Penem

A penem is a type of β-lactam with an unsaturated five-member heterocycle containing a sulfur atom in a pentacyclic ring fused to the β-lactam ring. Penems do not occur naturally; all are synthetic.¹ Related to penems are carbapenems, which have a carbon atom in place of the sulfur atom.²

An example is faropenem.³

Penem molecules do not occur naturally, and production of penems is an entirely synthetic process.⁵

Five main penem subgroups — thiopenems, oxypenems, aminopenems, alkylpenems, and arylpenems — have been produced and are distinguished by the side chain (at position 2) of the unsaturated five-membered ring.⁵ One structurally distinct penem is BRL 42715. This molecule has no substitution at the above position, but has a bulky group attached to the β-lactam ring, and it displays effective inhibition of class C β-lactamases, but no antimicrobial activity.⁶

One possible consequence of these structural differences of penems from other β-lactams may be reduced immunogenicity and immunogenic cross-reactivity.