*Cancer Drug Design: DNA

Question 1

What is cancer?

Answer 1

Approximately 200 different diseases
Abnormal cellular growth, resulting in tumour development
Invasive growth (metastasis) often results in death

Question 2

Cancer treatment overview

Answer 2

Surgery and radiotherapy, usually first if possible
Followed by chemotherapy
Molecular biology of cancer is extremely complex
Rationally designed anti-cancer agents are making an increasing impact on treatment and survival
Cancer is often considered a disease of malfunctioning DNA, with mutations leading to excessive cell division
Many traditional drugs work by targeting DNA or associated enzymes

Question 3

Alkylating agents

Answer 3

Oldest class of anti-cancer agents, still widely used
Target DNA- damage to DNA results in cell death
Problems with selectivity- tumour DNA vs. normal DNA

Question 4

Dacarbazine overview

Answer 4

Synthetic compound developed from the observation that aromatic triazenes are carcinogenic and mutagenic
Alkylated DNA is cytotoxic
Many side effects: n+v, diarrhoea
Alkylating agents contain a good leaving group, attached to an alkyl group
Chemically reactive, therefore non-selective, causing many side effects

Question 5

Dacarbazine MoA

Answer 5

Metabolic activation and decomposition make N-dimethyl metabolite
Tautomerism
Loss of nitrogen (gas) is the driving force of the reaction
DNA is alkylated with a methyl group
Activated dacarbazine alkylates guanosine
The alkyl group disrupts interstrand hydrogen bonding, destabilizing DNA leading to cell death

Question 6

Temozolomide

Answer 6

Dacarbazine analogue with same active species
Developed in 2000 for brain tumours
Entirely stable at acidic pH (oral delivery/5mg caps) but unstable/activated above pH7 (brain slightly alkaline)

Question 7

Doxorubicin overview

Answer 7

Used for treatment of solid tumours (breast, head, neck, thyroid, bladder, prostate, ovarian), leukaemia and lymphomas
Pharmacophore is a planar aromatic ring system and the amino group
Major side effect: dose dependent cardiotoxicity, drug levels must be monitored

Question 8

Doxorubicin: DNA interactions

Answer 8

Doxorubicin is an intercalating agent- binds tightly to DNA between adjacent base pairs
Disrupts DNA secondary structure inhibiting replication
Sugar portion interacts with topoisomerase enzymes

Question 9

Doxorubicin analogues

Answer 9

Epirubicin- stereocentre changed, equal activity to doxorubicin, less cardiotoxic, same indications
Idarubicin- methoxy group removed, more potent than danurobicin, less cardiotoxic than doxorubicin, advanced breast cancer, acute leukaemia

Question 10

Mitoxantrone- rationally designed doxorubicin analogue

Answer 10

Contains same pharmacophore, designed in silico, easy to synthesize, less cardiotoxic than doxorubicin

Question 11

Enediynes

Answer 11

Discovered in 1980s, calicheamicin is a bacterial natural product with potent anti-cancer activity
Complex structure and multi-site pharmacophore, including a DNA binding region
Targets DNA by binding to the major groove, not an intercalating agent
Disadvantage: very difficult to synthesize and manufacture
New therapeutics are under development utilizing the enediyne warhead approach

Question 12

Calicheamicin MoA

Answer 12

Binding occurs specifically at TCCT, TCTC and TTTT
Cellular activation produces nucleophilic S
Formation of new S ring causes enediyne ring to shrink
Cycloaromatisation
Forming a benzene diradical which cleaves both strands of DNA = cell death

Question 13

Calicheamicin drug delivery

Answer 13

Two antibody conjugates of calicheamicin have been investigated, developed and marketed
Inotuzumab: in clinical trials

Question 14

Cancer selective intercalators

Answer 14

Telomeres are a possible target: the extreme ends of chromosomes have a guanine rich 3’ single strand overhanging with the repeat sequence TTAGGG
DNA polymerases cannot replicate the extreme ends of DNA
50-200 bases are lost from the telomere at every cell division

Question 15

Telomeres as a cancer selective target

Answer 15

After many cell divisions, telomeres reach a critically short length: the crisis point
Cells enter an irreversible phase of slowed growth, and eventually die
Telomerase, a reverse transcriptase enzyme, maintains telomere length in immortal cancer cells
Telomerase is not normally expressed in human cells, but is present in 85% of human tumour cell lines: therefore selective anti-cancer target

Question 16

G-quadruplex

Answer 16

There is strong evidence that the 3’ single stranded G rich DNA of the T loop forms an unusual secondary feature: four planar guanine residues stabilised by hydrogen bonding and pi stacking

Question 17

Design of telomerase inhibitors

Answer 17

A range of compounds with diverse structures have been identified as telomerase inhibitors
All have large, often symmetrical, planar aromatic pharmacophores

Question 18

G-quadruplex interaction

Answer 18

RHPS4 has been shown to intercalate with a G-quadruplex in a quadruple helix by NMR spectroscopy
This is a complex and difficult experiment and is only a model, but in vitro activity of telomerase inhibitors correlate well with predictions