BIOCHEMISTRY Dr. G Murugananthan, Dr. Upama N. Trivedi & Anuradha Singh pdf free download

BIOCHEMISTRY Dr. G Murugananthan, Dr. Upama N. Trivedi & Anuradha Singh


The principal xanthines of medical interest include caffeine, theophylline and aminophylline. Caffeine is synthesized by several plants and was originally isolated from tea in 1838. It is a methylxanthine (Figure 1.12) which stimulates the central nervous system, increasing mental alertness. It also acts as a diuretic and stimulates gastric acid secretion. It is absorbed upon oral administration and is frequently included in drugs containing an analgesic, such as aspirin or paracetamol.

Theophilline is also a minor constituent of tea, but is prepared by direct chemical synthesis for medical use. It functions to relax smooth muscle and, therefore, can be used as a bronchodilator in the treatment of asthma and bronchitis. Aminophylline is a derivative of theophilline (theophylline ethylenediamine), which is often used in place of theophilline due to its greater aqueous solubility.

Terpenes are polymers of the 5-carbon compound isoprene (Figure 1.12) and, as such, generally display properties similar to those of hydrocarbons. Terpenoids are substituted terpenes (i.e. contain additional chemical groups, such as an alcohol, phenols, aldehydes, ketones, etc.). Only a few such substances could be regarded as true drugs. Terpenes, such as limonene, menthol and camphor, form components of various essential oils with pseudo-pharmaceutical uses. A number of these molecules, however, exhibit anti-tumour activity, of which taxol is by far the most important.

The diterpenoid taxol (Figure 1.12) was first isolated from the pacific yew tree (Taxus brevifolia) in the late 1960s. Its complete structure was elucidated by 1971. Difficulties associated with the subsequent development of taxol as a useful drug mirror those encountered during the development of many plant-derived metabolites as drug products. Its low solubility made taxol difficult to formulate into a stable product, and its low natural abundance required large-scale extraction from its native source.

Despite such difficulties, encouraging in vitro bioassay results against transformed cell lines fuelled pre-clinical studies aimed at assessing taxol as an anti-cancer agent. Initial clinical trials in humans commenced in 1983 using a product formulated as an emulsion in a modified castor oil. Initial difficulties associated with allergic reactions against the oil were largely overcome by modifying the treatment regimen used. Large-scale clinical trials proved the efficacy of taxol as an anti-cancer agent, and it was approved for use in the treatment of ovarian cancer by the US Food and Drug Administration (FDA) in 1992.

Direct extraction from the bark of T. brevifolia yielded virtually all of the taxol used clinically up to almost the mid-1990s. The yield of active principle was in the range 0.007–0.014%. Huge quantities of bark were thus required to sustain taxol production (almost 30 000 kg bark were extracted in 1989 to meet requirements during large-scale clinical trials). A major (late) intermediate in the biosynthesis of taxol is 10-decacetylbaccatin (10-DAB). This can be obtained from the leaves (needles) of many species of yew, and at concentrations in excess of 0.1%. Chemical methods have been developed allowing synthesis of taxol from 10-DAB, and much of the taxol now used therapeutically is produced in this way. Semi-synthesis of taxol also facilitates generation of taxol analogues, some of which have also generated clinical interest. Although semi-synthesis of taxol is relatively straightforward, its total de novo synthesis is extremely complex. The cost of achieving de novo synthesis ensures that this approach will not be adopted for commercial production of this drug.

An alternative route of taxol production under investigation entails the use of plant cell culture techniques. Plant cell culture is considered to be an economically viable production route for plant-derived drugs, if the drug commands a market value in excess of $1000–2000/kg. While many commonly used plant-derived metabolites fall into this category, plant cell culture has notgenerally been adopted for their industrial production. In many cases, this is because the plant cell lines fail to produce the desired drug, or produce it in minute quantities. Several cultures of T. brevifolia, however, have been shown to produce taxol. Interestingly, a fungus, Taxomyces andreanae (isolated growing on T. brevifolia) also produces taxol, although at very low levels. Genetic manipulation of this fungus may, however, yet yield mutants capable of synthesizing taxol in quantities rendering production by this means economically viable.

Betulinic acid is a five-ringed triterpene which has recently generated interest as an anti-cancer agent. It is produced in relatively substantial quantities in the bark of the white birch tree, from which it can easily be isolated. Initial studies indicate that betulinic acid is capable of selectively destroying melanoma cells by inducing apoptosis. Over the past number of years, the incidence of melanoma has increased at a faster rate than any other cancer. In the region of 7000 patients die annually from this condition in the USA alone. Although early surgery produces a 10-year survival rate of greater than 90%, treatment of late (metastatic) melanoma is more problematic. The current most effective drug (dacarbazine) is only effective in 25% of cases. A more effective drug would be a valuable therapeutic tool in combating advanced cases of this cancer.

Cardiac glycosides and coumarins

Cardiac glycosides are steroids to which a carbohydrate component is attached. Although produced by a variety of plants, the major cardiac glycosides that have found medical use have been isolated from species of Digitalis (foxgloves). ‘Digitalis’ in pharmaceutical circles has also come to mean a crude extract of dried foxglove leaves. This contains two glycoside components — digoxin and digitoxin — which increase heart muscle contraction. These drugs are in widespread use in the treatment of heart failure; both can be administered either orally or by injection. Digoxin induces an immediate but short-lived effect, whereas digitoxin is slower-acting but its effects are prolonged.

