Drug Discovery andDevelopmentHow are drugs discovered anddeveloped?
Basic Steps• Choose a disease• Choose a drug target• Identify a “bioassay”bioassay = A test used to determine biologicalactivity.• Find a “lead compound”“lead compound” = structure that has some activityagainst the chosen target, but not yet goodenough to be the drug itself.• If not known, determine the structure of the “leadcompound”• Synthesize analogs of the lead• Identify Structure-Activity-Relationships (SAR’s)
• Structure-Activity-Relationship (SAR) = How doesthe activity change as structure is systematicallyaltered?• Identify the “pharmacophore”pharmacophore = the structural features directlyresponsible for activity• Optimize structure to improve interactions withtarget• Determine toxicity and efficacy in animal models.• Determine pharmacodynamics andpharmacokinetics of the drug.• Pharmacodynamics explores what a drug does tothe body, whereas pharmacokinetics exploreswhat the body does to the drug.
Basic steps (cont.)• Patent the drug• Study drug metabolism• Test for toxicity• Design a manufacturing process• Carry out clinical trials• Market the drug
Choosing aDisease• Pharmaceutical companies arecommercial enterprises• Pharmaceutical companies will,therefore, tend to avoid products with asmall market (i.e. a disease which onlyaffects a small subset of the population)
Choosing aDisease• Pharmaceutical companieswill also avoid products thatwould be consumed byindividuals of lower economicstatus (i.e. a disease whichonly affects third worldcountries)
Choosing a Disease (cont.)• Most research iscarried out on diseaseswhich afflict “first world”countries: (e.g. cancer,cardiovasculardiseases, depression,diabetes, flu, migraine,obesity).
The Orphan Drug Act• The Orphan Drug Act of 1983 was passedto encourage pharmaceutical companies todevelop drugs to treat diseases whichaffect fewer than 200,000 people in the US• Under this law, companies who developsuch a drug are entitled to market it withoutcompetition for seven years.• This is considered a significant benefit,since the standards for patent protectionare much more stringent.
Identifying a Drug Target• Drug Target = specific macromolecule,or biological system, which the drug willinteract with• Sometimes this can happen throughincidental observation…
Identifying a Drug Target (cont.)•Example: In addition to their being able to inhibit the uptakeof noradrenaline, the older tricyclic antidepressants wereobserved to “incidentally” inhibit serotonin uptake. Thus, it wasdecided to prepare molecules which could specifically inhibitserotonin uptake. It wasn’t clear that this would work, but iteventually resulted in the production of fluoxetine (Prozac).NH2NHHOserotoninOHNprozacNN CH3H3CImipramine(a classical tricyclic antidepressant)F3C
The mapping of the humangenome should help!• In the past, many medicines (and leadcompounds) were isolated from plant sources.• Since plants did not evolve with human beings inmind, the fact that they posses chemicals whichresults in effects on humans is incidental.
• Having the genetic code for the production of anenzyme or a receptor may enable us to over-express that protein and determine its structureand biological function. If it is deemed importantto the disease process, inhibitors (of enzymes), orantagonists or agonists of the receptors can beprepared through a process called rational drugdesign.
Simultaneously, Chemistry is Improving!• This is necessary, since,ultimately, plants and naturalsources are not likely toprovide the cures to alldiseases.• In a process called“combinatorial chemistry”large numbers of compoundscan be prepared at one time.• The efficiency of syntheticchemical transformations isimproving.
Selectivity is Important!• e.g. targeting a bacterial enzyme, whichis not present in mammals, or whichhas significant structural differencesfrom the corresponding enzyme inmammals
The Standards are Being Raised• More is known about the biologicalchemistry of living systems• For example: Targeting one subtype ofreceptor may enable the pharmaceuticalchemist to avoid potentially troublesomeside effects.
Problems canarise• Example: The chosen target, may over time,lose its sensitivity to the drug• Example: The penicillin-binding-protein (PBP)known to the the primary target of penicillin in thebacterial species Staphylococcus aureus hasevolved a mutant form that no longer recognizespenicillin.
Choosing the Bioassay• Definitions:– In vitro: In an artificial environment, as in a testtube or culture media– In vivo: In the living body, referring to testsconductedin living animals– Ex vivo: Usually refers to doing the test on atissue taken from a living organism.
Choosing the Bioassay (cont.)In vitro testing• Has advantages in terms of speed and requiresrelatively small amounts of compound• Speed may be increased to the point where it ispossible to analyze several hundred compoundsin a single day (high throughput screening)• Results may not translate to living animals
Choosing the Bioassay (cont.)In vivo tests• More expensive• May cause suffering to animals• Results may be clouded by interferencewith other biological systems
Finding the LeadScreening Natural Products• Plants, microbes, the marine world, andanimals, all provide a rich source ofstructurally complex natural products.
