Anthracyclines and Mitotic Inhibitors Flashcards
what are anthracyclines?
they are antitumour antibiotics
how do anthracyclines work?
substances produced by microorganisms that exert their anticancer activity by
interacting with DNA
they all intercalate preferentially into major groove of DNA
what are the main anthracyclines available?
– Doxorubicin (+ liposomal)
* main anticancer anthracycline
– Daunorubicin (+ liposomal)
– Idarubicin
– Epirubicin
– Mitoxantrone
– pixantrone
how were daunorubicinis/ docorubicins found?
isolated soil samples with a new strain
of Streptomyces peucetiusthat produced an antibiotic with good activity against
murine tumors
what clinical condition should danorubicin not be used in?
cardiac- could produce fatal cardiac toxicity
what is doxorubicin?
Doxorubicin is a 14-hydroxylated version of daunorubicin, the immediate precursor
of doxorubicin in its biosynthetic pathway
what was better daunorubicin or doxorubicin?
docorubicin
– better activity than daunorubicin, particularly against solid tumors
– relatively higher therapeutic index
– cardiotoxicityremaine
how is danorubicin produced?
produced by a number of different wild type strains of streptomyces
what is the first step in anti-cancer action?
involves sliding between the bases of DNA
what are the general features of the anthracyclines?
– polycyclic aromatics
– positively charged groups
* ionised amine
* induces stability
* makes molecule more water soluble
* binds strongly to DNA
– hydrogen bonding sites
* H-bond donors / acceptors
* -OH, NH,C=O
what should intercalators be?
planar
is DNA static? why?
DNA is not a static structure
* Gaps between the base-pairs open and close
* ‘DNA breathing’
* Intercalation into DNA in the space between two adjacent base pairs becomes
possible
what are DNA strands linked by?
DNA strands are linked by hydrogen-bonds
– directional
– non-covalent
– can be disrupted
what must the distance be between the dna strands for an intercalator to fit?
the latter must separate by over 0.3 nm
what happens when an intercalator fits between base pairs?
– local structural changes to the DNA strand
* unwinding of the double helix
* lengthening of the DNA strand
* Structural modifications lead to functional changes
– inhibition of transcription and replication processes
what is the effect of intercalation?
- More rigid structure
– viscosity mof DNA increase
– melting temperature Tm of DNA increases - DNA breathing is reduced
- Hydrogen-bonding in DNA fails at point of intercalation
as bases/strands separated - DNA structure partially unwinds and elongates
what are the modes of intercalation?
Parallel Intercalation:
– long axis of drug lies parallel to the base-pair axis
– maximum overlap between drug and DNA bases
– most stable form of intercalation
* Perpendicular Intercalation:
– long axis of drug lies perpendicular to the base-pair axis
– most common form of intercalation
what happens after the intercalation into DNA? what is its origin?
ring D of the anthracyclineprotrudes into the major
groove of the double helix, while ring A reaches out into the minor groove
– the anthracyclineis at right angles to the direction of the DNA bases
(perpendicular intercalation)
* The amino sugar, linked to ring A of the anthracycline, interacts with groups in the
minor groove, serving as an anchor that stabilises the anthracycline-DNA complex
what is doxorubicin formulated as? why?
Formulated as hydrochloride salt, minimal oral activity
what is docorubicin sensitive to? what is done about this?
Photosensitive(aluminium/brownwaxpaperbag
how is docorubicin administered?
- Administered by intravenous infusion
– rapid flowing vein, fine cannula into vein
– most often a central line into vein near collarbone
what is docorubicin irritating towards?
- Very irritating to tissue
– vesicant side-effects
– leakage into tissue around the vein (extravasation), causes tissue damage
– gloves must be worn for administration
what does admin of docorubicin via fast flowing vein cause?
- Fast flowing vein causes rapid dilution by blood volume
– reduces damage to vein walls
– minimises drug leakage
what does admin of docorubicin via fast flowing vein cause?
- Intramuscular/subcutaneous administration causes severe tissue necrosis
how is docorubicin made up?
- Sterile red-orange lyophilised (freeze-dried) powder in single dose flip-top vials
– 10, 50 mg vial
– reconstitute with 0.9% saline to 2 mg / mL doxorubicin hydrochloride
– stable (7 d at RT, 15 d refrigerated) and protected from exposure to sunlight
– 2 mg / mL pre-prepared
why are liposomal products beneficial?
- Ubiquitous body distribution
- Little or no preferential accumulation in some tumor tissues
- Improved by modifying its mode of delivery –liposomes
what are the liposomoal products?
- Doxorubicin (Caelyx), Daunorubicin (DaunoXome)
how are doxorubicin liposomes formulated?
