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ALKYLATING AGENTS
AND ANTIMETABOLITES
OBJECTIVE
TO UNDERSTAND THE TYPE AND ROLE OF
ALKYLATING AGENTS
TO UNDERSTAND THE TYPE AND ROLE OF
ANTIMETABOLITES
Alkylating Agent
alkylating agent, any highly reactive drug that binds
to certain chemical groups (phosphate, amino,
sulfhydryl, hydroxyl, and imidazole groups) commonly
found in nucleic acids and other macromolecules,
bringing about changes in the DNA and RNA of cells.
Alkylating agents were the first anticancer drugs used
The types of molecular changes induced by alkylating
agents include cross-linking between strands of DNA
and the loss of a basic component (purine) from or the
breaking of the nucleic acid.
Con’t The result is that the nucleic acid will not be
replicated. Either the altered DNA will be unable to
carry out the functions of the cell, resulting in cell
death (cytotoxicity), or the altered DNA will change
the cell characteristics, resulting in an altered cell
(mutagenic change).
This change may result in the ability or tendency to
produce cancerous cells (carcinogenicity).
Normal cells may also be affected and become
cancer cells.
cont
These molecules may either bind twice to one strand of DNA (intrastrand crosslink) or
may bind once to both strands (interstrand crosslink).
If the cell tries to replicate crosslinked DNA during cell
division, or tries to repair it, the DNA strands can break.
This leads to a form of programmed cell death called
apoptosis.
classes
The alkylating agents are the largest group of
anticancer agents.
Five main sub-groups.1. Nitrogen mustards
2. Alkyl sulphonates
3. Nitrosoureas
4. Ethyleneimine
5. Thiazines
Nitrogen mustards
Mechlorethamine: first nitrogen mustard.
Used for Hodgkin and non-Hodgkin lymphomas
It is also used topically for treatment of cutaneous T-
cell lymphoma .
Toxicity: Block reproductive functions, First trimester
of pregnancy and later stage of pregnancy it should
not be used.
Methclorethamine ,Cyclophosphamide,
Melphalan,Chlorambucil and Ifosfamide.
Alkyl sulphonates
Busulfan – Highly specific for myeloid elements
and granulocyte .it is not curative.
Toxicity: hyperuricaemia is common; pulmonary
fibrosis and skin pigmentation are specific
adverse effects.
Drug of choice for chronic myeloid leukaemia .
Dose: 2-6 mg/ day, orally
Nitrosoureas
It is lipid soluble alkylating agents with a wide range of antitumor activity
. Nitrosoureas: interfere with enzymes needed for DNA repair.
They are able to cross the blood-brain barrier.
used to treat:
1. brain tumors 2. non-Hodgkin’s lymphoma,
3. multiple myeloma, and
4. malignant melanoma.
Major toxicities occur in the hematopoietic and gastrointestinal systems.
Carmustine, lumustine, and streptozocin are examples of nitrosoureas.
Thiazines
Have primary inhibitory action on RNA and
protein synthesis (others mainly affect DNA).
Ideal for malignant melanoma; also used in
Hodgkin's disease.
Nausea and vomiting are prominent side effects.
Alkylating agents can cause:
severe nausea and vomiting
as well as decreases in the number of red blood
cells and white blood cells.
The decrease in the number of white cells results
in susceptibility to infection.
Alkylating agents have found use in the
treatment of lymphoma, leukemia, testicular
cancer, melanoma, brain cancer, and breast
cancer. They are most often used in combination
with other anticancer drugs.
Antimetabolite
similar in structure to a metabolite, or enzymatic
substrate, so competes with or inhibits the
metabolite.
Their maximal cytotoxic effects are in S-phase
specific. readily become incorporated into either DNA or RNA
cause toxicity In cells that are rapidly dividing
o Skin disorder, hair loss, anemia, mucositis,
myelosuppression and thrombocytopenia are common.
Antimetabolite as chemotherapeutic
1. Folate antagonist
2. Pyrimidine antagonist
3. Purine antagonist
4. Thymidylate synthase inhibitors
5. Multitargeted antifolate
6. Glycinamide ribonucleotide formyltransferase
and aminoimadazole carboxamide
ribonucleotide formyl transferase
Folate antagonist
Methotrexate
DHFR - conversion of dihydrofolate to
tetrahydrofolate and ultimately to 10-formyl
tetrahydrofolate which
provides the formyl group for glycinamide
ribonucleotide formyl- transferase (GARFT) and
aminoimidazole carboxamide ribo- nucleotide
formyl transferase (AICARFT).
Thus, the inhibition of DHFR results in depletion of
intracellular pools of reduced folates and ultimately
in reduced synthesis of purines and pyrimidines.
