Tamethicillin is a basic ester pro-drug of methicillin (MET) which is converted in the body by non-specific esterases to MET. Tamethicillin was used as an antibacterial agent in the veterinary as a useful alternative for the treatment of mastitis in livestock, especially in mastitis due to beta-lactamase-producing Staphylococcus.

Oxetacillin was developed as an antibacterial drug that has never been marketed.

Fenbenicillin (phenoxybenzylpenicillin) is an acid-stable, orally active penicillin. Fenbenicillin demonstrated antibacterial activity in patients suffering from streptococcal, pneumococcal, and penicillin-sensitive staphylococcal infections.

Pirbenicillin is a broad-spectrum semisynthetic penicillin with activity against both gram-positive cocci and gram-negative bacilli. Pirbenicillin is less active than either carbenicillin or ticarcillin against Proteus spp. It is as active as carbenicillin against Serratia spp. and Enterobacter spp. Pirbenicillin demonstrated eight- and fourfold better minimal bactericidal concentration values towards Pseudomonas isolates than those of carbenicillin and ticarcillin, respectively. Pirbenicillin binds to and inactivates penicillin-binding proteins (PBPs) located on the inner membrane of the bacterial cell wall. The drug is somewhat more stable in serum than carbenicillin. Pirbenicillin does not appear to inactivate gentamicin as rapidly as carbenicillin.

ISOPROPICILLIN is semisynthetic penicillin.

Piridicillin is the semi-synthetic penicillin. It has exhibited broad-spectrum activity in vitro against gram-positive cocci, except penicillin G-resistant Staphylococcus aureus, and against gram-negative bacilli. Piridicillin is reported to be more active in-vitro than piperacillin, azlocillin or ticarcillin against Ps. aeruginosa. It is unstable at alkaline pH and displays marked inoculum independence.

ROTAMICILLIN was developed as an antibiotic that has never been marketed.

Benethamine penicillin is a medium-long-acting insoluble penicillin salt. After a single injection, this salt acts and bacteriostatic level of penicillin is maintained in the blood for three to six days.

Phenyracillin is the 2,5-diphenylpiperazine salt of benzylpenicillin. It is antibiotic from Pencillium spp. and also other fungal spp. Shows activity against gram-positive bacteria. Phenyracillin was well tolerated.