Antibiotics and Penicillins - Introduction PDF

Antibiotics  In 1942, Waksman proposed the widely cited definition that “an antibiotic or antibiotic substance is a substance produced by microorganisms, which has the capacity of inhibiting the growth and even of destroying other microorganisms.” (bacteria, mycobacteria, fungi, protozoa or...

Antibiotics  In 1942, Waksman proposed the widely cited definition that “an antibiotic or antibiotic substance is a substance produced by microorganisms, which has the capacity of inhibiting the growth and even of destroying other microorganisms.” (bacteria, mycobacteria, fungi, protozoa or viruses). This definition restricts to antibiotics as metabolic products of microorganisms.  In 1947, Benedict and Langlykke defined antibiotic as “a chemical compound derived from or produced by a living organism, which is capable, in small concentration, of inhibiting the life processes of microganism” according to this broader definition, the sources of antibiotics are the living organisms in general which may be higher plants or even mammals.  Nowadays the synthesized compounds included under the name antibiotics  In short antibiotics are specific chemical substances derived from, or produced by living organisms, as well as their structural analogs obtained by synthesis, capable of inhibiting in low concentration, the growth and even destruction of other microrganism.  Therefore, a substance is classified as an antibiotic if the following conditions are met: 1. It is a product of metabolism (although it may be duplicated or even have been anticipated by a chemical synthesis). 2. It is a synthetic product produced as a structural analog of a naturally occurring antibiotic. 3. It antagonizes the growth or survival of one or more species of microorganisms. 4. It is effective in low concentrations. PENICILLINS O H C N H H S Me R N Me O CO2H INTRODUCTION TO PENICILLINS Antibacterial agents which inhibit bacterial cell wall synthesis Discovered by Sir Alexander Fleming from a fungal colony (1928) e.g benzyl penicillin and phenoxymethylpenicillin produced by fungi penicillium notatum and P. chrysogenum Shown to be non toxic and antibacterial Isolated and purified by Florey, Chain and Abraham at Oxford University (1938) First successful clinical trial (1941) Produced by large scale fermentation (1944) Structure established by X-ray crystallography (1945) Full synthesis developed by Sheehan (1957) Isolation of 6-APA by Beechams (1958-60). development of semi-synthetic penicillins Discovery of clavulanic acid and b-lactamase inhibitors STRUCTURE R= O CH2 H H H C N S Me Benzyl penicillin (Pen G) R 6-Aminopenicillanic acid N (6-APA) R= Acyl side Me chain O O CH2 CO2H Thiazolidine ring Phenoxymethyl penicillin (Pen V) Side chain varies depending on carboxylic acid present in fermentation medium. However the method is of limited use because its tedious and time consuming CH2 CO2H Penicillin G present in corn steep liquor (is a by-product of corn wet-milling. A viscous concentrate of corn solubles, it is an important constituent of some growth media. It was used in the culturing of Penicillium during research into penicillin. OCH2 CO2H Penicillin V (first orally active penicillin)  The penam ring (β-lactam fused to thiazolidine) can be considered as a dipetide composed of a cysteine and a valine residues. H N S CH3 cysteine residue CH3 N valine residue O COOH SH CH3 H2N H CH2 C CH3 O OH H2N COOH  Shape of Penicillin G The β-lactam ring and the thiazolidine ring are not co-planar but in a folded structure with an angle of 117° Between the 2 rings. This non-planarity suppresses normal amide resonance O Me C H R NH S Me O N CO2H H H.. The unshared es are far to interact with resonance Folded ‘envelope’ shape The β –lactam ring is highly reactive, unstable and ring open easily because: 1. Four membered ring and hence highly strained 2. Have another ring fused to it (5-membered) and this increase the strain. 3. The unshared pair of es on the β – lactam N are not in a plane with the C=O as in normal amide and therefore there is no resonance and this makes penicillins sensitive to nucleophilic attack (unlike normal amides which are less sensitive) 4. Susceptible to internal nucleophilic attack from side chain amide.  