2002 Alfred Burger Award Address in Medicinal Chemistry. Natural products and design: interrelated approaches in drug discovery

Prodrugs of pyrophosphates and bisphosphonates: disguising phosphorus oxyanions

Pyrophosphates have important functions in living systems and thus pyrophosphate-containing molecules and their more stable bisphosphonate analogues have the potential to be used as drugs for treating many diseases including cancer and viral infections. Both pyrophosphates and bisphosphonates are polyanionic at physiological pH and, whilst this is essential for their biological activity, it also limits their use as therapeutic agents. In particular, the high negative charge density of these compounds prohibits cell entry other than by endocytosis, prevents transcellular oral absorption and causes sequestration to bone. Therefore, prodrug strategies have been developed to temporarily disguise the charges of these compounds. This review examines the various systems that have been used to mask the phosphorus-containing moieties of pyrophosphates and bisphosphonates and also illustrates the utility of such prodrugs.

Toxin to medicine and bioisosterism in drug development: a study of the discovery and development of ACE inhibitors from snake venom

The advent of the angiotensin-converting enzyme (ACE) inhibitors is a landmark in drug discovery and a breakthrough in the management of hypertension. Their clinical introduction has led to appreciable increase in the lifespan of hypertensive patients. And their development initiated a new era of structure-based or rational drug design that has subsequently been applied successfully for development of drugs for many other disorders. This paper presents an account of the discovery, design and development of ACE inhibitors from an academic perspective and possibly, as a guide to future research. The paper highlights the milestones and recounts the challenges encountered and the strategies applied in the search for ACE inhibitors. This exposition also expounds some of the concepts and intricacies of drug discovery, design and development. Keywords: drug development, ACE inhibitors, snake venom peptide, bioisosterism, antihypertensive agents

Dialkylation of Indoles with Trichloroacetimidates to Access 3,3-Disubstituted Indolenines

2-Substituted indoles may be directly transformed to 3,3-dialkyl indolenines with trichloroacetimidate electrophiles and the Lewis acid TMSOTf. These reactions provide rapid access to complex indolenines which are present in a variety of complex natural products and medicinally relevant small molecule structures. This method provides an alternative to the use of transition metal catalysis. The indolenines are readily transformed into spiroindoline systems which are privileged scaffolds in medicinal chemistry.

Importance of microbial natural products and the need to revitalize their discovery

Microbes are the leading producers of useful natural products. Natural products from microbes and plants make excellent drugs. Significant portions of the microbial genomes are devoted to production of these useful secondary metabolites. A single microbe can make a number of secondary metabolites, as high as 50 compounds. The most useful products include antibiotics, anticancer agents, immunosuppressants, but products for many other applications, e.g., antivirals, anthelmintics, enzyme inhibitors, nutraceuticals, polymers, surfactants, bioherbicides, and vaccines have been commercialized. Unfortunately, due to the decrease in natural product discovery efforts, drug discovery has decreased in the past 20 years. The reasons include excessive costs for clinical trials, too short a window before the products become generics, difficulty in discovery of antibiotics against resistant organisms, and short treatment times by patients for products such as antibiotics. Despite these difficulties, technology to discover new drugs has advanced, e.g., combinatorial chemistry of natural product scaffolds, discoveries in biodiversity, genome mining, and systems biology. Of great help would be government extension of the time before products become generic.

Enantioselective cyclization of enamide-ynes and application to the synthesis of the kopsifoline core

We report the palladium-catalyzed enantioselective cyclization of 1,6-enamidynes to form spirocyclic ring systems. We applied this methodology to the concise synthesis of the skeletal core of the kopsifoline alkaloids.

Antibiotics: natural products essential to human health

For more than 50 years, natural products have served us well in combating infectious bacteria and fungi. Microbial and plant secondary metabolites helped to double our life span during the 20th century, reduced pain and suffering, and revolutionized medicine. Most antibiotics are either (i) natural products of microorganisms, (ii) semi-synthetically produced from natural products, or (iii) chemically synthesized based on the structure of the natural products. Production of antibiotics began with penicillin in the late 1940s and proceeded with great success until the 1970-1980s when it became harder and harder to discover new and useful products. Furthermore, resistance development in pathogens became a major problem, which is still with us today. In addition, new pathogens are continually emerging and there are still bacteria that are not eliminated by any antibiotic, e.g., Pseudomonas aeruginosa. In addition to these problems, many of the major pharmaceutical companies have abandoned the antibiotic field, leaving much of the discovery efforts to small companies, new companies, and the biotechnology industries. Despite these problems, development of new antibiotics has continued, albeit at a much lower pace than in the last century. We have seen the (i) appearance of newly discovered antibiotics (e.g., candins), (ii) development of old but unutilized antibiotics (e.g., daptomycin), (iii) production of new semi-synthetic versions of old antibiotics (e.g., glycylcyclines, streptogrammins), as well as the (iv) very useful application of old but underutilized antibiotics (e.g., teicoplanin).

Microbial drug discovery: 80 years of progress

Microbes have made a phenomenal contribution to the health and well-being of people throughout the world. In addition to producing many primary metabolites, such as amino acids, vitamins and nucleotides, they are capable of making secondary metabolites, which constitute half of the pharmaceuticals on the market today and provide agriculture with many essential products. This review centers on these beneficial secondary metabolites, the discovery of which goes back 80 years to the time when penicillin was discovered by Alexander Fleming.

