Penem  A penem is a type of unsaturated β-lactam with a sulfur atom in the five-membered ring. Penems do not occur naturally; all are synthetic. Related to penems are carbapenems. Where penems have a sulfur, carbapenems have another carbon. An example is faropenem. Penem molecules do not occur naturally, and production of penems is an entirely synthetic process. 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. 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. One possible consequence of these structural differences of penems from other β-lactams may be reduced immunogenicity and immunogenic cross-reactivity.