Q. How do drugs taken once a week, once a month or even once a year do their work?
A. There are three basic mechanisms, said Dr. Marcus M. Reidenberg, head of the clinical pharmacology division of NewYork-Presbyterian Hospital/Weill Cornell Medical Center.
First, “some of the medications are given in a very sustained-release formulation,” he said. Some are injections of tiny crystals that take days or weeks to dissolve and be absorbed. For some, “the chemical is encased in a plastic cylinder and slowly diffuses through the plastic.” Norplant, a birth-control implant, takes five years to diffuse, he said, the longest period he knows of.
For some drugs, a single dose has a long-lasting effect where it ends up in the body. For example, “the ones for osteoporosis go to the bone and bind there and stay,” Dr. Reidenberg explained. “They go to the bone mineral crystal, where they inhibit the action of cells called osteoclasts that normally break down bone.” Other cells, osteoblasts, build bone in a continuing cycle.
“The drug changes the balance of the cycle, so that the total bone mass stops going down,” Dr. Reidenberg said.
Still other drugs prompt a series of events in the body that take much longer for completion, he said. “The drugs used in chemotherapy all produce their effects on the target cells in a matter of hours,” he said. “But the consequences develop that produce the therapeutic effect” over a long period.