medchem of cytotoxic drugs

8/2/2019 Medchem of Cytotoxic Drugs

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Intro

Otherwise, the chemo can kill the patient before it kills the cancer

We want to specifically kill cancer cells over normal cells (selective toxicity)

Compared to antibiotics, which exploit differences in biochemical pathways

between us and them

Problem is, cancerous cells tend to use the same biochemical pathways as normalcells

Some normal cells are rapidly dividing (gut, bone marrow, liver) so they are

affected my chemo

But some cancerous cells won't divide rapidly, so chemo won't work well

against them

So we tend to target rapidly dividing cells to target the cancer

Oral forms are desired

Finally, we also want to have them in a form which is easy to administer

Vesicant- a substance which is able to cause blistering. Quite a few chemo drugs tend to

be vesicants. This is why patients need to be told to look out for redness, swelling,

discomfort or pain around the infusion site.

Alkylating agents

Can't copy or transcribe information from DNA

Trigger apoptosis due to damage

Guanine is generally targeted due to its nucleophilic properties (see below)

Designed to interfere with DNA function

Guanine is alkylated, so there's this massive group attached to it

This can lead to the elimination of the group as well (the base comes

off the DNA, see below)

Either way, the DNA can't work this way, so excision repair enzymes

are activated, to cut the DNA to replace these faulty bases

Because repairs can be made, this isn't effective

Modification of DNA bases (mono-alkylation)

See below for the structure a sulphur mustard

Notice it has two chlorines, so it can alkylate twice

Between chains

Within chains (more common)

It can form covalent bonds either:

This will prevent the DNA from coming apart normally for normal

function

Effective

Cross-linking within and between DNA strands (di-alkylation)

Normally, we'd except A goes with T and C goes with G in DNA

But alkylated G can go with T, which is a mistake

This will lead to mutations

Which can lead to a malfunctioning cell, and apoptosis

Nucleotide mispairing

How does it work? (mode of action):

Janus is the Roman god of doors. He has two heads, one pointing inside, and

the other pointing out.

Why is this important? Because alkylating agents will kill cancerous cells BUT

because they interfere with DNA, they can also CAUSE cancer.

Exhibits a 'Janus' effect

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People may have secondary tumours which are completely different

to the original tumour after a few years of treatment

Below is a sulphur mustard, where there's a two carbon bridge between the

S and the chlorines. THIS IS IMPORTANT. RECOGNISE THIS.

Sulphur mustards are gases, due to low intermolecular bond strength (they

don't H-bond to each other) so they are too dangerous to work with

So nitrogen mustards were investigated, because they can H-bond to each

other, so it's not a gas anymore, so it's safer to handle

The alkylating agents will always have a specific moiety

The two nitrogens in the right side ring presents a electron rich region

This makes it nucleophilic, attacking the alkylating agent (seen as R)

Can lead to the ring opening which permanently binds the agent to

the base

Or can cause the entire group to come off as a leaving group from the

DNA

This addition will lead to a positive charge on the nitrogen, which needs to

be removed.

But remember: monoalkylation (shown below) can easily be repaired

Why is guanine (N7 nitrogen) targeted specifically?

The electronegative chlorine atoms will draw electrons towards itself,

causing the adjacent carbons to become slightly positive

Chlorine is a good leaving group as it comes of neutral with respects to

acid-base chemistry (i.e. even though it's negatively charged, it

doesn't have acid base chemistry)

The non-bonding electron pair (NBP electrons) on the nitrogen is attracted

to the positive charges, leading to intramolecular nucleophilic attack (SNi)

Bond angles are strained (at 60 degrees instead of normal 108)

Both carbons are positively charged

The SNi leads to the formation of the aziridium ion, which is a highly reactive

electrophile (i.e. susceptible to nucleophilic attack)

After nucleophilic attack with the nucleophile (which is likely to be guianine),

The mechanism of action of alkylating agents is all the same (and we need to

memorise it)

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Remember: the molecule is bifunctional as it has two carbons, so it

can crosslink DNA (deals more damage)

the base is now alkylated.

