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Learning Objectives:
Nucleic Acid therapeutics
Reading Chapter 4, p. 160-172 from Blackburn & Gait
Know the general mechanisms of the anti-cancer drugs that are
nucleoside analogs
5-fluoroU, methotrexate, cyclophosphamide
Know the general mechanisms of the anti-viral drugs that are
nucleoside analogs
acyclovir, 5’-substituted pyrimidines, ribavirin,
phosphonoformic acid, phosphonoacetic acid, AZT,
ddI, ddC, nevirapine
Structure of HIV RT and how drugs inhibit it
Nucleic Acid therapeutics
Review nucleic acid biosynthesis - de novo and salvage pathways
Cancers and viruses treated by targeting NA biosynthesis
Some vaccines for viruses (Hep B) but doesn’t help those already
infected so need treatments
Another problem results from latency of viruses
Most common viral diseases
influenza virus
rhinovirus (cold)
enterovirus
change rapidly so vaccine doesn’t work well
Nucleotides & Nucleic Acids
Biosynthesis and Degradation
Biosynthesis:
De novo pathways - uses metabolic precursors (AAs, ribose 5phosphate, CO2, NH3)
Salvage pathways - recycle free bases and nucleosides released from
nucleic acid breakdown
Nucleotides & Nucleic Acids
Biosynthesis:
De novo pathways
Nucleotides &
Nucleic Acids
Biosynthesis:
De novo pathways
Nucleic Acid therapeutics
Anti-cancer chemotherapy
Rate of replication in cancer cells is high compared to normal cells so
many chemo drugs target DNA replication
5-Fluorouracil
5-fU is converted to a
nucleotide in the body
Potent inhibitor of thymidylate synthetase and so it prevents de novo
synthesis of dTMP, dTTP and DNA
Suicide inhibitor blocks abstraction of H from C5 in enzyme-cofactorsubstrate complex of thymidylate synthetase
cofactor
Used to treat common solid tumors
Toxic, suppresses immune system
Nucleic Acid therapeutics
Anti-cancer chemotherapy
Methotrexate
Prevents de novo synthesis of dTTP by inhibiting biosynthesis of
tetrahydrofolate (required as cofactor and donor of -CH3)
Selectivity of methotrexate for cancer cells due to its preferential uptake
of the drug
Nucleic Acid therapeutics
Anti-cancer chemotherapy
Methotrexate and 5-fU
Nucleic Acid therapeutics
Anti-cancer chemotherapy
5-fU
Nucleic Acid therapeutics
Anti-cancer chemotherapy
Cyclophosphamide
Alkylating agent (lots of other chemo drugs too)
Thought to crosslink DNA and interfere with replication
Needs enzymatic oxidation (in liver) to form active species
Selective for tumor cells which may be due to more efficient transport of
active metabolite or lack of enzymatic degradation in tumor cells
Toxicity, patient prone to viral & bacterial infections
Alkylating agent
Nucleic Acid therapeutics
Anti-viral chemotherapy
Every virus is different (membrane tags, viral replication cycle)
No agent for ALL viruses like with bacterial antibiotics
Drug discovery takes long time (exception: AZT)
Very few antiviral drugs
lots for herpesvirus
all but 3 are nucleosides/analogs
Nucleic Acid therapeutics
Anti-viral chemotherapy
Acyclovir
Activated in the body, used to treat herpesvirus
Herpesviruses
1. dsDNA viruses that cause cold sores, eye infections,
genital sores, chicken pox, shingles, mononucleosis
2. Latent virus (after infection, virus goes into latent state in
nerve endings from where it can be reactivated by stress, UV
other viruses
3. Virus codes for many enzymes involved in its own repl
(DNA pol, TK)
4. Virus vulnerable b/c properties of virally encoded enzymes
are slightly different than corresponding host cell enzymes
5. Virus relies on salvage pathways for production on dTTP
for DNA syn.- virus encodes its own thymidine kinase (TK)
6. Viral TK not so specific so it phosphorylates many analogs
which once activated can inhibit viral replication
Acyclovir - purine analog (missing 2’ and 3’-OH) phosphorylated by
HV TK at 5’-OH
Nucleic Acid therapeutics
Anti-viral chemotherapy
Acyclovir
No metabolism of acyclovir in normal cells
However in virally infected cell, phosphorylated acyclovir is recognized
by guanylate kinase and it gets to the triphosphate stage - now it’s a
substrate for HV DNA pol and it is incorporated into viral DNA, then it
acts as a chain terminator, inhibiting DNA synthesis
6-deoxyacyclovir
Prodrug that gets converted into acyclovir
Has greater solubility and so get higher levels in plasma
Nucleic Acid therapeutics
Anti-viral chemotherapy
5’-substituted pyrimidine 2’-deoxynucleosides
5’-iodo-deoxyuridine was first anti-viral nucleoside drug to be marketed
Mode of action still not completely know but it is a substrate for cell &
viral TK
May cause an effect b/c it is incorporated into DNA or b/c it may inhibit
viral DNA pol
5’-vinyl-deoxyU (4.21b) more potent than 5’-iodo in cells but in
animals no effect
(E)-5-(2-bromovinyl)-2’-deoxyU (BVDU) (4.21c) effective against
HSV-1 b/c substrate for viral TK; triphosphate of BVDU is substrate
for & inhibitor of viral DNA pol
Nucleic Acid therapeutics
Anti-viral chemotherapy
Ribavirin
Used in aerosol form against RSV only even though it is active against
85% of all viruses studied
5’-triphosphate form needed in cells
1. thought to inhibit inosine monophosphate dehydrogenase,
which causes depletion of cell’s GTP pools
2. Competitive inhibitor of viral RNA pol
3. Competitive inhibitor of viral-specific mRNA capping
enzyme, so no viral protein syn.
Nucleic Acid therapeutics
Anti-viral chemotherapy
Phosphonoformic / phosphonoacetic acid
Antiherpes drug, selective inhibitor of viral DNA pol
Analog of pyrophosphate
No activation needed so have to rely on different affinities of viral &
cellular DNA pols
Nucleic Acid therapeutics
Anti-viral chemotherapy
Retroviral inhibitors
HIV - lentivirus, viral Reverse transcriptase
Target viral RT and replication
Chain terminator drugs
3’-azido-2’,3’-dideoxyT (AZT) - found to be least toxic
5’-phos needed for active AZT
ddI, ddC - toxicity and need to be phosphorylated
Nucleic Acid therapeutics
Anti-viral chemotherapy
Retroviral inhibitors
Structure of RT solved
active form is a heterodimer having 1 pol and 1 RNase H active site
Nucleic Acid therapeutics
Anti-viral chemotherapy
Retroviral inhibitors
Specific inhibitors of RT found
nevirapine - binds near RT pol active site
Combo therapy used
Nucleic Acid therapeutics
Anti-viral chemotherapy
Drug design
target adhesion of virus to host cell
target virus uptake
target uncoating of nucleic acid
target release of virion
HIV - use of protease inhibitors
Influenza virus - use of amantadine (ion channel inhibitor)