All penicillins contain 3 chiral centres i.e 8 optically active forms, the natural isomer with the biological activity is 2S, 5R, 6R.  Owing to the high strain of the β –lactam, it is easily cleaved as in the following chemical degradation reaction: 1. Alkaline hydrolysis (H2O/OH ) to penicilloic acid which is a stable sodium salt but readily decarboxylate in acidic media to penilloic acid 2. Other nucleophile such as ROH, RNH2. NH2OH can attack the carbonyl carbon of the β –lactam and result in ester, amide and hydrooxamic acid respectively.  H2 C H N S CH3 O CH3 H2 C H HN O amide N S CH3 NHR COOH O CH3 HN H2 O ester C H OR RNH2 N COOH S CH3 ROH O CH3 HN O H2 NHOH C H NH2OH COOH N hydrooxamic acid S CH3 O CH3 N 1) NaOH O COOH H2 C H2 H C H N N S CH3 CH3 S H2C O CH3 H+ O CH3 HN hot HN O O Na COOH COOH Penicilloic acid Acidified Na salt readily decarboxylate to penilloic acid  From a clinical point of view, the most significant transformation of penicillins are caused by gasteric acid (pH 2) and the microbial enzyme penicillinase. R HOOC H N S CH3 S CH3 pH 2 O CH3 N CH3 N N O COOH R COOH penillic acid Acids helps in influencing side chain amide paticipating in attacking the β – lactam ring, mechanism for this acid rearrangement is: R H R H N N S CH3 CH3 S CH3 N S H+ O CH3 R CH3 O CH3 CH NH N O O C HN O COOH COOH O COOH  HOOC S CH3 N CH3 N R COOH penillic acid Further reaction of penillic acid decarboxylate O O H H2 R C N C C H  Β –lactamase enzyme cleavage start with a nucleophilic attack, it contain a serine amio acid residue at its active site that initiate the attack of the cleavade. R H N R S CH3 H N S CH3 O CH3 HN O CH3 O H2 N O Enz C OH COOH O H2C COOH ENz Biosynthesis of Penicillins O H C N S Me R N Me O CO2H CYS VAL Properties of Penicillin G Active vs. Gram +ve bacilli and some Gram -ve cocci Non toxic Limited range of activity Not orally active - must be injected Sensitive to b-lactamases (enzymes which hydrolyse the b-lactam ring) Some patients are allergic Inactive vs. Staphylococci Drug Development Aims To increase chemical stability for oral administration To increase resistance to b-lactamases To increase the range of activity  SAR Amide essential Cis Stereochemistry essential O H C N H H S Me R N Me O Carboxylic acid essential bLactam essential CO2H Conclusions Bicyclic system essential Amide and carboxylic acid are involved in binding Carboxylic acid binds as the carboxylate ion Mechanism of action involves the b-lactam ring Activity related to b-lactam ring strain (subject to stability factors) Bicyclic system increases b-lactam ring strain Not much variation in structure is possible Variations are limited to the side chain (R)  Mechanism of action Penicillins inhibit a bacterial enzyme called the transpeptidase enzyme which is involved in the synthesis of the bacterial cell wall. Penicillins acylate a specific bacterial D-transpeptidase The b-lactam ring is involved in the mechanism of inhibition Penicillin becomes covalently linked to the enzyme’s active site leading to irreversible inhibition. O O O H H H H H H H H H C N C N C N S Me S Me S Me R R R Enz-Nu O C HN Nu N Me N Me Me -H Nu-Enz O O Enz CO2H CO2H CO2H H Covalent bond formed to transpeptidase enzyme Irreversible inhibition  Mechanism of action - bacterial cell wall synthesis NAM NAG NAM NAG NAM L-Ala NAM L-Ala NAG NAM L-Ala NAG NAM D-Glu D-Glu D-Glu L-Ala L-Ala L-Ala NAM NAG NAM NAG NAM L-Lys L-Lys L-Lys D-Glu D-Glu D-Glu L-Ala L-Ala L-Ala L-Lys L-Lys L-Lys D-Glu D-Glu D-Glu L-Lys L-Lys L-Lys Bond formation inhibited by penicillin NAM = N-Acetylmuramic acid NAG = N-Acetylglucosamine Mechanism of action - bacterial cell wall synthesis NAM NAG NAM NAG SUGAR BACKBONE L-Ala L-Ala D-Glu D-Glu L-Lys Gly Gly Gly Gly Gly L-Lys Gly Gly Gly Gly Gly D-Ala D-Ala D-Ala D-Ala PENICILLIN TRANSPEPTIDAS E D-Alanine NAM NAG NAM NAG SUGAR BACKBONE L-Ala L-Ala D-Glu D-Glu L-Lys Gly Gly Gly Gly Gly L-Lys Gly Gly Gly Gly Gly D-Ala D-Ala Cross linking Mechanism of action - bacterial cell wall synthesis Penicillin inhibits final crosslinking stage of cell wall synthesis It reacts with the transpeptidase enzyme to form an irreversible covalent bond Inhibition of transpeptidase leads to a weakened cell