The ability to treat bacterial infections with chemotherapeutic agents represents one of the most important medical achievements of the twentieth century. The modern era of chemotherapy began with the clinical use of sulphanilamide in humans in 1936. Antiinfective therapy began with the industrial production of penicillin in 1941 and was followed by the discovery and development of streptomycin in 1944, chloramphenicol in 1947, chlortetracycline in 1948, the macrolides in 1952, semi-synthetic penicillins, cephalosporins and glycopeptides from 1958 onwards, streptogramins and quinolones in 1962, fluoroquinolones in the 1980s and, finally, oxazolidinones and cationic peptides in the 1990s.
From the 1950s onward and parallel to the increasing use of antimicrobial agents to control disease in man, veterinary use has provided similar control in both livestock and companion animals. Their application in veterinary practice has contributed to significant improvements in animal health and welfare and assisted in enabling the production of meat and milk products which thereby became unlikely to present disease problems for the customer. The most recent developments in the field of veterinary medicine are the third and fourth generation cephalosporins, fluoroquinolones, tilmicosin and florfenicol.
However, even from the very earliest period of the antiinfective era the potential for the emergence of drug resistant bacteria has been recognized. In 1998 some 19766 tonnes of antiinfectives were used in human medicine and some 9920 tonnes in veterinary medicine. It seems reasonable to suppose that there is a connection between the amount of drug substances used and the extent and speed of resistance development. Based on this assumption numerous organisations, from the WHO to national bodies and pressure groups, have elaborated prudent use guidelines. A key message of these guidelines is the sentence: Antiinfectives should only be used when it is known or suspected that an infectious agent is present which will be susceptible to therapy.
There is no doubt that sensible measures to limit the therapeutic use of antiinfectives to valid indications and to ensure the susceptibility of the causative agent prior to any treatment will be of value in limiting the emergence of resistant organisms. However, the resistance problem already existing especially in human medicine requires renewed efforts by the pharmaceutical industry to discover and develop new products.
New antiinfective drugs
The driving force in the search for new drugs is the human health sector. We can therefore realistically expect that in the medium term this is where new antiinfectives for animals will come from. In the medium term, as in the past, the animal health industry will therefore rely primarily on spin-offs from the human sector.
Four principal drug discovery approaches are employed in the search for new antiinfectives, namely, (i) the expansion of known drug classes to cover organisms resistant to earlier members of the class, (ii) the reevaluation of un(der)explored molecules, (iii) the classical screening of synthetic compounds and natural compounds isolated from fermentation broths of microorganisms, plants or other organisms and (iiii) the identification of novel agents active against previously not-exploited or even unknown (novel) targets within the pathogen.
(i) Variations on an old theme
So far industrial approaches have been dominated by this first approach. This is obvious within the ß-lactam class of antiinfectives, e.g. the cephalosporins. Spectrum and activity of these compounds have been improved over the years and lead recently to the introduction of the fourth cephalosporin generation.
Several companies are working on the development of ß-lactam antibiotics with an improved Gram-negative and Gram-positive spectrum and of novel inhibitors of ß-lactamases. Examples are the carbapenems, which are being developed by Zeneca and others, and the trinems (e.g. sanfetrinem, GV143253) which are being investigated by Glaxo/Smithkline.
New generations of fluoroquinolones are also under development. These substances are characterised mainly by improved activity in the Gram-positive range and a more favourable pharmacokinetic profile. The first example, grepafloxacin (Glaxo/Smithkline), has since been withdrawn from the market (severe cardiovascular side effects). Substances that remain promising are gatifloxacin (Kyorin/BMS), moxifloxacin (Bayer) and gemifloxacin (Glaxo/Smithkline).
Macrolides are also being investigated. Here, too, improvements are made to the activity spectrum and the pharmacokinetic profile, i.e. primarily the duration of action. The ketolides are an example. The most advanced of these is telithromycin (HMR 3647) by Aventis, but Abbott, Kosan and Pfizer are also researching this class of substances.
Another example is the introduction of Synercid® by Aventis. Synercid® is a combination of quinupristin and dalfopristin, two streptogramin antibiotics. This activity class was discovered way back in 1962, and today synercid is a key weapon in the fight against multiresistant Gram-positive pathogens.
The glycylcyclines, a core research focus of AHP, should also be mentioned in this context. They are true broad-spectrum antiinfectives.
The main advantages of this approach for the industry are obvious: the resistance mechanisms are known, realistic assumptions regarding the possible product profile and commercial attractiveness can be made and the risks of unexpected efficacy and safety issues are quite low. However, a great disadvantage of this strategy is cross resistance. Unfortunately, the existence of resistance mechanisms to earlier members of the drug class often provides the organisms with a head-start for mutational adaptation by which expression of resistance to the newest member of the class also rapidly emerges. Thus, this approach can only be considered at best a temporary solution to the problem of resistance.