• It is necessary to have a quick assay forthe desired biological activity and to beable to separate the bioactivecompound from the other inactivesubstances• Lastly, a structural determination willneed to be made
Finding the Lead (cont.)Screening synthetic banks• Pharmaceutical companies haveprepared thousands of compounds• These are stored (in the freezer!),cataloged and screened on new targetsas these new targets are identified
Finding the Lead (cont.)Using Someone Else’s Lead• Design structure which is similar to existing lead, butdifferent enough to avoid patent restrictions.• Sometimes this can lead to dramatic improvementsin biological activity and pharmacokinetic profile.(e.g. modern penicillins are much better drugs thanoriginal discovery).
Finding the Lead (cont.)Enhance a side effectONHSOONHtolbutamideNH2SOOH2Nsulphanilamide(an antibacterial with the side effect oflowering glucose levels in the blood and alsodiuretic activity)(a compound which has been optimized to onlylower blood glucose levels. Useful in the treatmentof Type II diabetes.)SNHNO OSOOH2NClChlorothiazide(a compound which has been optimized to only display diureticactivity.)
Use structural similarity to a natural ligandNNH2HOHNN(CH3)2HSHNO OH3C5-Hydroxytryptamine (5-HT)Serotonin (a natural neurotransmittersynthesized in certain neurons in the CNS)Sumatriptan (Imitrex)Used to treat migrain headachesknown to be a 5-HT1 agonist
Finding the Lead (cont.)Computer-Assisted Drug Design• If one knows the precise molecular structure ofthe target (enzyme or receptor), then one canuse a computer to design a perfectly-fittingligand.• Drawbacks: Most commercially availableprograms do not allow conformationalmovement in the target (as the ligand is beingdesigned and/or docked into the active site).Thus, most programs are somewhat inaccuraterepresentations of reality.
Finding a Lead (cont.)Serendipity: a chance occurrence• Must be accompanied by an experimentalistwho understands the “big picture” (and is notsolely focused on his/her immediate researchgoal), who has an open mind towardunexpected results, and who has the ability touse deductive logic in the explanation of suchresults.• Example: Penicillin discovery• Example: development of Viagra to treaterectile dysfunction
Finding a Lead (cont.)Sildenafil (compound UK-92,480) was synthesized by agroup of pharmaceutical chemists working at PfizersSandwich, Kent research facility in England.It was initially studied for use in hypertension (high bloodpressure) and angina pectoris (a form of ischaemiccardiovascular disease).Phase I clinical trials under the direction of Ian Osterlohsuggested that the drug had little effect on angina, but thatit could induce marked penile erections.
Pfizer therefore decided to market it for erectile dysfunction, ratherthan for angina.The drug was patented in 1996, approved for use in erectiledysfunction by the Food and Drug Administration on March 27,1998, becoming the first pill approved to treat erectiledysfunction in the United States, and offered for sale in theUnited States later that year.It soon became a great success: annual sales of Viagra in theperiod 1999–2001 exceeded $1 billion.
Finding a Lead (cont.)NNSOONNNNHOOviagra(Sildenafil)
Structure-Activity-Relationships (SAR’s)• Once a lead has been discovered, it is important tounderstand precisely which structural features areresponsible for its biological activity (i.e. to identifythe “pharmacophore”)
The pharmacophore is the precise section of themolecule that is responsible for biological activity
• This may enable one to prepare a more active molecule• This may allow the elimination of “excessive” functionality, thusreducing the toxicity and cost of production of the active material• This can be done through synthetic modifications• Example: R-OH can be converted to R-OCH3 to see if O-H isinvolved in an important interaction• Example: R-NH2 can be converted to R-NH-COR’ to see ifinteraction with positive charge on protonated amine is animportant interaction
Next step: ImprovePharmacokinetic Properties• Improve pharmacokinetic properties.pharmacokinetic = The study of absorption,distribution, metabolism and excretion of adrug (ADME).• Video• exercise=MedicationDistribution&title=Medication%20Absorption,%20Distribution,%20Metabolism%20and%20Excretion%20Animation&publication_ID=2450
Metabolism of Drugs• The body regards drugsas foreign substances,not produced naturally.• Sometimes suchsubstances are referredto as “xenobiotics”•Body has “goal” of removing such xenobioticsfrom system by excretion in the urine•The kidney is set up to allow polar substancesto escape in the urine, so the body tries tochemically transform the drugs into more polarstructures.
