPLC (LOD = 1.25 µg/ml & LOQ =
4.15 µg/ml, between 1 & 100 µg/ml with a CV ≤ 2%).
Results and Discussion
3D modelling determined the number of hydrogen bonds
formed between salbutamol and sulphate (figure 3).
A peak height ratio (1606 cm-1 and 1617 cm-1)
change was detected when in the presence
of increasing sulphate concentration.
(a)
(b)
24
A log plot of salbutamol to sulphate ion-pair
binding was constructed from the peak height
ratio using GraphPad Prism (Fig. 2).
72
28; 44
42
41
74
43
SAL unbound (%)
100 pK = 1.505
R2 = 0.9877
75
25
0
-1
0
1
2
3
4
- Log free sulphate (M)
Figure 2. FTIR binding affinity plot for salbutamol (SAL) sulphate
(2:1) in solution. Data expressed as percentage (%) unbound
against the negative log of free sulphate concentration.
The affinity constant (pK) was used to
calculate the percentage of salbutamol
bound to sulphate in solution using speciation
software.
At 0.0673 sulphate concentration 68% of
salbutamol (0.00178 M) was bound to
sulphate (table 2).
Atoms
Atom number
Bond distance (Å)
H ammonium – O alkoxide
H(28)-O(42)
1.8609
H ammonium – O alkoxide
H(72)-O(43)
1.7691
H ammonium – O alkoxide
H(44)-O(42)
1.7168
H alcohol – O oxo
H(24)-O(41)
1.7925
H alcohol – O oxo
H(74)-O(41)
1.8284
SB:SS
1:0
500
450
400
SB:SS
1 : 0.006
350
300
SB:SS
1 : 0.7
250
200
150
100
50
0
0
50
100
150
200
250
At a 2:1 salbutamol to sulphate ion-pair complex 5 hydrogen
bonds are formed between the parent drug and counter ion.
The strength of the interaction was considered to be in-line with values
Molecular modelling of the bound complex showed multiple hydrogen bonds formed between the salbutamol sulphate ion-pair.
The transport data showed that the physicochemical properties of salbutamol had been modified by the presence of
sulphate.
Figure 4. Cumulative mass of salbutamol permeation per unit area for
salbutamol base (SB) and salbutamol in the presence of sulphate (SS) into de ionised water (pH 7.4) across RCM (n = 5). Each point represents the mean ± SD.
The steady state flux of salbutamol (0.00178 M) in the
presence of 0.0673 M sulphate concentration was 1.54 ±
0.17 µg/cm2/min between 30-120 minutes (P ≤ 0.0001, twotailed, unpaired, Student’s t-test) (table 2).
Figure 3. (a) 3D conformational model of salbutamol sulphate ion-pair (2:1),
(carbon atoms in grey, oxygen atoms in red, nitrogen atoms in blue, sulphate
atom in yellow & hydrogen atoms in white); (b) structural formula of
salbutamol sulphate with the atom numbers and (c) the intermolecular
hydrogen bonds formed between salbutamol sulphate and the bond
distance calculated using ChemBio 3D version 12.0.2..
Conclusion
Salbutamol formed an ion-pair in solution.
reported in the literature (Dai et al., 2005).
550
Time (min)
(C)
50
A reduction in salbutamol transport across RCM was
observed with an increase in sulphate concentration (fig.
4).
Cumulative Mass per Area (µg/cm2)
Using FT-IR the N - H bend of salbutamol (1606
cm-1) was shown to be sensitive to the
presence of sulphate (1617 cm-1).
Table 2. The flux (µg/cm2/min) of salbutamol base (SAL, 0.00178 M) in the
presence of increasing sulphate concentration (pH 7.4) through RCM over 4
hours. Fux represents mean ± SD (n = 5).
SAL concentration
(M)
0
0.00000673
0.0000213
0.0000673
0.000213
0.000673
0.0673
SAL bound to
sulphate (%)
0
0.02
0.06
0.20
0.64
2.00
67.87
Steady state flux of salbutamol in
the presence sulphate (µg/cm2/min)
3.09 ± 0.29
2.22 ± 0.22
2.23 ± 0.13
1.75 ± 0.47
2.09 ± 0.22
1.39 ± 0.14
1.54 ± 0.17
References
1.  Serajuddin AT (2007). Salt formation to improve drug solubility. Advanced Drug Delivery Reviews 59(7):
603-616.
2.  Dai J, Mendonsa SD, Bowser MT, Lucy CA, Carr PW (2005). Effect of anionic additive type on ion pair
formation constants of basic pharmaceuticals. Journal of Chromatography A 1069(2): 225-234.
Acknowledgements
Prof. Robert Hider and Dr. Kong Xiole
MedPharm Ltd. and the EPSRC for providing the funding for this research project