(ii) Recollection of the unexplored
A variety of peptides with pronounced antimicrobial properties were discovered back in the 1960s. These drugs were abandoned by the pharmaceutical industry for a variety of reasons:
Their large scale production and purification was considered difficult or non-viable, there were concerns about the therapeutic utilisation of proteins and, finally, the medical need was not recognised (pre-MDR).
Today biotech companies in particular are active in this area.
A typical example is AMBI, who is carrying out intensive research on Nisin and Lysostaphin. Nisin is considered to have potential mainly in the treatment of clostridial and enterococcal infections.
Lysostaphin is highly effective against staphylococci. Resistance development seems highly unlikely, especially in combination with ß-lactam antibiotics.
Another class of agents attracting considerable interest is the cationic peptides. They are ubiquitous in nature (>300 peptides known), evolving as a first-line defense mechanism in all higher organisms.
It is particularly interesting that these substances damage the bacterial cell membrane via a novel mechanism.
Most of the cationic peptides have broad-spectrum activity. Two examples of clinical candidates are Protegrin IB-367 (Intrabiotics) and pexiganan acetate (Magainin). An example of a substance with a narrower activity spectrum is BPI (human bactericidal permeability increasing protein), which XOMA is investigating. This peptide has the advantage of not only killing multiresistant Gram-negative organisms, but also of binding lipopolysaccharide. This leads to a significant reduction of the toxic effects of Gram-negative bacteria.
The reevaluation of unexplored or under-explored molecules has some obvious advantages. The discovery project starts with a defined molecule, thus enabling the sponsor to define a likely product profile. Furthermore, there is a distinct chance to establish a new class of antiinfectives with a new mode of action lacking any cross resistance to established compounds and ensuring intellectual property rights to the sponsor. However, the main shortcoming of this approach is the lack of clinical experience with the respective compound and the risk of unexpected efficacy and safety findings during development.
(iii) The classical approach
The classical screening approach determined by testing the inhibitory effect of synthetic and natural compounds or extracts in in vitro cultures has generated decreasing amounts of interesting structures. But pharmaceutical companies are adopting this approach in growing numbers. One reason is almost certainly the discovery of the oxazolidinones and daptomycin through using this very strategy.
The oxacolidinones are being investigated by Pharmacia & Upjohn. Linezolid is the first representative of this class. The characteristic features of this class of compounds are: synthetically manufactured, unique mode of action, spectrum includes multiply resistant Gram-positive bacteria. Linezolid binds specifically to the 50S ribosomal subunit and inhibits the formation of a functional initiation complex. Laboratory and clinical experiments to date have shown that it is very difficult to induce resistance in bacteria to these drugs. There is no cross-resistance with known antiinfectives.
Daptomycin is being investigated by Cubist. Daptomycin is a fermentation product of Streptomyces roseosporus. Chemically it is a lipopeptide. It, too, is characterised by a broad activity against Gram-positive pathogens, including resistant staphylococci and enterococci. Daptomycin has rapid bactericidal activity, a novel mode of action without cross-resistance to known antiinfectives and is mostly excreted actively via the kidneys. As a special side effect it confers protection against the nephrotoxic action of aminoglycoside antibiotics. In terms of effectiveness and safety Daptomycin could therefore be an ideal combination partner for this traditional group of compounds.
(iv) Target based high throughput screening
The discovery of novel agents active against existing targets or novel targets is based on the following paradigms: The genomic paradigm: genomics can identify targets on the basis of sequence comparisons and select the best targets with regard to spectrum of activity and selectivity. The target based paradigm: high throughput screening can identify molecules that bind to and inhibit a target protein, medicinal chemistry can solve problems of spectrum, cell permeability and potency of the so identified compounds. In principle, this approach allows for the identification and differentiation of in vitro and in vivo expressed genes, thus, enabling the identification of housekeeping and virulence targets and the discovery of compounds active against the respective targets. Overall, this strategy might offer the best chance to establish new classes of antiinfectives with completely new modes of action lacking any cross resistance to established compounds and ensuring intellectual property rights to the sponsor. However, the risk to fail is also significant. So far, there is no indication in the scientific literature that targets related to genes of unknown function would be more productive than known targets. Furthermore, sequence homology of a target is not necessarily predictive for the spectrum and selectivity of its inhibitors. A key success factor and thus a severe limitation of this approach will be the diversity of the chemical library used in high throughput screening.
With prudent use it will be possible to control the spread of the resistance problem also in the long term. The therapeutic arsenal will grow in the short and medium term by traditional approaches (i-iii). In the long term, leading edge biochemical and molecular biological methods together with bioinformatics provide a good chance of controlling bacterial infections in entirely new ways. It should even be possible to conquer bacterial infections with bacteria-friendly substances. To quote Joshua Lederberg:
"The most important change we can make is to supersede the 20th century metaphor of war for describing the relationship between people and infectious agents. A more ecologically informed metaphor which includes the germs´ eye view of infection might be more fruitful."