Drug Discovery Chem 195

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Drug Discovery Chem 195

• Lecture 2
• History of Pharmaceutical Drug Discovery - The
  Sulfonamides

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Early Developments

• Evolution of the conception of drugs as specific
  chemical entities
• Ehrlich and Chemotherapy - Trypanosomes (Sleeping
  sickness) and Syphilis
• Domagk/Woods and the Sulfonamides

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Last week

• General drug discovery processes
• Multidisciplinary
• Serendipity
• Observations
• Mechanism Based or Empirical
• Ehrlich
• In vivo testing and synthesis of compounds
• Chemotherapy - Salvarsan for Syphilis

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Last Week

• Reading
• Ganellin
• Sources of Drugs - Natural products, existing
  drugs - chemistry based
• Disease models - Biologically based
• Physiological models - molecularly based -
  chemistry and biology
• Issues - is the mechanism key? Does the drug get
  to the right site? What other effects occur?
  Selectivity - a two edged sword

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Last Week

• Reading
• Dimasi
• Has drug development improved from the 60s to the
  90s?
• What caused the increase in IND and NDA
  candidates?
• Success Rates?

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Early Modes of Drug Discovery

• Synthesis of Compounds and Blind Pharmacological
  Testing - Sulfonamide Derived Drugs
• Natural Products - Isolation and Identification -
  Salicylates
• Hoffmann/Vane/Needleman and NSAIDs
• Isolation of Biologically Active Substances -
  Insulin
• Banting and Best

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History of Drug Discovery

• Gerhard Domagk (1895-1964) and the Sulfanilamides
• Developed mouse model of sepsis with
  Streptococcus hemolyticus infection
• Lethal model with most mice dead in 24 hours
• Tested azo-dyes directly in this model.
• Others had shown some azo dyes to be active in
  vitro against a number of bacteria but not to
  have any in vivo activity

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Discovery of Sulfonamides

• Prontosil Red - Azo-dye
• Pre-treatment of bacteria before infection, no
  effect, but subsequent administration to mice -
  survival!
• In vivo activity but no in vitro activity!
• Pro-drug - active in vitro after reduction
• Sulfanilamide first shown active in vitro by
  Fourneau (1935)
• Specific for streptococci not other pathogenic
  bacteria
• Only tested in humans by necessity
• Nobel Prize in 1939 to Domagk

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Sulfonamides - Mechanism of Action

• Woods (1940)
• Followed up on observation that bacterial
  extracts blocked bacteriostatic effects of sulfa
• Suggested that sulfanilamide was a mimic for a
  bacterial metabolite and that it was PABA
• First example of a competitive enzyme inhibitor
  as a drug (inhibitor and substrate)

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Bacteriostatic Effect of Sulfanilamide can be
Competed by Bacterial Extracts
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Effect of Benzoic Acid Derivatives on
Sulfanilamide Induced Streptococcal Growth
Inhibition
Woods, 1940
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Sulfanilamide Mechanism
PABS
N

O
O
O
PABA
N
O
P
O
P
C
N
N
O
N
C
N
1 carbon transfer
PAPS derivative not A substrate for DHFR
DNA
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Mechanism of Sulfa Drugs

• Bacteriostatic not bacteriocidal
• Shown to block folic acid biosynthesis
• Essential metabolite synthesis inhibition
• Foundation of antimetabolite theory of
  antimicrobial agents later extended to
  anti-cancer drugs (Fildes 1940)
• First example of an enzyme inhibitor as a
  therapeutic

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Why are Sulfa Drugs Selective ?

• Selective toxicity
• Folic acid is a vitamin for humans - we dont
  make it
• Therefore, no dihydropteroate synthetase
• Bacteria dont have a folic uptake system since
  they make it
• Thus, a selective anti-bacterial agent!

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Sulfonamides - Resistance and Present Use

• Sulfonamide Resistance Mechanisms
• Increased synthesis of PABA
• Mutant enzyme - binds sulfonamides less well
• Decreased uptake of sulfa drugs
• Synergistic Therapy
• Sulfa DHFR inhibitor
• Used today to treat P. carinii pneumonia in HIV

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Non Antibiotic uses of Sulfa Drugs

• Some sulfa drugs were observed to be diuretics in
  mice
• Sulfanilamide shown to be a weak carbonic
  anhydrase inhibitor
• IC50 ca. 5 x10-6 M
• CO2 H2O - HCO3- H
• Effect on kidney function (pH of urine)
• Subsequent derivatives developed as diuretics for
  treatment of congestive heart failure

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Sulfonamides and Diuretics
Carbonic Anhydrase Inhibitor (uM)
Carbonic Anhydrase Inhibitor (nM)
Disulfonamides
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Sulfonamides and Diabetes

• Anti-typhoid compound (IPTD) showed hypoglycemia
  (to the point of killing patients!)
• Led to the sulfonylureas which are used today for
  the treatment of type II diabetes

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Evolution of Sulfonylureas
S
N
N
O
O
No Amino Group!
O
O
N
N
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Mechanism of Action of Sulfonyl Ureas

• Binding blocks K(ATP) channels in pancreatic Beta
  cells
• Evolution of molecules and targets
• Bacterial pteroate synthetase
• Human kidney carbonic anhydrase
• Human pancreatic K(ATP) channels

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Summary - Sulfonamides

• Random screening in vivo led to first
  anti-bacterial compound
• Mechanism of action determined biochemically as
  enzyme inhibition
• Preclinical and clinical observations led to
  evolution of new therapeutic classes of initially
  chemically related agents which act via
  completely different mechanisms

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References for This Lecture

• cchem.berkeley.edu/chem195/sulphonamides.pdf
• Lectures notes will be posted at /Lecture2.ppt
• Next Week - New Developments in Drug Discovery -
  the last 50 years.

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Project Outline
Final Project Define a molecular drug discovery
target and describe an assay which you would
propose to do in a pharma/biotech company.
Cover the following areas in describing your
choice. Specific Mechanism/Rationale/Target
Indication Market Intellectual
Property Doability Assay Methodology 5 pages
references and figures
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Possible Project Indications - Class Suggestions

• Gliomas (brain tumors)
• Allergy
• Polycystic kidney disease