Forms the aziridium ion too easily then, which will just react with all the cells

it comes into contact with

So we need to tie up those NBP electrons to stop forming the aziridium ion

as easily to reduce toxicity, to reduce side effects

If the nitrogen mustard shown above had an aliphatic R group, it is too toxic to use

in people

Alkylating agents- examples

Mephylan

The NBP electrons on the nitrogen are partially taken up into the aromatic ring

It's actually L-phenylalanine (amino acid) attached to the mustard

The sterochemistry on the carbon is R

But it's still actively taken up by all cells, leading to side effects

They thought the phenylalanine would allow the drug to be taken up into growing

cells because it's an amino acid

If the phenylalanine comes off, it's still active because the mustard is intact

If the amino or carboxylate groups are metabolised, again, the mustard is

still intact and it's still active

This drug has some activity, because the NBPs are somewhat available

This is the really important one because we use it quite often

Cyclophosphamide

It is a prodrug, it must be metabolised first:

R

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In fact, the structure shown on the right can be further broken down to form

just a bare nitrogen mustard, which is thought to have most of the activity

The NBP electrons in Cyclophosphamide are completely taken up into resonance,so no aziridium ion formation can occur, so there is no alkylation.

Need to co-administer with Mesna and make sure to keep the patient very

well hydrated with IV fluids and oral fluids

Mesna (pictured top left below) has a sulfate group purely for solubility and

salt formation, while the active area of the molecule is the thiol (SH) group,

which acts as a nucleophile to bind with the acrolein to form a non-toxic

compound

Problem with cyclophosphamide is acrolein is a side-product which is toxic

Some cancer cells produce a great amount of glutathione (GSH)

Remember: thiol is a nucleophile, the active aziridium ion form is very

attractive

GSH has a thiol group, which can react with the alkylating agent before it

reaches the DNA to deal damage

Therefore, these cells will be resistant to treatment

Thiol groups could also be a hindrance to treatment though

An alkylating agent may be conjugated to a steroid to help it get into specific

cells

It is only effective in cells which have low ALDH (aldehyde

dehydrogenase), which are the well-differentiated blood cells, while

the stem cells of the blood are quite high in ALDH, so they tend to be

protected

Cyclophosphamide is actually quite targeted if you think about it

Lastly, some forms are slightly selective

Alkylating agents- Methansulfonates

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The oxygens are strongly electronegative, causing a great positive charge on

the sulfur and adjacent carbon

Busulfan has two methanesulfonate groups (the two sulfur containing groups on

the sides)

The methanesulfonate group is a good leaving group, so the carbon with thepositive charge is able to attack guanine as well

But because it's got two groups, its able to cross link DNA

Alkylating agents- nitrosoureas

These are the drugs which tend to end in 'mustine'

Very useful for brain cancers, as they are lipophilic enough to pass through the

BBB

Because the non-bonding pairs of electrons on the nitrogen are completely

taken up into resonance, so the aziridium ion can't be formed

Although they look like normal alkylating agents, they don't have the same

mechanism

Instead, through a complicated mechanism, it breaks down to form two positively

charged carbocations which are the active molecules

Platins (alkylating-like agents)

These are not alkylating agents, but shows some similar action (crosslinking of

DNA)

The classical one is cisplatnin

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Cisplatnin is a square planar molecule, with 2 amine and 2 chloride groups in

a cis configuration:

This is because of this equilibrium reaction:

Interestingly, cisplatnin is formulated in normal saline (0.9% NaCl)

If the concentration of chlorine is high (as it is in the blood and in normal saline),

then the equilibrium lies to the left, which is the inactive form

Therefore, platnins are prodrugs

However, if cisplatnin moves into the cells, the chloride concentration is much

lower, the equilibrium moves to the right, and the activated 'aquated' form is

produced (pretty much water chucked on)

The H2O ligand is a very good leaving group

The aquated form is active, because the platinum atom can now attach to the N7

atom of guanine (just like alkylating agents)

However, this is a intra-strand (within strand) crosslink.

This will cause the DNA to have a 90 degree kink due to the shape of

cisplatnin (square planar)

This irregular shape means the DNA is now useless

Because there are two chloride groups, the same process will happen again, whichcauses DNA to become cross-linked

GSH will also bind to platnins to make them useless

We can try to shield the platnin with a bulky group, but this is ineffective

Again, another huge problem is with glutathione

Therefore, we have newer, second generation platnins which are less

reactive/toxic but still just as effective

Cisplatnin is too reactive, it is quite toxic

It has a bi-dentate ligand instead of the two chorines

This slows down the aquation of the platinum, leading to reduced toxicity

Pictured above is oxaliplatnin, a second generation platnin

Antimetabolites

Self directed learning

Up to now, we've looked at compounds which deliberately damage DNA

But antimetabolites will cause DNA damage by preventing the synthesis of DNA,

either by producing false metabolites, or interfering with the enzymes responsible

