Methicillin is a semisynthetic penicillin antibiotic.

In vitro: Similar to other β-lactam antibiotics, meticillin acts via inhibiting the synthesis of bacterial cell walls. Meticillin can block the cross-linkage between the linear peptidoglycan polymer chains by binding to and competitively inhibiting the transpeptidase enzyme or penicillin-binding proteins [1].

In vivo: In a previous animal study, the treatment with methicillin or gentamicin or both was started 3 days after infection to a experimental mouse model of foreign body infection. Results found that the treatment showed a significant effect, demonstrated as reduction of bacteria on the foreign body, for all three regimens with a reduction of up to 2 log units, but there was no synergism. However, the actual efficacy of the treatment was poor, though the local methicillin concentrations was greater than the MIC for at least 72 h [2].

Clinical trial: Meticillin was developed by Beecham in 1959 and it has been previously used to treat infections caused by susceptible gram-positive bacteria, particularly, penicillinase-producing organisms such as Staphylococcus aureus resistant to most penicillins. However, currently meticillin is no longer manufactured since the more stable and similar penicillins such as oxacillin, flucloxacillin, and dicloxacillin are identified [1].

References:
[1] https://en. wikipedia.org/wiki/Meticillin
[2] Espersen F, Frimodt-M ller N, Corneliussen L, Riber U, Rosdahl VT, Skinh j P. Effect of treatment with methicillin and gentamicin in a new experimental mouse model of foreign body infection. Antimicrob Agents Chemother. 1994 Sep;38(9):2047-53.