Electrophile-induced dearomatizing spirocyclization of N-arylisonicotinamides: a route to spirocyclic piperidines

Treatment of N-arylisonicotinamides with trifluoromethanesulfonic anhydride triggers intramolecular nucleophilic attack of the aryl ring on the 4-position of the pyridinium intermediate. The products are spirocyclic dihydropyridines which can be converted to valuable spirocyclic piperidines related to biologically active molecules such as MK-677.

Cationic polyamines inhibit anthrax lethal factor protease

Background

Anthrax is a human disease that results from infection by the bacteria, Bacillus anthracis and has recently been used as a bioterrorist agent. Historically, this disease was associated with Bacillus spore exposure from wool or animal carcasses. While current vaccine approaches (targeted against the protective antigen) are effective for prophylaxis, multiple doses must be injected. Common antibiotics that block the germination process are effective but must be administered early in the infection cycle. In addition, new therapeutics are needed to specifically target the proteolytic activity of lethal factor (LF) associated with this bacterial infection.

Results

Using a fluorescence-based assay to identify and characterize inhibitors of anthrax lethal factor protease activity, we identified several chemically-distinct classes of inhibitory molecules including polyamines, aminoglycosides and cationic peptides. In these studies, spermine was demonstrated for the first time to inhibit anthrax LF with a K_i value of 0.9 ± 0.09 μM (mean ± SEM; n = 3). Additional linear polyamines were also active as LF inhibitors with lower potencies.

Conclusion

Based upon the studies reported herein, we chose linear polyamines related to spermine as potential lead optimization candidates and additional testing in cell-based models where cell penetration could be studied. During our screening process, we reproducibly demonstrated that the potencies of certain compounds, including neomycin but not neamine or spermine, were different depending upon the presence or absence of nucleic acids. Differential sensitivity to the presence/absence of nucleic acids may be an additional point to consider when comparing various classes of active compounds for lead optimization.

Synthesis and antibacterial activities of 5-hydroxy-4-amino-2(5H)-furanones

Starting from the mucohalogen acids 1a and b 5-hydroxy-2(5H)-furanones 2a-h have been prepared and tested. These novel 4-amino-5-hydroxy 2(5H)-furananones have shown a broad antibiotic activity against Staphylococcus aureus ATCC 25923, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853 in the micromolar range. A one step synthesis from mucohalogen acids towards the antibacterials 2a-h was developed, in which the target was obtained from 1a and b under reflux in toluene in presence of a catalytic amount of sulfuric acid. The derivatives 2b and c displayed a MIC and MBC of 4/8mug/ml, against Staphylococcus aureus with a selectivity towards the resistant strains.

Molecular recognition: the fragment approach in lead generation

The successful practice of medicinal chemistry is crucially dependent on the principles of molecular recognition: the first and "fundamental" requirement for a drug is to bind to its target; specificity, or at least selectivity, of binding is also a must. Subsequent optimization steps to develop a lead compound into a drug are a complex mixture of processes that are not yet fully understood or predictable. Fortunately, criteria exist to discard leads that would be intractable for optimization. The concepts of non-lead-likeness and lead-likeness, in respect to drug-likeness and non-drug-likeness, have prompted a rich discussion in the recent medicinal chemistry literature. The fragment approach is an emerging philosophy in the process of lead compound discovery. The basic interactions responsible for binding affinity are defined from the "protein interactions world" and key structural fragments are combined according to the criteria of three-dimensional diversity to find new leads. New techniques in screening are used for the detection of the weaker interactions of fragments with their targets that might be undetectable in classical biological assays.

Ace revisited: a new target for structure-based drug design

Angiotensin-converting enzyme (ACE) is a chloride-dependent metalloenzyme that catalyses the hydrolytic cleavage of dipeptides from the carboxyl terminus of many regulatory oligopeptides. ACE is central to the renin–angiotensin system that regulates blood pressure, fluid homeostasis, and renal and vascular function. It is therefore a major target for cardiovascular therapies.

ACE inhibitors (for example, captopril, enalaprilat and lisinopril) have been on the market for more than 20 years. Side effects of treatment with ACE inhibitors include cough and angioedema.

ACE comprises an N- and a C-domain, each containing an active site with distinct substrates and activation properties. The design of domain-selective inhibitors might produce new drugs with improved safety and efficacy — this endeavour will be facilitated by the recent determination of the three-dimensional structure of ACE.

The C-domain seems to be primarily responsible for the regulation of blood pressure. Data indicate that C-domain-selective inhibitors will have less severe side effects than current-generation inhibitors, which generally target both the N- and C-domains.

In contrast to the C-domain, the N-domain seems to have relatively low affinity for the peptides that control blood pressure. It preferentially hydrolyses at least three other physiologically important peptides, so targeted inhibition of the N-domain might have novel therapeutic applications.

Current-generation angiotensin-converting enzyme (ACE) inhibitors are widely used for cardiovascular diseases, including high blood pressure, heart failure, heart attack and kidney failure, and have combined annual sales in excess of US $6 billion. However, the use of these ACE inhibitors, which were developed in the late 1970s and early 1980s, is hampered by common side effects. Moreover, we now know that ACE actually consists of two parts (called the N- and C-domains) that have different functions. Therefore, the design of specific domain-selective ACE inhibitors is expected to produce next-generation drugs that might be safer and more effective. Here we discuss the structural features of current inhibitors and outline how next-generation ACE inhibitors could be designed by using the three-dimensional molecular structure of human testis ACE. The ACE structure provides a unique opportunity for rational drug design, based on a combination of in silico modelling using existing inhibitors as scaffolds and iterative lead optimization to drive the synthetic chemistry.