Doxorubicin liposomes formulated with surface-bound methoxypolyethylene
glycol
what is the benefit of liposomal formulation of docorubicin?
ignificantly reduces toxicity, specifically
the cardiotoxic effects of doxorubicin
what is a frequent side effect of liposomal doxorubicin?
hand and foot syndrome,
managed by reducing dose and increasing the cycle duration
– leakage from capillaries at extremeties(hands, feet)
what is the MOA of docorubicin?
- Initially, intercalation with DNA
- Intercalation with DNA is non-specific between adjacent base-pairs
- Then … main effect is on function of topoisomerase II
– forms stable ternary complex with DNA and topoisomerase II at point of
intercalation
– functions as enzyme inhibitor
– Interferes with DNA processing/repair and, hence, prevents cell division - Also, involved in oxidation / reduction reactions
what is Topoisomerase II?
- Enzyme responsible for controlling topology of DNA
– number of times DNA is coiled/folded
how is top 2 involved in DNA processing?
– important during replication –DNA coils are unwound
– topoisomerase II interacts with DNA to form a binary covalent complex
(DNA-topoisomerase)
* complex may be cleaved by water
– topoisomerase II simultaneously breaks both DNA strands
– the ends of the DNA are separated, and a second DNA duplex is passed
through the break.
– following passage, the cut DNA is re-connected
– increases or decrease the linking number of a DNA loop by 2 units
– forms a tighter (or looser) coil
how does doorubicin stabilise the top 2- DNA complex?
– forms a rigid ternary complex (DNA-topoisomerase-doxorubicin)
– cannot be cleaved
– inhibits DNA replication
– apoptotic response in cells is attributed to inefficient repair of double
strand breaks
– inhibits cell replication -> cell death
how does TOP 2 work- short steps?
(1) topoisomerase II-DNA binding
(2) generation of a prestrandpassage cleavage complex
(3) DNA strand passage (ATP driven)
(4) re-ligation of the DNA break
(5) ATP hydrolysis
(6) DNA release and subsequent enzyme recycling
how does doxorubicin cause intracellular redox reactions?
– redox properties of quinonefunctionallity
– CYP450 reductase catalyses reduction to semiquinoneradicals
– generates free radicals
* superoxide radical O2٠-
* hydroxyl radical OH
what are the properties of the radicals generated from doxorubicin reactions?
– central role in cardiotoxicity
– react with cell membranes
– damage membrane of cancer cells
what does the hydroxyl radical do?
One of most destructive chemical species known
Mitochondrial dysfunction
Alters calcium ion transport
Causes DNA base oxidation
Causes DNA strand cleavage
Causes protein oxidation
Causes thiol oxidation
Causes lipid membrane oxidation
what are some tissues with protective enzymes that are resistant to doxorubicin?
- antioxidants
- superoxide dismutase
- glutathione peroxidase
what has low superoxide dismutase and thus is suceptiable to docorubicin?
- tumours (and cardiac tissue) have low superoxide dismutase
- accumulation of superoxide radical
– cardiac tissue also has low levels of catalase - unable to metabolise H2O2
- readily processed to hydroxyl radical
*
what should you to do prevent fentons reaction?
Administration of dexrazoxane before doxorubicin (a strong iron chelator) removes
the iron from the drug complex
what is doxorubicin resistant to? why?
drug efflux by P-glycoprotein, also cells rich in protective enzymes
what are the pharmacokinetic properties of doxorubicin?
– extensively plasma protein bound
– does not cross blood-brain barrier
– metabolism/excretion –hepatic and primarily biliary
– some renal excretion leads to red urine
doxorubicin and epirubicin are epimers, what is this?
wo isomers differ in configuration at
only one stereogeniccenter
what is better tolerated out of docorubicin and epirubicin?
Epirubicinis better tolerated than doxorubicin with less nausea and vomiting, myelosuppression, and cardiac toxicity
this is due to the distrubution of docorubicin being signifigantly higher
what are the outcome of the metabolism of epirubicin?
- Metabolism of epirubicinis complex to give relatively or totally inactive
metabolites - Transformation of epirubicin into its glucuronide represents the major detoxifying
pathway.
– also significant is reduction of the ketone moiety at C-13 to give epirubicinol
what are bleomycins?
- Naturally occurring glycoprotein
- From Streptomyces verticillus
Clinically used Blenoxaneis a mixture that contains predominantly bleomycins A2 and B2
what job does the intercalator do in bleomycins?
tabilised by H-bonding
– forms a complex with Fe(II)
– formation of hydroxyl and
superoxide radicals
– cleavage of phosphodiesterbon
what does the activity of bleomycins require?
- a metal ion (usually Fe2+ / Fe3+)
- oxygen
- one-electron reducing agent
what are the 4 signifigant molecular regions in bleomycins?