wall Cells swell due to water entering the cell, then burst (lysis) Penicillin possibly acts as an analogue of the L-Ala-g-D- Glu portion of the pentapeptide chain. However, the carboxylate group that is essential to penicillin activity is not present in this portion Mechanism of action - bacterial cell wall synthesis Alternative theory- Pencillin mimics D-Ala-D-Ala. Normal Mechanism Pe ptide Pe ptide Ch ain Ch ain Pe ptide Pe ptide Chain Pe ptide D-Ala Gly Ch ain Ch ain Gly D-Ala D-Ala CO 2 H D-Ala OH OH O H Mechanism of action - bacterial cell wall synthesis Alternative theory- Pencillin mimics D-Ala-D-Ala. Mechanism inhibited by penicillin Pe ptide Chain Blocked Blocked H2 O O H O O R C NH Gly S Me R C NH H R C NH H S S Me Me N Me O O O HN HN Me H CO2H Me CO2H O CO2H OH O Blocked Irreversibly blocked Mechanism of action - bacterial cell wall synthesis Penicillin can be seen to mimic acyl-D-Ala-D-Ala R R H H C N H H C N Me S Me O O H H H N N Me O O CH3 CO2H CO2H Penicillin Acyl-D-Ala-D-Ala R H C N Me H S Me But 6-methylpenicillin is inactive O despite being a closer analogue N Me O CO2H Mechanism of action - bacterial cell wall synthesis Penicillin may act as an ‘umbrella’ inhibitor D- A l a - D - A l a R H N Blocked C H S Me O R H N N H C Me S Me O Acylation O CO2H HN HO O O Me H CO2H Active Site OH OH Gram +ve and Gram -ve Cell Walls Penicillins have to cross the bacterial cell wall in order to reach their target enzyme Cell walls are porous and are not a barrier The cell walls of Gram +ve bacteria are thicker than Gram -ve cell walls, but the former are more susceptible to penicillins Gram +ve and Gram -ve Cell Walls Gram +ve bacteria Thick porous cell wall Cell membrane Cell Thick cell wall No outer membrane More susceptible to penicillins Gram +ve and Gram -ve Cell Walls Gram -ve bacteria Hydrophobic barrier Outer membrane Porin Lactamase L enzymes L L Periplasmic Thin cell wall space L Cell Cell membrane Thin cell wall Hydrophobic outer membrane More resistant to penicillins Resistance to Penicillins Factors Gram -ve bacteria have a lipopolysaccharide outer membrane preventing access to the cell wall Penicillins can only cross via porins in the outer membrane Porins (protein channel in the cell wall) allow small hydrophilic molecules such as zwitterions to cross High levels of transpeptidase enzyme may be present (saturation). The transpeptidase enzyme may have a low affinity for penicillins (e.g. PBP 2a (penicillin-binding proteins 2a) for S. aureus) Presence of β-lactamases Concentration of β-lactamases in periplasmic space Mutations Transfer of β-lactamases between strains Efflux mechanisms pumping penicillin out of periplasmic space through the porins Penicillin Analogues - Preparation 1) By fermentation Vary the carboxylic acid in the fermentation medium Limited to unbranched acids at the α-position i.e. RCH2CO2H Tedious and slow (see slide No. 5) 2) By total synthesis Only 1% overall yield Impractical  O O O O S NH2 O O H NH N + HS O N H HO O O O O O t-butyl-  - phthaliminomalonaldehyde D-penicillamine HCl A 1 H2N NH2 2 HCl C6H5OCHCHOCl H H S Cl H3N HN O O OH O couple amino acids during artificial peptide synthesis N,N'-Dicyclohexylcarbodiimide 3) By semi-synthetic procedures Use a naturally occurring structure as the starting material for analogue synthesis O H H C N S Me CH2 Penicillin G N Me O CO2H Penicillin acylase or chemical hydrolysis H H H2N S Me Fermentation N Me O 6-APA CO2H O C R Cl O H H H C N S Me R Semi-synthetic penicillins N Me O CO2H  Penicillin Analogues - Preparation Problem - How does one hydrolyse the side chain by chemical means in presence of a labile β-lactam ring? Answer - Activate the side chain first to make it more reactive O PhCH2 C NH Cl OR S PCl5 ROH H2O PhCH2 C N PhCH2 C N 6-APA N PEN PEN O CO2H Note - Reaction with PCl5 requires the involvement of a lone pair of electrons from nitrogen. Not possible for the β-lactam nitrogen as the unshared es are out the plane of the ring not available for resonance, therefore the reaction selective to side chain amide nitrogen. PCl4 H O O Cl4 S CH3 S CH3 P Cl R N R N H CH3 CH3 N Cl N O O COOH COOH  Cl OR S CH3 R N CH3 ROH S CH3 R N H2O N CH3 O N COOH O COOH Cl S CH3 H2N CH3 R O N Semisyntheic penicillins O COOH 6-aminopenicillanic acid (6APA) See table 1 semi synthetic penicillins Table 1 penicillins  Nomenclature  The first system used to penicillins which designated according to the Chemical Abstracts system as 5- acylamino-2,2- dimethylpenam-3-carboxylic acids (penam is the bicyclic penicillin nucleus).  