Suncillin is an antibacteria and antifungal agent produced in Phytera's laboratory from a cell culture of a plant. Suncillin was patented by Bristol-Myers Co. In 1968 for the Pseudomonas bacteria treatment but was never marketed.

Quinacillin is semisynthetic penicillase-resistant penicillin patented by Boots Pure Drug Co. Ltd. For the treatment of bacterial infection. Quinacillin binds to and inactivates penicillin-binding proteins (PBPs) located on the inner membrane of the bacterial cell wall. Inactivation of PBPs interferes with the cross-linkage of peptidoglycan chains necessary for bacterial cell wall strength and rigidity. This interrupts bacterial cell wall synthesis and results in the weakening of the bacterial cell wall, eventually causing cell lysis. In clinical trials Staph. aureus was eradicated from all but two patients during treatment but recurred in 4 after withdrawal. The antibiotic was especially useful in the treatment of staphylococcal respiratory infections, as it has little effect on the normal bacterial flora of the chest.

Fuzlocillin is a penicillin antibiotic. This semisynthetic acylureidopenicillin with antibacterial activity binds to and inactivates penicillin-binding proteins (PBPs) located on the inner membrane of the bacterial cell wall. Fuzlocillin (furazlocillin), has been shown to be highly specific for the FtsI gene. Inactivation of PBPs interferes with the cross-linkage of peptidoglycan chains necessary for bacterial cell wall strength and rigidity. This interrupts bacterial cell wall synthesis and results in the weakening of the bacterial cell wall, eventually causing the cell to break down (cell lysis). Among the acylureidopenicillins, furaclocillin exerts increased activity against Escherischia coli, Klebsiella, Proteus and Pseudomonas aeruginosa.

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.