Almecillin

Almecillin
alt=Top: Glass bottle set in front of marketing box, with labels bearing the text Cer-O-Cillin along with additional information on amount, preparation, manufacturer, and indications. Bottom: Drawing of Almecillin's chemical structure
Clinical data
Trade namesCer-O-Cillin
Other names[(Allylthio)methyl]penicillin
ATC code
  • None
Legal status
Legal status
  • Withdrawn from Market
Identifiers
  • (2S,5R,6R)-3,3-dimethyl-7-oxo-6-[(2-prop-2-enylsulfanylacetyl)amino]-4-thia-1-azabicycloheptane-2-carboxylic acid
CAS Number
PubChem CID
ChemSpider
UNII
ChEBI
ChEMBL
CompTox Dashboard (EPA)
Chemical and physical data
FormulaC13H16N2O4S2
Molar mass328.40 g·mol−1
3D model (JSmol)
  • CC1(C)S[C@@H]2[C@H](NC(=O)CSCC=C)C(=O)N2[C@H]1C(=O)O
  • InChI=1S/C13H18N2O4S2/c1-4-5-20-6-7(16)14-8-10(17)15-9(12(18)19)13(2,3)21-11(8)15/h4,8-9,11H,1,5-6H2,2-3H3,(H,14,16)(H,18,19)/t8-,9+,11-/m1/s1
  • Key:QULKGELYPOJSLP-WCABBAIRSA-N

Almecillin (INN), also known as penicillin O, was a penicillin antibiotic used to treat a number of bacterial infections in those with a penicillin allergy. This included a wide range of streptoccic, staphylococcic, pneumococcic, or gonococcic infections, with similar effect and effectiveness as Benzylpenicillin. It is obtained by isolation from Penicillium chrysogenum.

Medical uses

Antimicrobial potency

As it was designed as a functional analogue to Benzylpenicillin, its antimicrobial profile is functionally identical, possessing effectiveness against Gram-positive bacteria and some limited action against Gram-negative bacteria such as Neisseria gonorrhoeae and Leptospira weilii. Despite this, some research indicated higher doses of Almecillin were required to achieve the same clearance rates, with 51% of samples in one study requiring a doubled dose to achieve growth inhibition.

Toxicity & allergic response

When compared to Benzylpenicillin, Almecillin was found to be profoundly less toxic (requiring 175,000 more units per kilogram of body weight to cause any incidence of death in rat models, and 250,000 more units to cause an irritative reaction), and found little incidence of cross-sensitivity in human case studies.

Following its market introduction; post marketing studies did not support the conclusions that had been made about allergic response, and found that many patients had a similar reaction to that of benzylpenicillin when treated with almecillin, calling into question the main benefit of its use (though the same studies continued to find evidence of reduced toxicity).

Production

Production of so-called "crude" Almecillin was fairly trivial, but yielded inconsistent, and often low concentrations, with the original patent noting that crude concentrations varied from "17 to 80 percent of penicillin O". These low concentration products were unsuitable for medical use due to their variable and often low active ingredient concentration. Previous to the discovery of the model which eventually led to the medications commercial viability, high-concentration production was possible only through partition chromatography utilizing a silica gel base and Phosphate buffered ether, a process which was expensive, time-consuming, and often self defeating.

It was discovered that mixing the impure Almecillin with Chloroprocaine led the almecillin to bond with it, forming a solid crystalline salt, which would spontaneously precipitate out of the solution, increasing purity to at or near 90%.

Marketing & market withdrawal

Beginning in 1951, and until 1957, Almecillin was marketed in the United States as an powder for reconstitution and injection known as Cer-O-Cillin by Upjohn. Following the withdrawal of the product by its manufacturer, it became unavailable in the United States, and approval was officially revoked by the FDA in 1997.

References

  1. ↑ "International Nonproprietary Names for Pharmaceutical Substances (INN). Recommended International Nonproprietary Names: List 40" (PDF). World Health Organization. p. 170.
  2. 1 2 3 "New Prescription Products". Journal of the American Pharmaceutical Association. XII (3): 134–138. March 1951.
  3. 1 2 3 4 Volini IF, Shlaes WH, Felsenfeld O (July 1950). "Use of penicillin O in patients hypersensitive to pencilling G". Journal of the American Medical Association. 143 (9): 794–797. doi:10.1001/jama.1950.02910440012006. PMID 15421819.
  4. ↑ "Penicillin O". ChEBI. EMBL-EBI. Retrieved 11 June 2015.
  5. ↑ Levitov MM, Inozemtseva II, Gotovtseva VA, Komokina ZF, Iudina OD, Kleiner GI, et al. (November 1961). "[Production and basic properties of almecillin (allylmercaptomethylpenicillin)]". Meditsinskaia Promyshlennost' SSSR (in Russian). 11: 12–19. PMID 14464705.
  6. 1 2 3 4 US 2647894, Ford JH, "2-chloroprocaine salt of penicillin omicron", published 1953-08-04, issued 1953-08-04, assigned to Upjohn Co and Pharmacia and Upjohn Co
  7. ↑ "Penicillin G" (PDF). Toku-E. 2010-10-10. Archived from the original (PDF) on 2016-03-03. Retrieved 2012-06-11.
  8. ↑ Siegal S (August 1951). "Allergy to penicillin O and the management of penicillin sensitivity". The American Journal of Medicine. 11 (2): 196–201. doi:10.1016/0002-9343(51)90105-2. PMID 14856988.
  9. ↑ Adair CV, Woodin WG, Bunn PA (August 1951). "Clinical and pharmacologic studies with allylmercaptomethyl penicillin (penicillin O)". The American Journal of Medicine. 11 (2): 188–195. doi:10.1016/0002-9343(51)90104-0. PMID 14856987.
  10. ↑ Marsh RR, Tillotson IG (July 1951). "The cutaneous toxicity and therapeutic effectiveness of penicillin O". The New England Journal of Medicine. 245 (1): 17–20. doi:10.1056/NEJM195107052450104. PMID 14843362.
  11. ↑ Orris C, Winkworth J (18 February 2025). "Listing of Upjohn Human Pharmaceutical Products 1957 - 1995" (PDF). UpJohn.
  12. ↑ PubChem. "Almecillin". PubChem. Retrieved 2026-08-03.
  13. ↑ "Federal Register, Volume 62 Issue 229 (Friday, November 28, 1997)". www.govinfo.gov. 1997-11-28. Retrieved 2026-08-03.