The second, seen more frequently in the medical literature, uses the name “penicillanic acid” to describe the ring system with substituents that are generally present (i.e., 2,2-dimethyl and 3- carboxyl).  A third form of nomenclature is to name the entire 6- carbonylaminopenicillanic acid portion of the molecule (penicillin) and then name the R as substituent e.g Penicillin G is benzyl penicillin in which R is benzyl see table (1) for more examples. Problems with Penicillin G It is sensitive to stomach acids It is sensitive to b-lactamases - enzymes which hydrolyse the b- lactam ring It has a limited range of activity  Problem 1 - Acid Sensitivity Reasons for sensitivity Factor 1: Ring strain O O O H H H Acid or C H N H H C H N H H C N S Me enzyme R S Me R S Me R HO HO2C N Me HN Me H2O N Me O O CO2H CO2H CO2H H Relieves ring strain  Problem 1 - Acid Sensitivity Reasons for sensitivity Factor 2: Reactive b-lactam carbonyl group Does not behave like a tertiary amide Tertiary amide R R R C NR2 C N Unreactive O O R b-Lactam Me S S Me Me O N H CO2H X O N Me Folded ring CO2H Impossibly system strained Interaction of nitrogen’s lone pair with the carbonyl group is not possible Results in a reactive carbonyl group Problem 1 - Acid Sensitivity Reasons for sensitivity Factor 3: Acyl side chain Neighbouring group participation in the hydrolysis mechanism R H C N H R N R N S S S -H O O N O HN N O O O H Further reactions Problem 1 - Acid Sensitivity Conclusions The β-lactam ring is essential for activity and must be retained Cannot deal with factors 1 and 2 (related to β –lactam ring) Can only deal with factor 3 (related to side chain) Strategy  Acid-catalyzed degradation in the stomach contributes strongly to the poor oral absorption of penicillin. Thus, efforts to obtain penicillins with improved pharmacokinetic and microbiological properties, have focused on acyl functionalities that would minimize sensitivity of the β -lactam ring to acid hydrolysis while maintaining antibacterial activity.  Vary the acyl side group (R) to make it electron-withdrawing to decrease the nucleophilicity of the carbonyl oxygen H H E.W.G. N S C O N O Decreases nucleophilicity  Problem 1 - Acid Sensitivity X HC  H H N Examples R C S H H O N PhO CH2 N S O C O N X = NH2, Cl, PhOCONH, electronegative O Heterocycles, CO2H oxygen Penicillin V (orally active) Very successful semi-synthetic penicillins e.g. Better acid stability and orally active α–aminobenzylpenicillin But sensitive to β-lactamases (ampicillin), and α- Slightly less active than penicillin G halobenzylpenicillin are Allergy problems with some patients significantly more stable than benzylpenicillin in acid solutions. Hetrocycles e.g Oxacillin (see tab 1 slide 33)  α-aminobenzylpenicillin (ampicillin) exists as the protonated form in acidic (as well as neutral) solutions, and the ammonium group is known to be powerfully electron- withdrawing. NH3 H H N S CH3 O CH3 N O COO  Problem 2 - Sensitivity to b-Lactamases b-Lactamases Enzymes that inactivate penicillins by opening b-lactam rings Allow bacteria to be resistant to penicillin Transferable between bacterial strains (i.e. bacteria can acquire resistance) Important w.r.t. Staphylococcus aureus infections in hospitals 80% Staph. infections in hospitals were resistant to penicillin and other antibacterial agents by 1960 Mechanism of action for lactamases is identical to the mechanism of inhibition for the target enzyme (transpeptidase) But product is removed efficiently from the lactamase active site O O H H H H C N C N S Me S Me R R N HO2C HN Me Me O b-Lactamase CO2H CO2H Problem 2 - Sensitivity to b-Lactamases Strategy Use of steric shields Block access of penicillin to the active site of the enzyme by introducing bulky groups to the side chain Size of shield is crucial to inhibit reaction of penicillins with b- lactamases, but not with the target transpeptidase enzyme O Bulky H H H C N group S Me R N Me Enzyme O