Penicillins semisynthetic production

These defects have spurred attempts to prepare analogs. The techniques used have been (1) natural fermentation (in which the penicillin-producing fungus is allowed to grow on a variety of complex natural nutrients from which it selects acids for incorporation into the side chain), (2) biosynthetic production (in which the fermentation medium is deliberately supplemented with unnatural precursors from which the fungus selects components for the synthesis of "unnatural" penicillins), (3) semisynthetic production (in which 6-aminopenicillanic acid (2) is obtained by a process involving fermentation, and suitably activated acids are subsequently reacted chemically with 6-APA to form penicillins with new side chains) and (4) total synthesis (potentially the most powerful method for making deep-seated structural modifications but which is at present unable to compete economically with the other methods). 

The amounts of penicilloylated proteins in commercial benzylpenicillin and semisynthetic penicillins seem to vary considerably. Whereas up to 200-300 parts/ 10 of penicilloy 1-protein in benzylpenicillin preparations were reported by Batchelor et al. (1967) and also by Butcher and Stewart (1970), other groups have only detected less than 10 parts/10 of proteinaceous impurities in commercial benzylpenicillin (Dursch 1968 Preud homme and Lunel 1969 Weidenmuller and Ziegler 1970 Walsh et al. 1971 Ottens et al. 1971 Vilim et al. 1976). As described by Ottens et al. (1971), the methods used for the determinations, i.e., dialysis, membrane filtration, and gel filtration followed by penicilloyl and protein analysis, have several drawbacks. Penicillin degradation products and polymers were found to influence the determinations, making these somewhat unreliable. 

Semisynthetic. In 1959, scientists at Beecham Researeh Laboratories sueceeded in isolating the penicillin nucleus, 6-aminopenicillanie aeid (6-APA Fig. 5.1 A R represents H). During the commercial production of benzylpenicillin, phenylacetic (phenylethanoic) acid (C6H5.CH2.COOH) is added to the medium in whieh the Penicillium mould is growing (see Chapter 7). This substance is a precursor of the side... 

The manufacture of benzylpenicillin (penieillin G, originally just penicillin ) is chosen as a model for the antibiotic production process. It is the most renowned of antibioties and is the first to have been manufactured in bulk. It is still universally prescribed and is also in demand as input material for semisynthetic antibiotics (Chapter 5). Developments associated with the penicillin fermentation process have been a significant factor in the development of modem bioteehnology. It was a further 30 years, i.e. not until the 1970s, before there were signifieant new advances in industrial fermentations. 

It is possible to convert penicillin V or benzylpenieillin to a cephalosporin by chemical ring expansion. The first-generation cephalosporin cephalexin, for example, can be made in this way. Most cephalosporins used in clinical practice, however, are semisynthetics produced from the fermentation product cephalosporin C. 

Fig. 2.2 illustrates the three types of chemicals by reference to the production of the semisynthetic penicillin antibiotic, ampicillin. 

The bacterium Staphylococcus aureus, which is a major cause of infection in the developed countries, is now resistant to most antibiotics. It is usually present on the skin, where it causes no problems, but it can invade the body through cuts and wounds, including those caused by surgery. These bacteria are now prevalent in many hospitals, so that infection is a major problem for the medical staff in hospitals. The resistant bacterium is known as methicillin-resistant Staphylococcus aureus (MRSA). It is also known in the mass media as the super bug . Penicillin kiUs bacteria because the P-lactam group in the antibiotic inhibits a reaction that is essential for bacterial ceU wall production. Consequently, the bacteria cannot proliferate. Resistance to penicillin in many bacteria is due to production of an enzyme, p-lactamase, that degrades P-lactams. The antibiotic methicillin is one of a group of semisynthetic penicillins in which the P-lactam group is not... 

Semisynthetic heterocycles are also important dmg molecules. These compounds attempt to capture the best of both worlds, being synthetic derivatives of natural products. The use of a natural product in the preliminary stages of the synthesis enables the elimination of numerous costly synthetic steps. The subsequent synthetic modifications enable further fine tuning of the natural product pharmacophore. There are a number of semisynthetic penicillin derivatives available. Similarly, there are also semisynthetic hormone analogs, especially of estrogens and gestagens. 

Semisynthetic Penicillins. However useful they may be, natural penicillins have several drawbacks. They have a relatively narrow activity spectrum, primarily inhibiting Gram-positive bacteria only. They are acid- and lactamase-sensitive, and in a small percentage of patients they cause allergie side effects. All of these limitations could potentially be overcome by molecular modifications during the biosynthesis of these drugs. Unfortunately, however, the fermentation process used in penicillin production is not very flexible and does not permit the incorporation of very many amide side chains into the molecule. 