Metabolism of Drugs (cont.)• Phase 1 Metabolism involves theconversion of nonpolar bonds (eg C-Hbonds) to more polar bonds (eg C-OHbonds).• A key enzyme is the cytochrome P450system, which catalyzes this reaction:RH + O2 + 2H++ 2e–ROH + H→ 2O
Phase I metabolism mayeither detoxify or toxify.• Phase I reactions produce a more polarmolecule that is easier to eliminate.• Phase I reactions can sometimes resultin a substance more toxic than theoriginally ingested substance.• An example is the Phase I metabolismof acetonitrile
The Liver• Oral administration frequently brings thedrugs (via the portal system) to the liver
Metabolism of Drugs (cont.)• Phase II metabolism links the drug to stillmore polar molecules to render them evenmore easy to excreteO OOHHOOHHOO PHOOOPHO OO OHOOHNNHOOR OHO OOHHOOHHOORGlucuronic AcidUDP Glucuronic AcidMore easily excreted than ROH itselfglucuronosyltransferaseenzymeDrugDrug
Metabolism of Drugs (cont.)• Another Phase II reaction is sulfation(shown below)R OHO NNNNNH2OHOOPOO-OSOOO-PO O-O-3-Phosphoadenosine-5-phosphosulfateDrugR OSO3-Sulfated Drug(more easily excreted)
Phase II Metabolism• Phase II reactions most commonlydetoxify• Phase II reactions usually occur at polarsites, like COOH, OH, etc.
Manufacture of Drugs• Pharmaceutical companies must make a profit to continue to exist• Therefore, drugs must be sold at a profit• One must have readily available, inexpensive starting materials• One must have an efficient synthetic route to the compound– As few steps as possible– Inexpensive reagents
• The route must be suitable to the“scale up” needed for the production ofat least tens of kilograms of finalproduct• This may limit the structural complexityand/or ultimate size (i.e. mw) of thefinal product• In some cases, it may be useful todesign microbial processes whichproduce highly functional, advancedintermediates. This type of processusually is more efficient than trying toprepare the same intermediate usingsynthetic methodology.
Toxicity• Toxicity standards are continually becomingtougher• Must use in vivo (i.e. animal) testing to screen fortoxicity– Each animal is slightly different, with different metabolicsystems, etc.– Thus a drug may be toxic to one species and not toanother
Example: ThalidomideThalidomide was developed by German pharmaceuticalcompany Grünenthal. It was sold from 1957 to 1961 in almost50 countries under at least 40 names. Thalidomide waschiefly sold and prescribed during the late 1950s and early1960s to pregnant women, as an antiemetic to combatmorning sickness and as an aid to help them sleep. Before itsrelease, inadequate tests were performed to assess the drugssafety, with catastrophic results for the children of women whohad taken thalidomide during their pregnancies.Antiemetic = a medication that helps preventand control nausea and vomiting
Example: ThalidomideFrom 1956 to 1962, approximately 10,000 children were born withsevere malformities, including phocomelia, because their mothers hadtaken thalidomide during pregnancy. In 1962, in reaction to the tragedy,the United States Congress enacted laws requiring tests for safetyduring pregnancy before a drug can receive approval for sale in theU.S.NOONHOOThalidomidePhocomelia presents at birth very short or absent long bonesand flipper-like appearance of hands and sometimes feet.
Example: Thalidomide•Researchers, however, continued to work with the drug. Soonafter its banishment, an Israeli doctor discovered anti-inflammatory effects of thalidomide and began to look for usesof the medication despite its teratogenic effects.•He found that patients with erythema nodosum leprosum, apainful skin condition associated with leprosy, experiencedrelief of their pain by taking thalidomide.Teratogenic = Causing malformations in a fetus
ThalidomideFurther work conducted in 1991 by Dr. Gilla Kaplan at RockefellerUniversity in New York City showed that thalidomide worked inleprosy by inhibiting tumor necrosis factor alpha. Kaplan partneredwith Celgene Corporation to further develop the potential forthalidomide.Subsequent research has shown that it is effective in multiplemyeloma, and it is now approved by the FDA for use in thismalignancy. There are studies underway to determine the drugseffects on arachnoiditis, Crohns disease, and several types ofcancers.
Clinical Trials• Phase I: Drug is tested on healthy volunteersto determine toxicity relative to dose and toscreen for unexpected side effects
Clinical Trials• Phase II: Drug is tested on small group of patientsto see if drug has any beneficial effect and todetermine the dose level needed for this effect.
Clinical Trials• Phase III: Drug is tested on much largergroup of patients and compared with existingtreatments and with a placebo
Clinical Trials• Phase IV: Drug is placed on the market and patientsare monitored for side effects