CO2H Problem 2 - Sensitivity to b-Lactamases Examples - Methicillin (Beechams - 1960) O ortho groups H H H important MeO C N S Me N Me OMe O CO2H Methoxy groups block access to b-lactamases but not to transpeptidases Binds less readily to transpeptidases compared to penicillin G Lower activity compared to Pen G against Pen G sensitive bacteria Poor activity vs. some streptococci Inactive vs. Gram -ve bacteria Poorer range of activity Active against some penicillin G resistant strains (e.g. Staphylococcus) Acid sensitive since there is no electron-withdrawing group Orally inactive and must be injected  Problem 2 - Sensitivity to b-Lactamases Examples - Oxacillin R' O H H H Oxacillin R = R' = H C N S Me Cloxacillin R = Cl, R' = H R N Flucloxacillin R = Cl, R' = F N Me O Me O Bulky and CO2H e- withdrawing Orally active and acid resistant Resistant to b-lactamases Active vs. Staphylococcus aureus Less active than other penicillins Inactive vs. Gram -ve bacteria Nature of R & R’ influences absorption and plasma protein binding Cloxacillin better absorbed than oxacillin Flucloxacillin less bound to plasma protein, leading to higher levels of free drug Problem 3 - Range of Activity Factors 1) Cell wall may have a coat preventing access to the cell 2) Excess transpeptidase enzyme may be present 3) Resistant transpeptidase enzyme (modified structure) 4) Presence of b-lactamases 5) Transfer of b-lactamases between strains 6) Efflux mechanisms Strategy The number of factors involved make a single strategy impossible Use trial and error by varying R groups on the side chain Successful in producing broad spectrum antibiotics Results demonstrate general rules for broad spectrum activity.  Problem 3 - Range of Activity Results of varying R in Pen G 1) Hydrophobic side chains result in high activity vs. Gram +ve bacteria and poor activity vs. Gram -ve bacteria 2) Increasing hydrophobicity has little effect on Gram +ve activity but lowers Gram -ve activity 3) Increasing hydrophilic character has little effect on Gram +ve activity but increases Gram -ve activity 4) Hydrophilic groups at the -position (e.g. NH2, OH, CO2H) increase activity vs Gram -ve bacteria. The significant advance arising from the preparation of semisynthetic penicillins was by the introduction of an ionized or polar group into the α-position of the side chain benzyl carbon atom of penicillin G confers activity against Gram-negative bacilli. Hence, derivatives with an ionized -amino group, such as ampicillin and amoxicillin, are generally effective against such Gram-negative as Escherichia, Klebsiella, Haemophilus, Salmonella, Shigella Problem 3 - Range of Activity Examples of Broad Spectrum Penicillins Class 1 - NH2 at the -position Ampicillin and amoxicillin (Beechams, 1964) H NH2 H NH2 C HO C H H C N H C N H O O O O Ampicillin (Penbritin) Amoxicillin (Amoxil) 2nd most used penicillin Problem 3 - Range of Activity Examples of Broad Spectrum Penicillins Properties Active vs Gram +ve bacteria and Gram -ve bacteria which do not produce b-lactamases Acid resistant and orally active Non toxic Sensitive to b-lactamases Increased polarity due to extra amino group Poor absorption through the gut wall Disruption of gut flora leading to diarrhoea Inactive vs. Pseudomonas aeruginosa  Incorporation of an acidic substituent at the -benzyl carbon atom of penicillin G also imparts clinical effectiveness against Gram-negative bacilli and, furthermore, extends the spectrum of activity to include organisms resistant to ampicillin. Thus, α-carboxybenzylpenicillin (carbenicillin) is active against ampicillin-sensitive, Gram-negative species and additional Gram-negative bacilli of the genera Pseudomonas, Klebsiella, Enterobacter COOH H H N S CH3 O CH3 N O COOH carbenicillin Problem 3 - Range of Activity Prodrugs of Ampicillin (Leo Pharmaceuticals - 1969) O C H NH2 R= CH2O CMe3 PIVAMPICILLIN C H O C N H H S R= O TALAMPICILLIN O Me N Me O O CO2R C R= CH O O CH2Me Properties Me BACAMPICILLIN Increased cell membrane permeability Polar carboxylic acid group is masked by the ester Ester is metabolised in the body by esterases to give the free drug  Problem 3 - Range of Activity Mechanism of prodrug activation O H H PEN PEN PEN C H C O CH2 O C OH C O CH2 O CMe3 O