Parenteral delivery routes drug absorption

Parenteral delivery routes are those that do not give rise to drug absorption into the splanchnic circulation. Thus, they avoid the possibility of hepatic first-pass metabolism. It should be noted that some parenteral routes do not avoid other first-pass metabolism effects (e.g., pleural metabolism for some inhaled drugs). Some major parenteral drug delivery routes are intraarterial, intrathecal, intravenous, intramuscular, trans-dermal, intranasal, buccal, inhalation, intraperitoneal, vaginal, and rectal. 

The IM and SC routes are by far the most frequently used extravascular parenteral routes of drug administration in farm animals. The less frequently used parenteral routes have limited application, in that they aim at directly placing high concentrations of antimicrobial agent close to the site of infection. These routes of administration include intra-articular or subconjuctival injection and intra-mammary or intra-uterine infusion. These local routes differ from the major parenteral routes in that absorption into the systemic circulation is not a prerequisite for delivery of drug to the site of action. The combined use of systemic and local delivery of drug to the site of infection represents the optimum approach to... 

Many drugs can now be delivered rectally instead of by parenteral injection (intravenous route) or oral administration. Generally, the rectal delivery route is particularly suitable for pediatric and elderly patients who experience difficulty ingesting medication or who are unconscious. However, rectal bioavailabilities tend to be lower than the corresponding values of oral administration. The nature of the drug formulation has been shown to be an essential determinant of the rectal absorption profiles. The development of novel absorption enhancers with potential efficacy without mucosal irritation (low toxicity) is very important. The delivery of peptide and protein drugs by the rectal route is currently being explored and seems to be feasible. 

A required drug absorption profile is achieved by a variety of dosage routes. These routes may be divided into parenteral and enteral. Due to of the importance of the various routes of administration in drug delivery, and of recent advances in optimizing route-dependent drug delivery, they are briefly reviewed here. 

The lack of activity after oral administration for most peptides and proteins resulted in the past besides parenteral application into the utilization of nonoral administration pathways, for example, nasal, buccal, rectal, vaginal, percutaneous, ocular, or pulmonary drug delivery [27]. Drug delivery via these administration routes, however, is also frequently accompanied by presystemic degradation processes. Bioavailability of numerous peptides and proteins is, for example, markedly reduced after subcutaneous or intramuscular administration compared to their intravenous administration. The pharma-cokinetically derived apparent absorption rate constant is thus the combination of absorption into the systemic circulation and presystemic degradation at the absorption... 

It has been shown in a number of studies that the incorporation of drug in o/w nanosized emulsions significantly increased the absorption of the drug when compared with the equivalent aqueous solution administered orally [132-135], However, the use of emulsions for oral application is limited since other attractive alternatives, such as self-emulsifying oil delivery systems, which are much less sensitive and easy to manufacture, are available [136,137], Thus the potential of nanosized emulsions after administration with parenteral and traditional nonparenteral topical routes such as ocular, percutaneous, and nasal is covered in this section. 

Other semi-liquid or liquid preparations can be used rectally (gel, enemas). Rectal drug delivery should not be overlooked in certain therapeutic situations, when oral and parenteral routes are not available, or when the child is unconscious (e.g. postoperative), vomiting or on continuous suction. The absorption is usually rapid and may avoid first-pass metabolism. 

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