The absorption of a drug is when it has dissolved into the solution of the GI tract (dissolution) and transferred across the membrane barrier into the blood (Smith et al., 2012) (16). The absorption rate between the compounds is greatly affected due to propranolol being a lipophilic and atenolol a hydrophilic compound. As propranolol is bound to first pass metabolism, it is all most completely absorbed in comparison to atenolol. Atenolol is hydrophilic, which has at most a bioavailability of 50% and is mostly passed through faeces (Borchard, 1998; Smith et al., 2012). (18,16)
Lipophilic compounds, such as propranolol, are heavily affected by first-pass metabolism, which occurs when the drug concentration decreases dramatically before it has reached systemic bloodstream and site of action. This takes place in the liver when the drug is administered orally, as well as other sites such as the gastrointestinal (GI) tract. Due to the first pass metabolism, the absorption of propranolol is very high and nearly 25% of the drug reaches the systemic circulation. (31) It is also more likely to be absorbed into the central nervous system (CNS) causing side effects such as hallucinations and depression. However, the ability to penetrate the CNS results in propranolol being used for more than its ?-blocker properties, and also for treating other conditions such as migraines (Elliott et al., 1991; Poirier and Tobe, 2014; Pond and Tozer, 1984; Quirke, 2006; Smith et al., 2012). (16, 12,9,22,27).
Atenolol on the other hand, due to its hydrophilic nature, does not move across membranes with as much ease as propranolol (Smith et al., 2012). Therefore, is less susceptible to first-pass metabolism, almost incompletely absorbed in the liver and excreted via the kidneys. Due to the lack of first-pass metabolism, the dosage administered to patients will have to be altered according to their renal function. Atenolol was deemed a safer compound than propranolol as it would not be easily penetrated by the CNS and therefore there were no side effects related to this (Borchard, 1998; Poirier and Tobe, 2014; Smith et al., 2012). (16,18,27)
Therefore, it can be seen that the more lipophilic a drug is the better rate of absorption is will have. This can clearly be seen in propranolol and atenolol, as the absorption rates are 90+% and 50% retrospectively (Smith et al., 2012 (16, 37).