After the first successful attempts in 1928 to identify the active biochemicals found in antibacterial molds, followed the rediscovery of penicillin by Fleming, identification of its chemical structure by Hodgkin, and subsequent synthesis by Chain, Heatley, and Florey, which led to the commercial production of penicillin in the mid 1940s [1], Since then, other families of (3-lactam antibiotics have been developed [2, 3], and their massive use worldwide continues to be a forefront line of action against infectious pathogens [4-6]. In recent years, (3-lactams have found other biomedical applications, such as inhibitors of serine protease ([7, 8] for a review, see [9]) and inhibitors of acyl-CoA cholesterol acyltransferasa (ACAT) [10]. Encouraged by their bioactivity, the synthesis and chemistry of (3-lactam antibiotics have been the focus of active research, and chemical modification of some basic structures available from biosynthesis (semisynthetic approaches) as well as the discovery of fully chemical routes to de novo synthesis of (3-lactam... 

The manufacture of several semisynthetic oral cephalosporin antibiotics involves the chemical ring expansion of penicillin V to 7-aminodeacetoxycephalosporanic acid (7-ADCA Fig. 2 [68]). This is a costly and potentially environmentally damaging process [69], Cloning of the C. acremonium cefEF gene (DAOCS/ DACS [47]) and the S. clavuligerus cefD (IPNE) and cefE gene (DAOCS [43]) opened the possibility for biosynthetic/enzymatic processes for production of 7-ADCA. 

The industrial production of 6-aminopenicillanic acid (6-APA), the key building block for all semisynthetic variations (1957) 6-APA is readily available from high-producing strains of Penidllium chrysogenum by enzymatic cleavage of penicillin G with penicillin acylase (see Section 2.03.11). 

Since the target enzymes of penicillins are membrane-bound proteins, an essential condition of antibacterial activity is that the antibiotic must be able to penetrate the outer spheres of the bacterial cell and reach its target in an active form. This problem is closely linked to the phenomenon of bacterial resistance (production of /3-lactamases), and justify the development of semisynthetic penicillins varying in the nature of the acylamino side chain at position C-6, and more recently the development of totally synthetic penems related to thienamycin (see Section 2.03.12.3). 

Semisynthetic Penicillins and Cephalosporins Adventures in Product Quality (Bristol-Myers)... 

The natural penicillins, primarily G and V, have a relatively narrow spectrum. They act mostly on gram-positive organisms. The fact that proper selection of precursors could lead to new variations in the penicillin side chain offered the first source of synthetic penicillins. Penicillin V, derived from a phenoxy-acetic acid precursor, attracted clinical use because of its greater acid tolerance, which made it more useful in oral administration. Also, the widespread use of penicillin eventually led to a clinical problem of penicillin-resistant staphylococci and streptococci. Resistance for the most part involved the penicillin-destroying enzyme, penicillinase, which attacked the beta-lactam structure of the 6-aminopenicillanic acid nucleus (6-APA). Semisynthetic penicillins such as ampicillin and carbenicillin have a broader spectrum. Some, such as methicillin, orafi-cillin, and oxacillin, are resistant to penicillinase. In 1984, Beecham introduced Augmentin, which was the first combination formulation of a penicillin (amoxicillin) and a penicillinase inhibitor (clavulanic acid). Worldwide production of semisynthetic penicillins is currently around 10,000 tons/year, the major producers are Smith Kline Beecham, DSM, Pfizer, and Toyo Jozo. 

Although investigations to determine the structures and establish the beneficial properties of semisynthetic penicillins were aimed first and foremost at the production of more and more of such compounds, a number of examples can be found in the literature concerning their functional modifications (term suggested by Heusler) and nuclear analogues . Work on these unusual /3-lactam structures has been on the increase since the middle of the sixties, and the search for new cephalosporin syntheses has produced a vast number of compounds. 

Over 30 penicillins have been isolated from fermentation mixtures. Some of these occur naturally others have been biusynthc.sizcd by altering the culture medium to provide certain precursors that may be incorporated as acyl groups. Commercial production of biosynthetic peniciliins today depends chielly on various strains of Pcnicillium notatum and P. chrysogenum. In recent years, many more penicillins have been prepared semisynthetically, and undoubtedly, many mure will be added to the list in attempts to find superior products. 

The biosynthesis of penicillins has been extensively studied, and the general biosynthetic pathway from a-aminoadipic acid (33), cysteine (34) and valine (35) is shown in Scheme 7. Enzymic removal of the side chain of the product (36) affords 6-APA which may then be chemically modified to form the other semisynthetic penicillins in Figure 5. 

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