Chemotherapy

1
Chemotherapy

• Bacteria
• Many antibiotics
• Highly selective
• Viruses
• Use host cell metabolism
• Selectivity difficult
• Toxicity

2
Chemotherapy

• Key is selectivity
• Other problems
• Toxicity
• Rapid excretion
• Rapid metabolism
• Poor absorption

3
Chemotherapy
Ideal Drug

• Water soluble
• Chemically and metabolically stable
• Easily absorbed (apolar)
• NOT
• Toxic
• Carcinogenic
• Allergenic
• Mutagenic
• Teratogenic

4
Chemotherapy
Therapeutic index (T.I.) Minimum dose toxic to
cell Minimum dose toxic to virus
Effective drug T.I. 100-1000
5
Chemotherapy
Other consideration Disease severity Rhinovirus
v. Symptomatic rabies or Lassa fever
6
Chemotherapy

• Reasons for continuing search for anti-virals
  versus vaccines
• For many established disease there is still no
  effective vaccine
• Rapid mutation (retroviruses)
• Reassortment (influenza)
• New and emerging diseases - no vaccine
  available
• Vaccine development takes many years
• Disease that involve immunosuppression (AIDS,
  cancer, transplantation)
• At present no drug completely suppresses viral
  replication (with possible exception of anti-HIV
  protease inhibitors)

7
Chemotherapy

• Interfere with
• A specific viral function e.g. enzyme
• A cellular function that the virus needs so that
  it cannot replicate
• If interfere with cellular function either
• It must be crucial to virus but not the cell
• or
• Only the virus-infected cell must be killed
• (activation of drug in the infected cell only?)

8
Chemotherapy

• Viral enzymes
• Nucleic acid polymerases
• DNA-dependent DNA polymerase - DNA viruses
• RNA-dependent RNA polymerase - RNA viruses
• RNA dependent DNA polymerase (RT) -
  Retroviruses
• Protease
• Integrase
• Neuraminidase

9
Chemotherapy

• 1962 Idoxuridine
• Pyrimidine analog
• Toxic
• Topical - Epithelial herpetic keratitis

• 1983 Acyclovir
• Purine analog
• Sugar modification
• Chain terminator
• Anti-herpes
• Selective to virus-infected cells

1990s Protease inhibitors
10
(No Transcript)
11
Chemotherapy

• Binding to surface receptor
• At present no effective drugs in this class
• Soluble CD4
• May make HIV more infective as results in
  chemokine receptor being the only necessary
  receptor

CD4-Ig2 (PRO542) A more stable version of
soluble CD4 is a tetrameric fusion protein of
immunoglobulin G and CD4. It can reduce levels of
virus in vivo.
AMD3100 - appears to bind to CXCR4 (fusin)
12
Chemotherapy
Peptides derived from gp41 can inhibit
infection Probably block a) interaction of gp41
with cell membrane proteins during fusion b)
by stopping conformational change that results
from the association of two gp41 molecules and
which is necessary for fusion. T-20 block this
conformational change. In clinical trials, a
nearly two log reduction in plasma viral levels
achieved.
13
Chemotherapy

• Membrane Fusion
• Endosome low pH often necessary for membrane
  fusion
• Lysosomotropic agents
• Uncoating of core of membrane and non-membrane
  viruses

• Uncoating of picornavirus e.g. polio, echo,
  rhino
• Stabilize coated virus?

14
Chemotherapy
N
O
CH3
O
WIN 71711 (Disoxaril)
O
N

• Stabilizes picornaviruses - coated virus remains
  in cytoplasm
• 3-methylisoxazole group inserts in capsid VP1
  and covers ion channel

Similar mechanism Pleconaril
15
Chemotherapy
VP1
VP1
Human rhinovirus with WIN V1
16
Chemotherapy

• Amantadine 1966
• Rimantadine 1993
• Only effective against flu A (200mg/day)
• Marginally effective therapeutically
• Prophylaxis Reduce flu by 90
• Since disease usually mild and avoidable not used
  much here

• Good alternative to vaccine for
• Elderly
• Immunocompromized
• Allergic
• Where causative strain not the vaccine strain

Lysosomotropic drugs
May also affect Golgi ion channel involved in
activation of virus before release from cell
17
Chemotherapy
Nucleic Acid Synthesis Polymerases are often
virally encoded Other enzymes in nucleic acid
synthesis e.g. THYMIDINE KINASE in Herpes Simplex
18
Thymidine Kinase
Viral or cellular thymidine kinase adds first
phosphate
PO4
PO4
PO4
Cellular kinases add two more phosphates to form
TTP
19
Chemotherapy
Why does Herpes simplex code for its own
thymidine kinase? TK- virus cannot grow in neural
cells because they are not proliferating (not
making DNA) Although purine/pyrimidines are
present, levels of phosphorylated nucleosides are
low Allows virus to grow in cells that are not
making DNA Thymidine kinase is a
misnomer Deoxynucleoside kinase
NON-SPECIFIC
20
Chemotherapy

• Thymidine kinase will phosphorylate any
  deoxynucleoside including drugs as a result of
  its necessary non-specificity
• Nucleoside analog may be given in
  non-phosphorylated form
• Gets drugs across membrane
• Allows selectivity as only infected cell has
  enzyme to phosphorylate the drug

ACG
P
P
P
21
Chemotherapy

• Need for activation restricts drug to
• Viruses such as HSV that code for own thymidine
  kinase
• Virus such as cytomegalovirus and Epstein-Barr
  virus that induce cells to overproduce their own
  thymidine kinase
• In either case it is the VIRUS-INFECTED cell
  that activates the drug