Mechanism
HN
N N
N
O
H2N
NH
O
NH
CO2H
CO2H
Folic acid
DHFR
HN
N NH
N
O
H2N
NH
R
Dihydrofolate
DHFR
HN
N NH
HN
O
H2N
NH
R
Tetrahydrofolate
PABA
Microorganisms
HN
N NH
N
O
H2N
N
R
Thymine synth
predominant mechanism of action of methotrexate is uncertain, as
polyglutamated forms of the drug also inhibit TS and AICARFT
Resistance
Nonproliferating cells are resistant to MTX,
probably because of a relative lack of DHFR.
Decreased levels of the MTX polyglutamate due to
its decreased formation or increased breakdown.
Resistance in neoplastic cells can be due to:
amplification (production of additional copies) of the
gene that codes for DHFR, resulting in increased
levels of this enzyme.
The enzyme affinity for MTX may also be
diminished.
Resistance can also occur from a reduced influx of
MTX,
Indications
MTX, effective against:
acute lymphocytic leukemia,
choriocarcinoma,
Burkitt's lymphoma in children,
breast cancer, and
head and neck carcinomas.
Toxicity
Renal damage: is a complication of high-dose MTX by
precipitating in the tubules.
Hepatic function: Long-term use of MTX may lead to
cirrhosis.
Pulmonary toxicity: This is a rare complication.
Neurologic toxicities: associated with intrathecal
administration and include subacute meningeal
irritation, stiff neck, headache, and fever.
Long-lasting effects, disabilities, have been seen in
children.
Another DHFR inhibitor that has shown activity in
humans is piritrexim. This is compound does not
rely on the RFC, but enters the cell by means of
passive diffusion. It has oral bioavailability of 75%
Trimetrexate is more lipophilic than methotrexate
and is not dependent on the RFC for entry into
the cell. This leads to higher concentrations of trimetrexate within
the cell, although the drug does not undergo
polyglutamylation.
Pyrimidine analogs
It is widely used in colon cancer
They include:-
Fluorouracil
CAPECITABINE
CYTARABINE
GEMCITABINE
5-fluorouracil (5-FU)
replacing nucleosides in one or more normal cell
functions because of their similarity
Main use is in leukaemias, lymphomas,
colorectal cancer and solid tumors
They may fall into one of two main classes
either being incorporated into DNA and RNA
synthesis or
being responsible for inhibition of one of the
enzymes essential to cell metabolism.
5- FU prodrug capecitabine
cytidine analogue is administered as an oral
formulation
passes unchanged through the intestinal mucosa.
activated through a series of enzymatic steps in
the liver and in tumour cells conversion to 5-FU in a potentially tumour-selective manner
by the enzyme thymidine phosphorylase
Dose-limiting toxicities (DLT) included nausea,
mucositis, diarrhoea and neutropenia.
MOA
HN
N
O
O
O
HO
OP
O
OH
HO
5-FU metabolite
ThymidylateSynthetase
HS
NHN
N
HN
N
O
NH2
R
BH
HN
N
O
O
O
HO
OP
O
OH
HO
H
F
ThymidylateSynthetase
SB
NHN
N
HN
NH
O
NH2
R
F
Enzyme inhibition
HN
NH
O
O
Fin vivo
HN
N
O
O
F
O
HO
OP
O
OH
HO
InhibitorThymidylate synthetase
locks the enzyme into an inhibited conformation resembling the transition state
formed in the process of conversion of dUMP to thymidine by TS.
Resistance and adverse effects
Resistance is encountered when the cells have
lost their ability to convert 5-FU into its active form
(5-FdUMP) or when they have altered or
increased thymidylate synthase levels.
Adverse effects: anorexia, oral toxicity. A
dermopathy (erythematous desquamation of the
palms and soles) called the hand-foot syndrome
seen after extended infusions.
purines analogs 6-THIOPURINES
6-Mercaptopurine
6-Thioguanine
FLUDARABINE
CLADRIBINE
6-Mercaptopurineis the thiol analog of hypoxanthine
Primarly serves in the treatment of childhood
acute leukemia
Mode of action is:-
Inhibition of purine synthesis:
Incorporation into nucleic acids:
Nucleotide formation
6-Mercaptopurine
Resistance:
1. an inability to biotransform 6-MP to the corresponding nucleotide because of decreased levels of HGPRT
2. increased dephosphorylation
3. increased metabolism of the drug to thiouric acid or other metabolites.
4. Increased thiopurine methyltransferase (TMPT)activity
Adverse effects:
Bone marrow depression- principal toxicity. Side effects (anorexia, nausea, vomiting, and diarrhea) hepatotoxicity in one-third of adult patients.
REFERENCE
LIPPINCOAT’S PHARMACOLOGY
LANG’S PHARMACOLOGY
JOURNAL ON ANTIMETABOLITES
WIKIPEDIA
Cancer Research Campaign Department of
Medical Oncology, Beatson Oncology Centre,
Any queries????
QUERIES
Visit :www.bpharmstuf.com
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