Medicinal Chemistry for

infectious diseases

MSc.(F)

INFECTIOUS DISEASES

Infectious diseases, also known as contagious diseases or transmissible diseases, and include communicable diseases, comprise clinically evident illness (i.e., characteristic medical signs and/or symptoms of disease) resulting from the infection, presence and growth of pathogenic biological agents in an individual host organism.

The pathogen can be a bacteria, virus, fungus or a protozoan

TUBERCULOSIS : A GLOBAL EPIDEMIC

Tuberculosis (TB) is an ancient disease that has caused inestimable suffering and claimed millions of lives over the centuries.

and close to 1.8 million deaths annually

Caused by various strains of mycobacteria usually Mycobacterium tuberculosis, an airborne pathogen, that infects macrophages in the lungs

Two possible outcomes : Infected macrophage can be recognized by effectors of immune system and

eradicated

Bacilli may further multiply in the cell leading to its destruction and the infection of new macrophages drawn to the site of infection. This initiate T cell mediated adaptive immunity to eradicate the bacilli which if fails then it grows and spread to extra pulmonary sites.

Most infections  asymptomatic, latent infection

About one in ten latent infections eventually progress to active disease, which, if left untreated, kills more than 50% of those infected. 

HIV increases the risk of developing a full –borne disease.

CELL WALL OF MYCOBACTERIUM TUBERCULOSIS

MEDICAL HISTORY OF CURRENT TB CHEMOTHERAPY

TB drugs introduced in 1940’s and 1950’s

Streptomycin Isoniazi

d

Pyrazinamide

p-aminosalicyclic acid

Others like kanamycin,viomycin,cycloserine,ethionamide

TB drugs introduced in 1960’s and 1970’s

rifampicin

Ethambutol

Thioacteazone

Others like capreomycin,clofazimine

CLASSIFICATIONFirst line

Ethambutol is EMB or E,isoniazid is INH or H,pyrazinamide is PZA or Z,rifampicin is RMP or R,

Streptomycin is no longer considered as a first line drug by ATS/IDSA/CDC because of high rates of resistance)

Second line it may be less effective than the first-line drugs (e.g., p-

aminosalicylic acid it may have toxic side-effects (e.g., cycloserine) or it may be unavailable in many developing countries

(e.g., fluoroquinolones)

aminoglycosides: e.g., amikacin (AMK), kanamycin (KM);polypeptides: e.g., capreomycin, ;Fluoroquinolones  e.g.,  ciprofloxacin (CIP) ,levofloxacin, moxifloxacin (MXF);thioamides: e.g. ethionamide, prothionamide

Third Line Other drugs that may be useful, but are not on the WHO list of

SLDs: rifabutin macrolides: e.g., clarithromycin (CLR); linezolid (LZD); thioacetazone (T); thioridazine; arginine; vitamin D; R207910. they are not very effective (e.g., clarithromycin) because their efficacy has not been proven (e.g., linezolid,

R207910). Rifabutin is effective, but is not included on the WHO list

because for most developing countries, it is impractically expensive.

EMERGENCE OF DRUG RESISTANT TB

Combination therapy used to limit development of resistance

Who developed DOTS

Even then high relapse rates and 1990’s marked a period of increasingly resistant TB from mono to MDR-TB(resistant to INH and RIF)

Treatment with second line drugs with unproven efficacy and use of broad spectrum agents like fluoroquinolones

Five per cent of all TB cases are now estimated to be MDR

If cases are there which are resistant to first and second line drugs , then third line agents that are non-WHO approved are given.

Emergence of XDR-TB .

