Mecillinam (INN) or amdinocillin (USAN) is an extended-spectrum penicillin antibiotic that binds specifically to penicillin binding protein 2 (PBP2),^[2] and is only considered to be active against Gram-negative bacteria. It is used primarily in the treatment of urinary tract infections, and has also been used to treat typhoid and paratyphoid fever.^[3][4] Because mecillinam has very low oral bioavailability, an orally active prodrug was developed: pivmecillinam. Neither drug is available in the United States.^[5]

Mecillinam is used in the treatment of infections due to susceptible gram-negative bacteria, especially urinary tract infections which are most commonly caused by Escherichia coli.^[6] Mecillinam is active against most pathogenic Gram-negative bacteria, except Pseudomonas aeruginosa and some species of Proteus.^[5] Several studies have also found it to be as effective as other antibiotics for treating Staphylococcus saprophyticus infection, though it is Gram-positive, possibly because mecillinam reaches very high concentrations in urine.^[1]

Worldwide resistance to mecillinam in bacteria causing urinary tract infection has remained very low since its introduction; a 2003 study conducted in 16 European countries and Canada found resistance to range from 1.2% (Escherichia coli) to 5.2% (Proteus mirabilis).^[7] Another large study conducted in Europe and Brazil obtained similar results — 95.9% of E. coli strains, for instance, were sensitive to mecillinam.^[8]

The adverse effect profile of mecillinam is similar to that of other penicillins.^[2] Its most common side effects are rash and gastrointestinal upset, including nausea and vomiting.^[1]

With the codename FL 1060, mecillinam was developed by the Danish pharmaceutical company Leo Pharmaceutical Products (now LEO Pharma). It was first described in the scientific literature in a 1972 paper.^[9][10]