The short answer is that it doesn’t: molecules can’t transport themselves through the body, and they have no control over where they end up. But researchers can chemically modify drug molecules to ensure that they bind strongly where we want them and weakly where we don’t.
Pharmaceuticals contain more than the active drug that directly affects the body. Drugs also include “inactive ingredients,” or molecules that enhance stability, absorption, flavor, and other qualities essential to allowing the drug to do its job. For example, the aspirin you swallow also contains ingredients that both prevent the tablet from fracturing during transport and help it break down in your body.
As a pharmaceutical scientist, I have studied drug delivery for 30 years. That is, to develop methods and design non-drug components that help get a drug to where it needs to go in the body. To better understand the thought process behind the design of different drugs, let’s follow a drug from its entry into the body to where it eventually ends up.
How drugs are absorbed into the body
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When you swallow a tablet, it initially dissolves in your stomach and intestines before the drug molecules are absorbed into your bloodstream. Once in the blood, it can travel throughout the body to access different organs and tissues.
Drug molecules affect the body by binding to different receptors on cells that can trigger a particular response. Even though drugs are designed to target specific receptors to produce a desired effect, there is no way to prevent them from continuing to circulate in the blood and binding to off-target sites that can cause unwanted side effects.
Drug molecules circulating in the blood also break down over time and eventually leave the body in your urine. A classic example is the strong smell your urine can have after eating asparagus due to how quickly your kidneys eliminate asparagus acid. Likewise, multivitamins usually contain riboflavin, or vitamin B2, which causes your urine to turn bright yellow when eliminated. Since the efficiency with which drug molecules can cross the intestinal lining can vary depending on the chemical properties of the drug, some of the drugs you swallow are never absorbed and are eliminated in your stool.
Because not all of the drug is absorbed, which is why some drugs, such as those used to treat high blood pressure and allergies, are taken repeatedly to replace the drug molecules that are removed and maintain a high enough level of drug in the blood to maintain its effects in the body.
Getting medications to the right place
Compared to pills and tablets, a more efficient way to get the drug into the bloodstream is to inject it directly into a vein. In this way, all the medicine circulates throughout the body and avoids degradation in the stomach.
Many intravenously administered drugs are “biologics” or “biotechnology drugs,” which include substances derived from other organisms. The most common of these are a type of cancer medicine called monoclonal antibodies, proteins that bind to tumor cells and kill them. These drugs are injected directly into a vein because your stomach can’t tell the difference between digesting therapeutic protein and digesting the protein in a cheeseburger.
In other cases, drugs that require very high concentrations to be effective, such as antibiotics for serious infections, can only be given by infusion. Although increasing drug concentration can help ensure that enough molecules bind to the appropriate sites to have a therapeutic effect, it also increases binding to non-target sites and the risk of side effects.
One way to get a high concentration of medication in the right place is to apply the medication where it’s needed, such as applying ointment to a rash or using allergy eye drops. Although some drug molecules will eventually be absorbed into the bloodstream, they will be diluted enough that the amount of drug that reaches other sites is very small and unlikely to cause side effects. Similarly, an inhaler delivers the medicine directly to the lungs and avoids affecting the rest of the body.
Finally, a key aspect of any drug design is simply getting patients to take the right amount of drugs at the right time.
Because remembering to take a drug several times a day is difficult for many people, researchers try to design drug formulations so that they only need to be taken once a day or less.
Likewise, pills, inhalers, or nasal sprays are more convenient than an infusion that requires traveling to a clinic to have a trained clinician inject it into your arm. The less difficult and expensive it is to administer a drug, the more likely it is that patients will take their drugs when they need them. However, infusions or injections are sometimes the only effective way to deliver certain medications.
Even with all the science needed to understand a disease well enough to develop an effective drug, it’s often up to the patient to make everything work as intended.
(This is a syndicated PTI story via The Conversation)