Now we have TDR-TB for which no chemotherapeutic options

SPECIAL CHALLENGES IN TB DRUG DEVELOPMENT

Why we need improved stategies ? improved (shorter and simpler, but still affordable) multidrug

regimens for DS-TB to improve adherence and prevent development of more resistant strains of M. tuberculosis

shorter, more efficacious, less toxic and less expensive regimens

for MDR-TB and XDR-TB

short,simple, easily tolerable and safe regimens for LTBI

TB drugs with minimal interactions with the cytochrome P450 (CYP)enzyme and other metabolic systems

PROBLEMS

Problems with rifampicin

Malabsorption : patients with this disease are malnourished and weight loss is a common symptom

Heterogeneity of TB pathology- differences in clinical manifestation, host and pathogen physiology

Each lesion a distinct microenvironment

Drug penetration is limited

Drug should not only penetrate cell wall of bacteria but should be able to reach it ,within the fibrous necrotic lesion harboring the persistent organisms.

DEVELOPMENT OF THE TWO MOST COMMONLY USED FIRST LINE AGENTS

Rifampicin

Isoniazid

RIFAMYCINS

Most effective and widely used

Rifampin was developed in the Dow-Lepetit Research Laboratories( Milan, Italy) as part of an extensive program of chemical modification of the rifamycins, the natural metabolites of Nocardia mediterranei.

Developed in 1960 after extensive SAR performed on rifamycin B

Rifamycin B was the least active component of the rifamycin complex but showed an extremely low level of toxicity and a moderate level of therapeutic activity in infections in animals

First compound with ansa structure consisting of an aromatic nucleus spanned by an aliphatic bridge, therefore, known as ansamycins

Rifamycin SV is active compound

FROM RIFAMYCIN SV TO RIFAMPICIN

Extensive chemical modifications were made better oral absorption; more prolonged antibacterial levels in blood; and greater activity against mycobacterial infections and

infections due to gram-negative bacteria

Changes in ansa chain –less active

Subsitution or elimination-less active

Essential Subsitutuion to keto groups –no effect

ACTIVITY REQUIRED

two free hydroxyls in positions C-21 and C-23 on the ansa chain

two polar groups (either free hydroxyl or carbonyl) at positions C-1 and C-8 of the naphthoquinone nucleus

conformation of the ansa chain that resulted in certain specific geometric relations among these four functional groups.

Rifamycin derivatives with substitutions in position C-3 and/or position C-4 di-alkylamino-4-deoxyrifamycins; phenazino- and phenoxazinorifamycins; 3-

dialkylamino-alkylrifamycins. Extensive studies on the 3-dialkylaminomethyl derivatives of rifamycin SV.

the hydrazone of 3-formylrifamycin SV with N-amino-N'-methylpiperazine, designated rifampicin or rifampin was the most active and least toxic

RIFAMPICIN

MECHANISM OF ACTION

Rifampicin inhibits DNA-dependent RNA polymerase in bacterial cells by binding its beta-subunit, Rifampicin acts directly on messenger RNA synthesis.

Much of this acid-fast positive bacteria's membrane is mycolic acid complexed with peptidoglycan, which allows easy movement of the drug into the cell.

cannot stop the elongation of mRNA once binding to the template-strand of DNA has been initiated.

The Rifampin-RNA polymerase complex is extremely stable and yet experiments have shown that this is not due to any form of covalent linkage. It is hypothesized that hydrogen bonds and π-π bond interactions between naphthoquinone and the aromatic amino acids are the major stabilizers,

It is this last hypothesis that explains the explosion of multi-drug-resistant bacteria: mutations in the rpoB gene that replace phenylalanine, tryptophan, and tyrosine with non-aromatic amino acids result in poor bonding between rifampicin and the RNA polymerase.

Rifampicin-resistant bacteria produce RNA Polymerases with subtly different β subunit structures which are not readily inhibited by the drug.

SYNTHESIS

wherein rifamycin S is reacted with a 1,3,5-trisubstituted hexahydro-1,3,5-triazine in an aprotic dipolar solvent and optionally in the presence of formaldehyde, the reaction preferably being carried out without modifying the pH of the medium and preferably in the presence of certain acid substances, using controlled time and temperature conditions

1-amino-4-methylpiperazine is then added directly to the reaction mixture, while keeping the pH value in the range of from 5 to 7, and then isolating the rifampicin formed.

