Semisynthetic Products of Penicillin ( part-1)

The aim of producing semisynthetic penicillins has been to overcome the short comings of penicillin G (pnG), which are:

1.       Poor oral efficacy

2.       Susceptibility to penicillinase

3.       Narrow spectrum of activity.

Classification:

1.  Acid-resistant alternative to penicillin G: Phenoxymethyl penicillin(penicillin V)

2.  Penicillinase-resistant penicillins: Methicillin, Cloxacillin, Dicloxacillin

3.  Extended spectrum penicillins:

a)      Aminopenicillins: Ampicillin, Bacampicillin, Amoxicillin

b)      Carboxypenicillins: Carbenicillin

c)       Ureidopenicillins: Piperacillin, Mezlocillin

I am giving few features and its importances, to make it short, at the same time, make it adequate.

A. Phenoxymethyl Penicillin (penicillin V):

          1. Unlike pnG, it is acid stable,

          2. Plasma t_1/2 is 30-60 minutes,

          3. Oral absorption is better,

          4. It is used only for streptococcal pharyngitis, sinusitis, otis media, prophylaxis of rheumatic fever, less serious pneumococcal infections and trench mouth.

B. Cloxacillin/Dicloxacillin:

     1.  It is acid resistant,

     2.  Well absorbed from oral route, if taken in empty stomach,

     3.  Plasma t_1/2 is about 1 hour,

     4.  It is used in case of those infections caused by staphylococci, except in areas where methicillin resistant staphylococci aureus has become prevalent.

Extended Spectrum Penicillins: thesesemisynthetic penicillins are active against a variety of gram-negative bacilli as well. They can be grouped according to their spectrum of activity.

A. Aminopenicillins: This group, led by Ampicillin, has an amino substitution in the side chain. Some Prodrugs and all have quite similar antibacterial spectra. None is resistant to penicilinase or to other beta-lactamases.

     Ampicillin:

1. Oral absorption is incomplete but adequate,

2. Plasma t_1/2 is 1 hour,

3. It is more active than PnG for strep. Viridians, enterococci and Listeria, certain gram-negative bacilli are H. influenza, E.coli, proteus etc.

Bacampicilin: it is an ester prodrug of ampicillin which is nearly completely absorbed from the g.i.t.; and is largely hydrolysed  during absorption. Thus, higher plasma levels are attained. Incidence of diarrhoea is claimed to be lower, because of lesser alterartion in intestinal ecology.

Dose: 400-800 mg BD, PENGLOBE 200,400 mg tablets.

Amoxicillin: it is a close congener of ampicillin but not a prodrug; similar to it in almost all aspects, except it is moe active against penicillin resistant  Strep. pnemoniae and incidence of diarrhoea is lower.

Dose: 0.25-1 g TDS oral/i.m. AMOXYLIN, NOVAMOX, SYNAMOX 250, 500 mg capsules.

Uses: 1. Ampicillin has been the drug of choice for most acute urinary tract infections.

 2. In case of Meningitis, Ampicillin has been a first line drug, it is now used in combination with third generation cephalosporin with or without another antibiotic.

3. In case of Gonorrhoea, Cholecystitis it has been proven very effective.

4. Ampicillin is mostly used for eradicaiton of H.pylori from stomach and duodenum.

Rest will be laid out in the next part, untill then learn and help others to learn.