Ideally, most drugs would be in the form of a pill taken once a day (or less frequently), but not all drugs can be given this way. Some drugs, such as protein therapeutics, would be digested in the gut, so they have to be given in other ways — perhaps intravenously or injected under the skin (subcutaneously).
Other drugs need to get into specific tissues of the body at high concentrations, and the best way to get them there is to inject them directly (this is often the case where drugs are needed in the brain and spinal cord, such as forALS or spinal muscular atrophy (SMA), as there are barriers between these tissues and the rest of the body which are impermeable to many molecules).
The MDA-sponsored study of ISIS-SOD1Rx in ALS is an example of a drug that needs to be in the central nervous system, so it took the investigators some time to determine how best to administer it. This drug is now in clinical trials using “intrathecal” administration, directly to the cerebrospinal fluid.
Sometimes a low level of a drug could be achieved by swallowing a pill, but you need the drug at high concentrations in a particular tissue, such as an individual muscle, so the best bet is to directly inject the drug into that muscle. So, developers have to carefully evaluate how the drug is best delivered — if the drug doesn’t get to high enough levels where it is needed, it won’t work. If the drug reaches too-high levels, the developer risks it causing harmful effects.
At the end of this stage, the developer knows how the drug will be administered.Next: Pharmacokinetics & Pharmacodynamics