- C-terminal domain (heterocyclic bithiazole)
provides the DNA binding affinity, both by
intercalation and minor groove-binding
(electrostatics) - N-terminus domain (b-hydroxyhistidine, b-
aminoalanineand pyrimidine) constitutes
the metal-chelating and oxygen activation
domain (free-radicals and single/double
strand breaks) - flexible central linker gives the metal-
chelating domain necessary flexibility to
cleave both strands without dissociation
from DNA - glycoside region role is unclear, but shown
to be crucial for the DNA and RNA cleaving
efficiency (may contribute to drug uptake)
how do mitotic inhibitors work?
suppress cell division by inhibiting mitosis
* Mitotic spindle -vital for equal partitioning of DNA into daughter cells
* Spindle is composed of protein, tubulin -part of larger intracellular skeleton
* Mitotic inhibitors -disrupt balance between polymerised and depolymerised
microtubules
* Cell cycle specific
what are microtubules?
polymers of tubulin
integral components of mitotic spindle
what is tubulin?
Microtubules are composed of the protein
tubulin
* Tubulin forms dimers, which consist of an
a and a b subunit (450 aa monomers)
* Dimers stack together into protofilaments,
which are linear strings
how do profilaments bind?
bind laterally to form
hollow, cylindrical microtubules
what does a microtubule scaffold usually do?
normally positions
chromosomes, then collapses as the
replicated chromosomes are pulled apart
during cytokinesis
what is the equilibrium process of a tubulin?
Binding of a/b-tubulin to guanosine-5’-triphosphate (GTP) promotes assembly
Binding of b-tubulin to guanosine-5’-diphosphate (GDP) promotes disassembly
(hydrolysis of GTP -> GDP)
what are the various products in the taxane group?
Paclitaxel (Taxol)
Docetaxel (Taxotere)
* Cabazitaxel
what is the mode of action of taxanes?
– Microtubulin antagonist
* bind tubulin dimers and microtubule filaments reversibly
* induces assembly of tubulin into microtubules, inhibits depolymerisation
to tubulin
* stabilises mitotic spindle by formation of abnormal bundles of
microtubules
* microtubules do not depolymerise leading to disruption of mitosis
* equilibrium is shifted to formation of microtubules
* stabilise microtubules to the extent that mitosis is disrupted
* dysfunctional, cell stopped in metaphase
* cell death
how do taxanes interact laterally?
Taxolbinds very near the M loop of b-tubulin, which
makes lateral contacts between adjacent
protofilaments
Stabilisesmicrotubules by strengthening lateral
interactions between protofilments
Stabilisesa conformation of M loop that favors lateral
contacts
Counteracts destabilisingeffects of GTP induced
hydrolysis
Microtubule with GTP bound to end β-tubulin is stable
and will continue to grow
Microtubule with GDP molecule to end β-tubulin will
be unstable and will depolymerise
how do vinca alkaloids work?
Cell-cycle specific –blocks mitosis by metaphase arrest
how do vina alkaloids interact with tubulin?
– binds to tubulin
– inhibits polymerisation to microtubules
* inhibits development of mitotic spindle
– shifts equilibrium toward microtubledisassembly and shrinkage
– prevents formation of the mitotic spindle
* spindle poisons
– metaphase arrest
– cell division inhibited
– effects only apparent during mitosis
where do vinca alkaloids and taxanes bind?
- Vincaalkaloids bind to high-affinity sites
of b-tubulin at the microtubule (+)-end
to suppress assembly microtubule
dynamics - Taxolbinds along the interior surface of
the microtubule, suppressing its dis-
assembly dynamics
what is eribulin used for?
As mesylatesalt for metastatic breast cancer and liposarcoma
what MOA does eribulin have?
- Has both cytotoxic and non-cytotoxic mechanisms of action
– antimitotic activities
– decreased capacity for migration and invasion leads to reduced metastatic capacity
is eribulin synthetic?
fully synthetic
– macrocyclic ketone
– analogue of marine natural product halichondrinB (sponge genus Halichondria)
– halichondrinB is a potent naturally occurring mitotic inhibitor
what is the MOA of eribulin
– binds predominantly to a small number of high affinity sites at the plus ends of
existing microtubules
– binding site is distinct from vincadomain
– also creates non-functional aggregates of tubulin blocking microtubule
polymerisation
– apoptosis is induced following prolonged and irreversible mitotic blockade
what is the binding site of eribulin shaped by?
- Binding site is shaped by hydrophobic and polar residues
- “Cage” structure is in contact with the ribose moiety of the guanosine nucleotide
- Acts as a molecular “lid” to inhibit nucleotide exchange and tubulin-tubulin-
induced hydrolysis - Wraps around the side chain of Tyr224
how do mandrake plant extracts work?
– Teniposide
– Etoposide
* breakdown of microtublesand prevents polymerisation to produce new ones
* binds enzyme-DNA complex and maintains transiently cleavable form
* generates irreversible DNA strand breaks
how do the campotheca tree extracts work?
Semi-synthetic versions of extract of bark camptothecin
– Irinotecan(urethane prodrug)
– Topotecan
* Inhibition of topoisomerase I