O O ENZYME Formaldehyde Extended ester is less shielded by the penicillin nucleus Hydrolysed product is chemically unstable and degrades Methyl ester of ampicillin is not hydrolysed in the body Bulky penicillin nucleus acts as a steric shield for methyl ester Problem 3 - Range of Activity Examples of broad spectrum penicillins Class 2 - CO2H at the -position (carboxypenicillins) CO2R Examples CH H R=H Carbenicillin C N H H S R = Ph Carfecillin O Me N Me O CO2H Carfecillin = prodrug for carbenicillin Active over a wider range of Gram -ve bacteria than ampicillin Active vs. Pseudomonas aeruginosa Resistant to most b-lactamases Less active vs Gram +ve bacteria (note the hydrophilic group) Acid sensitive and must be injected Stereochemistry at the -position is important CO2H at the -position is ionised at blood pH Problem 3 - Range of Activity Examples of broad spectrum penicillins Class 2 - CO2H at the -position (carboxypenicillins) Examples CO2H H H H N S Me TICARCILLIN S O N Me O CO2H Administered by injection Identical antibacterial spectrum to carbenicillin Smaller doses required compared to carbenicillin More effective against P. aeruginosa Fewer side effects Can be administered with clavulanic acid Problem 3 - Range of Activity Examples of broad spectrum penicillins Class 3 - Urea group at the -position (ureidopenicillins) O Examples Azlocillin HN O N O R2N NH MeO2S H H H Mezlocillin N N S N Me O N Et N N Me Piperacillin O O O CO2H Administered by injection Generally more active than carboxypenicillins vs. streptococci and Haemophilus species Generally have similar activity vs Gram -ve aerobic rods Generally more active vs other Gram -ve bacteria Azlocillin is effective vs P. aeruginosa Piperacillin can be administered alongside tazobactam (β – lactamase inhibitor) The strategy of using a -lactamase inhibitor in combination with a β- lactamase–sensitive penicillin in the therapy for infections caused by β- lactamase–producing bacterial strains has, until relatively recently, failed to live up to its obvious promise. Early attempts to obtain synergy against such resistant strains, by using combinations consisting of a β- lactamase–resistant penicillin (e.g., methicillin or oxacillin) as a competitive inhibitor and a β-lactamase– sensitive penicillin (e.g., ampicillin or carbenicillin) to kill the organisms, met with limited success. Factors that may contribute to the failure of such combinations to achieve synergy include (a) the failure of most lipophilic penicillinase-resistant penicillins to penetrate the cell envelope of Gram-negative bacilli in effective concentrations,. (b) the reversible binding of penicillinase-resistant penicillins to β- lactamase, requiring high concentrations to prevent substrate binding and hydrolysis (c) the induction ofβ -lactamases by some penicillinase- resistant penicillins. The discovery of the naturally occurring, mechanism-based inhibitor clavulanic acid, which causes potent and progressive inactivation of -lactamases (Fig. 8.4), has created renewed interest in -lactam combination therapy. Read more in Wilson and Gisvold’s textbook  Most penicillins are acids with pKa values in the range of 2.5 to 3.0, but some are amphoteric. The free acids are not suitable for oral or parenteral administration. The sodium and potassium salts of most penicillins, however, are soluble in water and readily absorbed orally or parenterally.  Salts of penicillins with organic bases, such as benzathine, procaine, and hydrabamine, have limited water solubility and are, therefore, useful as depot forms to provide effective blood levels over a long period in the treatment of chronic infections. Procaine penicillin benzathine penicillin Various designations have been used to classify penicillins, based on their sources, chemistry, pharmacokinetic properties, resistance to enzymatic spectrum of activity, and clinical uses (Table 2). Thus, penicillins may be biosynthetic, semisynthetic, or (potentially) synthetic; acid-resistant or not; orally or (only) parenterally active; and resistant to β- lactamases (penicillinases) or not. They may have a narrow, intermediate, broad, or extended spectrum of antibacterial activity and may be intended for multipurpose or limited clinical use.