Coumarins are also synthesized by a variety of plant species. Medically, the most significant coumarins are dicoumarol and its derivative, warfarin. Dicoumarol was initially discovered as the active substance in mouldy sweet clover hay, which could induce haemorrhagic disease in cattle. Dicoumarol and warfarin are now used clinically as anticoagulants, as discussed in Chapter 9.

Aspirin

Few drugs have gained such widespread use as aspirin. The story of aspirin begins in the annals of folk medicine, where willow bark and certain flowers (e.g. Filipendula ulmaria) were used to relieve rheumatic and other pain. The bark of the white and black willow was subsequently found to contain salicin (Figure 1.13), which is metabolized to salicylic acid when ingested by humans. The flowers of F. ulmaria (meadowsweet) were also found to contain salicylic acid, which possesses anti-pyretic, anti-inflammatory and analgesic properties. Although it was an effective pain reliever, it irritates the stomach lining, and it was not until its modification to acetylsalicylate by Bayer chemists that it found widespread medical application (Figure 1.13). Bayer patented its acetylsalicylate drug under the trade name ‘Aspirin’ in 1900.

Pharmaceutical substances of microbial origin

Microorganisms produce a wide variety of secondary metabolites, many of which display actual or potential therapeutic application. Antibiotics are by far the most numerous such substances and this family of pharmaceuticals arguably has had the greatest single positive impact upon human healthcare in history. A detailed discussion of antibiotics goes far beyond the scope of this text and, as such, only a brief overview is presented here. The interested reader is referred to the further reading section at the end of this chapter.

Antibiotics are generally defined as low molecular mass microbial secondary metabolites which, at low concentrations, inhibit the growth of other microorganisms. To date, well in excess of 10 000 antibiotic substances have been isolated and characterized. Overall, antibiotics are a chemically heterogeneous group of molecules, although (as described later) many can be classified into different families based upon similarity of chemical structure.

While chemically heterogeneous, it is interesting to note that in excess of half the antibiotic substances described to date are produced by a single bacterial order, the Actinomycetales. Within this order, the genus Streptomyces are particularly prolific producers of antibiotic substances. Although several fungal genera are known to produce antibiotics, only two (Aspergillus and Penicillium) do so to a significant extent. Indeed, the first antibiotic to be used medically — penicillin — was extracted from Penicillium notatum.

Sir Alexander Fleming first noted the ability of the mould P. notatum to produce an antibiotic substance (which he called penicillin) in 1928. However, he also noted that when penicillin was added to blood in vitro, it lost most of its antibiotic action, and Fleming consequently lost interest in his discovery. In the late 1930s, Howard Florey, Ernst Chain and Norman Heatley began to work on penicillin. They purified it and, unlike Fleming, studied its effect on live animals. They found that administration of penicillin to mice after their injection with lethal doses of streptococci protected the mice from an otherwise certain death.

The Second World War conferred urgency to their research and they began large-scale culture of Penicillium in their laboratory (mostly in bedpans). However, low levels of antibiotic production rendered large-scale medical trials difficult. The first human to be treated was Albert Alexander, a policeman. Although dying from a severe bacterial infection, he responded immediately to penicillin treatment. Supplies were so scarce that medical staff collected his urine to extract traces of excreted penicillin. He was almost cured 4 days after commencement of treatment, but supplies ran out. He relapsed and died. The first serious clinical trials in 1940–1941 proved so encouraging that massive effort was expended (particularly in the USA) to develop large-scale penicillin production systems. This succeeded and, before the end of the war,

The aminoglycosides are a closely related family of antibiotics produced almost exclusively by members of the genus Streptomyces and Micromonospora (Table 1.19). Most are polycationic compounds, composed of a cyclic amino alcohol to which amino sugars are attached. They all induce their bacteriocidal effect by inhibiting protein synthesis (apparently by binding to the 30 S and, to some extent, the 50 S, ribosomal subunits). Most are orally inactive, generally necessitating their parenteral administration.

The aminoglycosides are most active against Gram-negative rods. Streptomycin was the first aminoglycoside to be used clinically. Another notable member of this family, gentamicin, was first purified from a culture of Micromonospora purpurea in 1963. Its activity against Pseudomonas aeruginosa and Serratia marcescens renders it useful in the treatment of these (often life-threatening) infections.

The macrolides and ansamycins

The macrolides are a large group of antibiotics. They are characterized by a core ring structure containing 12 or more carbon atoms (closed by a lactone group), to which one or more sugars are attached. The core ring of most anti-bacterial macrolides consists of 14 or 16 carbon atoms, while that of the larger anti-fungal and anti-protozoal macrolides contain up to 30 carbons. This family of antibiotics are produced predominantly by various species of Streptomyces. Antibacterial macrolides induce their effects by inhibiting bacterial synthesis (anti-fungal/protozoal macrolides appear to function by interfering with sterols, thus compromising membrane structure). The only member of this family that enjoys widespread therapeutic use is erythromycin, which was discovered in 1952.


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