22
Chemotherapy
Specificity of thymidine kinase from various
sources CYTOSOL MITO HSV SUBSTRATE dTHY 100
100 100 IDU 87 115 ARA-T 33 82 82 BV
DU 5 15 112 ACG low 0 28
23
Chemotherapy

• Thymidine kinase activates drug but
  phosphorylated drug inhibits the polymerase
• Nucleotide analogs
• Sugar modifications
• Base modifications
• Selectivity
• Viral thymidine kinase better activator
• Cellular enzyme may not be present in
  non-proliferating cells
• Activated drug is more active against viral DNA
  polymerase that against cell polymerase

24
Chemotherapy

• Guanine analogs
• Acyclovir acycloguanosine Zovirax
• Ganciclovir Cytovene
• Activated by viral TK
• Activated ACV is better (10x) inhibitor of viral
  DNA polymerase than cell DNA polymerase

Excellent anti-herpes drug
25
Chemotherapy

• ACV acts in two ways
• Competes with GTP
• Chain terminator
• Good anti-herpes drug

Normal DNA synthesis
26

• ACV acts in two ways
• Competes with GTP
• Chain terminator

Termination

• Selective
• Virus phosphorylates drug
• Polymerase more sensitive

• Also inhibits
• Epstein Barr
• Cytomegalovirus

27
Chemotherapy

• Acyclovir very effective against
• Herpes simplex keratitis (topical)
• Latent HSV (iv)
• Fever blisters Herpes labialis (topical)
• Genital herpes (topical, oral, iv)
• Resistant mutants in thymidine kinase or DNA
  polymerase
• Appears not teratogenic or carcinogenic
• Ganciclovir very effective against
  cytomegalovirus viral DNA polymerase is very
  sensitive to drug activated by cell TK

28
Chemotherapy

• Adenine arabinoside (Ara-A)
• Problems Severe side effects
• Resistant mutants (altered polymerase)
• Chromosome breaks (mutagenic)
• Tumorigenic in rats
• Teratogenic in rabbits
• Insoluble
• Use topical applications in ocular herpes simplex

Competitive inhibitor of virus DNA pol which is
much more sensitive than host polymerase
29
Chemotherapy

• Adenine arabinoside
• HSV encephalitis
• Neonatal herpes
• Disseminated herpes zoster
• Hepatitis B
• Poor in vivo efficacy
• DEAMINATION

30
Chemotherapy
Other sugar modifications
AZTazidothymidine
DDCdideoxycytidine
DDIdideoxyinosine
31
Chemotherapy
32
Chemotherapy
33
Prodrugse.g. Famciclovir
Available as topical cream
P
P
P
Taken orally
Converted by Patients metabolism
HSV thymidine kinase
Host kinase
SmithKlein-Beecham
34
Chemotherapy
Non-nucleoside Non-competitive RT
inhibitors Combination therapy with
AZT Resistance mutations will be at different
sites The most potent and selective RT
inhibitors Nanomolar range Minimal toxicity (T.I.
10,000-100,000) Synergistic with nucleoside
analogs (AZT) Good bio-availability Resistant
mutants - little use in monotherapy
35
Chemotherapy
DuPont
Sustiva
(S) -6- chloro-4-(cyclopropylethynyl)-1,4-dihydro-
4-(trifluoromethyl)-2H-3, 1-benzoxazin-2-one.
36
Chemotherapy
Nevirapine Approved for AIDS patients Pyridinone
derivatives (L-697) poor clinically Piperazine
compounds ( DLV) Considerable increases in CD4
cells in combination therapy with AZT and
3TC Efavirenz (Sustiva, DMP266) Phase 3 data
show in combination therapy will suppress viral
load as well as HAART and may be better
Approved for AIDS patients
37
Chemotherapy
38
Chemotherapy

• Ribavirin
• Guanosine analog
• Non-competitive inhibitor of RNA polymerase in
  vitro
• Little effect on flu in vitro
• Often good in animals but poor in humans
• Aerosol use respiratory syncytial virus
• i.v./oral reduces mortality in Lassa fever,
  Korean and Argentine hemorrhagic fever

39
Chemotherapy
May induce mutations in RNA viruses
40
Chemotherapy
Ribozymes RNA molecules that have catalytic
properties among which are the specific cleavage
of nucleic acids. Heptazyme Ribozyme that
cleaves hepatitis C RNA at highly conserved
regions Recognizes and cuts all known types of
the hepatitis C virus, thereby stopping viral
replication
41
Chemotherapy
Viral Protein synthesis No inhibitors
42
Chemotherapy

• Protein processing
• Proteolysis
• Glycosylation

43
Chemotherapy
GAG/POL polyprotein
GAG
Integrase
Polymerase
Protease
44
Chemotherapy
GAG
Integrase
Polymerase
Protease folds and cuts itself free
45
Chemotherapy
GAG
Integrase
Polymerase
Protease cuts at a site between the integrase and
polymerase
46
Chemotherapy
GAG
Integrase
polymerase
47
Chemotherapy
Saquinavir
48
Chemotherapy
49
Chemotherapy
50
Chemotherapy
Influenza Requires neuraminidase to escape from
cell Requires neuraminidase to penetrate
mucus Zanamivir - RELENZA (fall
1997) Neuraminidase inhibitor Active against
Influenza A and Influenza B
51
Chemotherapy
After neuraminidase inhibition, flu
hemagglutinin binds to sialic acid on other virus
particles virus clumps OR virus sticks to mucous
in respiratory tract
52
Virus hemagglutinin sticks new virus particle to
sialic acid on cell surface Virus cannot escape
from infected cell
53
Neuraminidase of virus removes sialic acid from
cell surface thereby releasing virus
54
Chemotherapy
Zanamivir - Relenza Neuraminidase inhibitor Nasal
spray Shortens symptoms by a few days
Tamiflu Oral neuraminidase inhibitor