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| 146P London, UK Pharmacology 2016 |
The effect of the N-alkyl substituent on the affinity of alprenolol and oxprenolol analogues at the human β1 and β2 adrenoceptor (AR)
Introduction: Alprenolol (1a) and oxprenolol (2a) are non-selective β-blockers that bind to both β1- and β2-ARs with high affinity. These compounds show interesting pharmacological characteristics as they can activate two agonist conformations of the β1-AR but only one β2-AR agonist conformation1. As a preliminary investigation of this phenomenon we aimed to study the effect of the amine substituent on the affinity of a series of alprenolol and oxprenolol analogues for both receptors.
Method: CHO cells stably expressing the human β1-AR or β2-AR were used throughout the study2. Ligand affinity for the β1- and β2-AR was assessed using 3H-CGP 12177 whole cell binding. The KD values for 3H-CGP 12177 was determined by saturation binding and values of β1 0.26 ± 0.02 nM n=10 and β2 0.20 ± 0.02 nM n=10 obtained.
Results: All analogues bound to both receptors (Table 1). Analogues containing a primary amine (1c/2c) yielded the lowest affinities. Methylation of the primary amine (1d/2d) slightly increased affinity for β2-AR and further homologation increased the affinity for both receptors with an ethyl substituent (1e/2e) as the optimal length. Branched substituents yielded the highest affinities, where analogues containing a tert-butyl group (1b/2b) improved affinity over the parent compounds (1a-b). Introduction of a more rigid substituent as cyclopropyl (1h/2h) and cyclopentyl (1j/2j) decreased affinity for both receptors. Interestingly, whilst all other analogues had β2-selectivity, the addition of the 3,4-dimethoxyphenethyl group (1l/2l) made the compounds non-selective.
Table 1 Binding affinities for human β1- and β2-ARs of alprenolol and oxprenolol analogues assessed using 3H-CGP 12177 whole cell binding assays. Data are mean±SEM for n experiments. 1a/2a are the parent compounds.
| Substituent (R) | Alprenolol analogues (1a-l) | Oxprenolol analogues (2a-l) | ||||||
| LogKDβ1 | n | LogKDβ2 | n | LogKDβ1 | n | LogKDβ2 | n | |
| A | -7.94 ± 0.02 | 6 | -9.01 ± 0.04 | 6 | -7.89 ± 0.02 | 6 | -8.77 ± 0.03 | 6 |
| B | -8.51 ± 0.05 | 6 | -9.74 ± 0.09 | 5 | -8.37 ± 0.03 | 5 | -9.51 ± 0.06 | 6 |
| C | -6.56 ± 0.03 | 6 | -7.18 ± 0.03 | 6 | -6.41 ± 0.01 | 6 | -7.07 ± 0.02 | 5 |
| D | -6.58 ± 0.03 | 6 | -7.66 ± 0.02 | 6 | -6.44 ±0.05 | 6 | -7.48 ± 0.04 | 5 |
| E | -7.23 ± 0.02 | 6 | -8.23 ± 0.04 | 6 | -7.31 ± 0.04 | 6 | -8.20 ± 0.05 | 5 |
| F | -7.07 ± 0.04 | 6 | -8.09 ± 0.11 | 5 | -6.93 ± 0.03 | 6 | -7.83 ± 0.04 | 6 |
| G | -6.86 ± 0.02 | 6 | -7.75 ± 0.04 | 6 | -6.75 ± 0.03 | 6 | -7.70 ± 0.03 | 5 |
| H | -7.36 ± 0.03 | 6 | -8.35 ± 0.06 | 6 | -7.50 ± 0.05 | 5 | -8.53 ± 0.04 | 6 |
| I | -7.04 ± 0.02 | 6 | -8.04 ± 0.02 | 6 | -6.78 ± 0.06 | 5 | -7.91 ± 0.05 | 5 |
| J | -7.66 ± 0.03 | 6 | -8.69 ± 0.03 | 6 | -7.46 ± 0.03 | 5 | -8.52 ± 0.08 | 6 |
| K | -6.61 ± 0.04 | 5 | -7.55 ± 0.06 | 5 | -6.44 ± 0.04 | 6 | -7.77 ± 0.03 | 6 |
| L | -8.14 ± 0.03 | 6 | -8.24 ± 0.04 | 6 | -8.22 ± 0.04 | 6 | -8.15 ± 0.04 | 6 |
Conclusion: Alprenolol and oxprenolol analogues were shown to follow the same trend. With regards to linear N-alkyl groups, ethyl substituent yielded the best affinity. In a similar manner, branching increased the affinities for both sets of analogues. Overall, the replacement of alprenolol and oxprenolol iso-propyl moiety by a tert-butyl group yielded the highest affinity for both receptors.
References:
1. Baker JG et al. (2003) Mol Pharmacol 63: 1312–1321.
2. Baker JG. (2005) Br J Pharmacol. 144: 317-322.