INTERACTIONS

Rifampicin is an inducer of many enzymes of the cytochrome P450 family Other possible interactions which may not be listed include antiretroviral agents, everolimus, atorvastatin, rosiglitazone/pioglitazone, celecoxib, clarithromycin, caspofungin,

ADVERSE EFEECTS

Influenza like symptoms Hepatotoxicity Altered liver function

ISONIAZID 

also known as isonicotinylhydrazine (INH). discovered in 1912, and later in 1951 it was found to be

effective against tuberculosis by inhibiting its mycolic acid(wax coat).

never used on its own to treat active tuberculosis because resistance quickly develops.

Isoniazid also has an antidepressant effect, and it was one of the first antidepressants discovered

SAR Analog of anti-tubercular drug thiacetazone which had limited use because of toxic effects

Phenyl ring was replaced with pyridine ring as nicotinamide had growth inhibitory effect on Mtb. isonicotinaldehyde thiosemicarbazone more active

Other intermediates in synthesis were evaluated leading to discovery of INH

Hundreds of derivatives synthesised bt none improved on activity .N acetyl INH inactive

N alkyl derivatives such as iproniazid and hydrazones such as verazide

Invivo metabolite is INH

SYNTHESIS

Isoniazid may be prepared by the base hydrolysis of 4-cyanopyridine to give the amide, followed by displacement of ammonia by hydrazine

MECHANISM OF ACTION

activated by a bacterial catalase-peroxidase enzyme that in M. tuberculosis is called KatG.

INH Isonictinoyl radicalacyclic isonicotinoyl –NAD(P) adducts

NAD(P)

Inhibits NADH dependent enoyl ACP reductase InhA involved in fatty acid biosynthesis

OTHER DRUGS

PYRAZINAMIDE –intracellular acidification following hydrolysis by Mtb nicotinamidase, inhibition of fatty acid synthesis

CYCLOSERINE-prevents D –alanine incorporation into peptidoglycan by inhibiting enzyme alanine racemase

CAPREOMYCIN-inhibit protein synthesis by binding at interface of 30S and 50S subunit of bacterial ribosome

http://en.wikipedia.org/wiki/File:Rifampicin.png

ANTIFUNGAL AGENTS

are increasingly common in immunocompromised and other vulnerable patients.

The use of antifungal drugs, primarily azoles and polyenes, has increased in parallel.

azoles are fungistatic and vulnerable to resistance, whereas polyenes cause toxicity

echinocandins, pneumocandins, and improved azoles.

Promising novel agents in preclinical development include several inhibitors of fungal protein, lipid and cell wall syntheses

AZOLES

An azole is a class of five-membered nitrogen heterocyclic ring compounds containing at least one other non-carbon atom of either nitrogen, sulfur, or oxygen

cklotrimazole

ketoconazole

fluconazole

POLYENES

ECHINOCANDINS

PNEUMOCANDINS

EMERGING TARGETS

Microtubulin inhibitors like griseofulvin Topoisomerase inhibitors Phosphnribosylaminoimidazole carboxylase, an enzyme of

them purine pathway Amino acid analogs to interfere with amino acid synthesis Proton ATPases and efflux pumps

REFERENCES Third world diseases by Richard Elliot

Foyes’ Medicinal Chemistry

History of the development of azole derivatives . J. A. Maertens ,Clinical Microbiology and Infection, Volume 10 Supplement 1, 2004

The discovery and development of amphotericin B. James Dutcher ,Dis Chest 1968;54;296-298

Antifungals: mechanism of action and resistance, established and novel drugs . Nafsika H Georgopapadakou Current Opinion in Microbiology 1998, 1:547-557

History of the Development of Rifampin . P.sensi ,From the Dow-LepetitR esearchL aboratories, Milan, Italy. Reviews of infectious diseases * vol. 5, supplement 3 * july-august 1983

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THANKS