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Therefore, this class of drugs are S phase specific for the cell cycle

for production

Responsible for producing thymine from uracil, uses tetrahydrofolate (THF)

as a co-factor

Disruption will mean thymine synthesis cannot occur, and the cell will

apoptose due to a thymineless death

The primary target enzyme is thymidylate synthase

Antimetabolites- 5-fluorouracil

5-flurouracil has a strongly electronegative group on the 5 position, which makes it

quite attractive to the enzyme

Note: he doesn't think it's a prodrug, because prodrugs tend to be

catabolised (broken down) to its active form. 5-FU is anabolised (built up) to

its final form due to the addition of ribose and phosphate

It is a prodrug (even though Schmerer disagrees), it must first be converted to its

deoxyribonucleotide form (pretty much just attach some phosphates to it to makeit look something like a nucleotide)

Normally the THF would react with the uracil to form thymine, but this can't

happen due to electrical repulsion between the fluorine and the nitrogen 10

of THF

When it enters the thymidylate synthase enzyme, it causes the formation of a

false complex with tetrahydrofolate (THF) and thymidylate synthase

This effectively prevents thymidylate synthase from being regenerated, which

stops thymine production

This causes the elongation of DNA to be stopped, leading to apoptosis

Additionally, these false nucleotides may also be incorporated into the DNA and

RNA

Anti-metabolites- folate metabolism

As stated above folate (as THF) is an important co-factor for thymidylate synthase

It needs to be reduced back to THF to be used again

After thymine is produced from uracil, the THF is oxidised to dihydrofolate (DHF)

It is able to be inhibited

Also causes a thymineless death

May be used as a synergistic drug with 5-FU, as they both target the same

process

The enzyme folate reductase is responsible for this function

Increases the electron density on the nitrogen at the bottom of the

ring, which is essential for binding

Therefore it will be able to outcompete folic acid

Better substrate compared to the endogenous substrate, folic acid due to

the amine group

Methotrexate will inhibit folate reductase

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Bleomycin

This is a problem, as it is hard to scale up to get large yields

Massive molecule which is synthesised by bacteria

Although a part of the molecule is cut off, DNA binding sites lie to the right

of the molecule shown below. However, it is unable to intercalate with DNA

due to too much 3D structure (need to be flat to intercalate)

The important bit is the iron in a square planar structure

Notice how the oxygen is bound to the iron, it displaces the carbamate

group

The oxygen is reduced to oxygen free radicals, which then damage the DNA

The action of bleomycin is to bind to the DNA and cause DNA breakages

The molecule is enzymatically cleaved by hydrolase, which reduces DNA binding

and damage

The copper is removed to inactivate the molecule to reduce toxicity (it will

find iron to chelate to in the body)

Normally, it comes as a blue copper complex (the copper sits where the iron is

sitting below)

It is amazing to see such a large molecule being able to enter the nucleus. The

sugars may be used as a recognition site to gain access to the nucleus

Because it converts oxygen into free radicals, the compound is associated with

oxygen toxicity, leading to pulmonary fibrosis. Need to monitor patients carefully

Actinomycin

Flat ring system which isn't fully aromatic. It is still able to intercalate to the

DNA

Composed of three basic parts:

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Two large lactones made of 5 amino acids, they may differ or be the same

Planar rings allow pi-stacking

Lactones will bind via hydrogen bonding (contains amino and carboxyl

groups) and Vander Waals' forces (because it's big)

The entirety of the molecule will be able to bind to the DNA, causing it to bend out

of shape completely

Bending it out of shape this badly prevents topoisomerase II from unwinding the

DNA properly, so the cell can't replicate or transcribe DNA, leading to death

Anthracyclines

Doxorubicin

Epirubicin

There are a few anthracyclines in use e.g.

Although they have 4 rings, they are not tetracyclines

Tip: rubor is redness, can't forget it's red now

They are red coloured compounds, and they are renally excreted, causing urine to

go red

Note: formaldehyde naturally formed by the body will attack the sugar,

which can cause covalent bonding of the anthracycline to the DNA

That is a good thing

The 4 flat rings allow for intercalation into the DNA, while the daunosamine sugar

(seen at the bottom of the image) will aid with binding to the DNA

Prevents transcription and duplication of DNA

After intercalating with the DNA, it stabilises the interaction of topoisomerase II

with the DNA, preventing it from doing anything else

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Causes free radical formation, which does have some effect against DNA,

but the problem is it also occurs in the cytosol of cells

This is why it might be causing cardiotoxicity, as the cells of the heart cannot

divide to form new cells, so the cells will take gradual damage over use

Therefore, there is a maximum cumulative lifetime dose for all the

molecules in the anthracycline family

They are also a target for reductase enzymes, this is a major issue

medchem of cytotoxic drugs

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