The Wyeth pharmaceutical company announced on Thursday that it has federal approval for a new antibiotic. The drug, Tygacil, seems to be effective against a wide range of bacteria, including those with a developed resistance to conventional antibiotics. Wyeth calls it the first in a new class of medicines. What makes Tygacil so special?
A distinctive, dangling chemical chain. Tygacil is derived from the tetracycline family of antibiotics that’s been around for decades. Like the tetracyclines, it works by clogging up the bacteria’s ribosomes, the parts of the cell that produce new proteins. But when scientists at Wyeth added a little tail of glyco-amino acids to a familiar tetracycline called minocycline, they got a new and more powerful drug.
Tetracyclines used to be more potent than they are now. In the decades since tetracyclines hit the market, bacteria have evolved several kinds of resistance. Some bacteria, for example, produce a protein “pump” that sits on the membrane and flushes antibiotics out of the cell. Others make protective proteins that pull the antibiotic away from their ribosomes.
Tygacil appears to get around both of these obstacles. The bacterial pumps seem less able to push it around, and the protective proteins aren’t as good at shaking it off the ribosome. As a result, Tygacil can be used to treat patients with certain drug-resistant infections.
It’s not clear if Tygacil can be placed in an existing class of medicines. Antibiotics are grouped informally according to what they look like and what they do. Penicillins, for example, contain a structural feature called a beta-lactam ring; they kill bacteria by inhibiting the formation of bacterial cell walls. Quinolones, like Cipro, have a different structure and work on a bacterium’s DNA.
Many major classes of antibiotics were developed in the 1970s and before. Since then, drug companies have developed numerous variations within each class. Sometimes a more effective version can be created by doing little more than adding a single fluorine atom—doctors can prescribe fluoroquinolones for infections on which regular quinolones would have no effect.
Is Tygacil the first of a “new class of antibiotics,” as Wyeth and several newspapers have reported? That depends on how you define “class.” No official body determines whether a given variant represents a whole new class of antibiotics. Since there is no official list, drug companies can boast of having developed new classes whenever they like. Tygacil might be considered a member of the tetracycline class, since it is such a close derivative of minocycline and works in a similar way. On the other hand, the addition of a glyco-amino side chain has led Wyeth to proclaim it a member of a new class, the “glycylcyclines.”
Explainer thanks Dr. Evan Loh of Wyeth, Dr. George Talbot, and Alexander Mankin of the University of Illinois.
