The Pharmacokinetics of Drugs Delivered to the Upper Nasal Space.

Abstract

Pharmacokinetics (PK) includes how a drug is absorbed, distributed, metabolized and eliminated. The compartment providing this information is usually the plasma. This is as close to the tissue of interest that we can get, although biopsies may be obtained to give "tissue levels" of drugs. Ultimately, the goal of PK is to understand how long the drug is actually engaged with the target in the tissue of interest after a dose has been administered. Most drugs at some point in their development will have been administered intravenously (IV), which acts as the standard for 100% bioavailability. By comparing various routes of administration to IV, the percentage of drug delivered to the plasma, on a dose-normalized basis, can be calculated and is referred to as the "absolute bioavailability". As pharmacology has advanced and more drugs have become available, many older products have been reformulated to be given by routes other than those originally intended (often oral). As the drawbacks of oral (or IV) administration have become better appreciated, non-oral, non-IV formulations and methods of administration have become more popular. Nasal administration is one route that has historically been overlooked as an alternative to oral administration-particularly for products needing rapid and non-invasive access to the target tissue-mostly via the blood stream. But attention is now focused on nasal administration for direct access to the brain, as that has the potential to bypass the blood-brain-barrier (BBB), which not even IV administration can always achieve. Assessing PK for these drugs targeting the brain may require serial sampling of the cerebrospinal fluid (CSF), making PK assessments of CNS drugs more invasive and complex, but still possible in future product development. However, we are now seeing more drugs reformulated for nasal delivery to gain faster systemic levels than oral administration (especially in patients with known or suspected gastrointestinal dysmotility), while avoiding the use of needles. For example, in recent years several different formulations and delivery methods for an old drug, dihydroergotamine (DHE), have been developed and these show very different characteristics, suggesting that delivery to different parts of the nose may have very different PK profiles. This review summarizes the systemic PK of different nasal DHE options that have been, or are being, developed and suggests that delivery of drugs to the upper nasal space (UNS) may represent an optimal target. Further research is required to ascertain if this route could also be utilized for direct administration to the CNS (as an attractive alternative to intrathecal delivery) via the olfactory or trigeminal nerves-but already preclinical data (and some human data) suggest this is entirely possible.