Lecture 5 - Alkylating Agents and Platinum Compounds

Question 1

Alkylating agents

Answer 1

drugs that generate reactive eletrophilic (electron deficient) intermediates that react with nucleophilic (electron rich) groups on DNA and proteins –> result in attachment of alkyl group to DNA and protein
major MOA involved alkylation of purine bases in DNA; guanine N7 is most common site of alkylation

Question 2

Most effective anti-cancer drugs are bifunctional alkylating agents that produce

Answer 2

DNA intra- and interstrand linkages
cross-links inhibit DNA replication as well as transcription
not cell-cycle phase specific

Question 3

Purine bases in DNA

Answer 3

all ring nitrogens have some reactivity, as well as the exocyclic oxygens
nucleophilicity is controlled by steric, electronic and hydrogen bonding effects

Question 4

Effects of DNA alkylation

Answer 4

intrastrand linking between two bases on the same strand and interstrand crosslinking of two separate strands

Question 5

Effects of alkylation

Answer 5

alkylating agents react with many nucleophiles other than DNA bases - thiols, amines, cysteine, lysine, glutathione
toxicity to cancer cells results from DNA alkylation and DNA cross-linking; DNA protein cross-links also inhibit DNA function
cells are more susceptible in late G1 and S phases of cell cycle but alkylating agents are considered non cell cycle specific

Question 6

Side effects of alkylation

Answer 6

measurable incidence of second malignancies in long-term survivors following chemotherapy with alkylating agents
most second malignancies arise in bone marrow (leukemias)

Question 7

Crosslinkers cause various types of DNA damage

Answer 7

prevent replication or transcription
mispairing
DNA fragmentation

Question 8

Mechlorethamine

Answer 8

extremely reactive compound
rapidly alkylates all nucleophiles - modifies DNA, RNA, and protein

Question 9

All alkylator SEs

Answer 9

myelosuppression, N/V, carcinogenic and teratogenic

Question 10

Two strategies have been employed in mechlorethamine to reduce reactivity and increase selectivity of nitrogen mustards

Answer 10

1. decrease nucleophilicity of nitrogen by adding aryl groups: chlorambucil, bendamustine, melphalan; this is because primary amines form aziridinium intermediates much faster, therefore they are more reactive than the ones with aromatic rings
2. prodrug strategy - cyclophoshamide

Question 11

Cyclophosphamide

Answer 11

most useful and commonly used alkylating agent
prodrug that requires hydroxylation by hepatic CYP450
the metabolite that forms is phosphoramide mustard, which cross-links DNA
metabolite is inactivated by aldehyde dehydrogenase –> elevated aldehyde dehydrogenase leads to reduced bone marrow toxicity

Question 12

Cyclophosphamide SEs

Answer 12

mild bone marrow toxicity
hemorrhagic cystitis - acrolein is the byproduct, which is toxic to bladder mucosa

Question 13

Ifosamide

Answer 13

has longer half life than cyclophosphamide but increased CNS toxicity

Question 14

Mesna

Answer 14

given with cyclophosphamide to decrease toxicity in bladder (block hemorrhagic cystitis)
mesna contains a charged anionic sulfonate group so it does not penetrate cells - anion transporters in proximal tubule excrete via kidney, mesna accumulates in urine and bladder
the free thiol on mesna reacts with and inactivates acrolein metabolites in urine

Question 15

Mitomycin C

Answer 15

aziridine containing natural product
functions as alkylating agent
myelosuppression is dose-limiting
can form bifunctional adducts (crosslinks) which make them good chemotherapies

Question 16

Platinum drugs

Answer 16

covalent crosslinkers, but not alkylating agents
cisplatin is original prototype square planar complex with leaving groups having cis geometry - 2 groups next to eachother crosslink

Question 17

Cisplatin

Answer 17

undergoes reversible hydrolysis in aqueous solution
equilibrium favors cisplatin in plasma (high Cl- conc) and equilibrium favors aquo form inside cell (low Cl- conc)
aquo form is highly reactive and potent electrophile
aquo form reacts rapidly with other nucleophiles, especially thiols (glutathione)

Question 18

Platinum crosslink geometry

Answer 18

aquo form reacts primarily at guanine N-7 and adenine N-7 in DNA
because of bond lengths and angles, cross-links are intrastrand - this imposes severe geometrical constraints on DNA –> introduces sharp bend in cross-linked strand, lesion not readily repaired by standard DNA repair enzymes

Question 19

Cisplatin highly effective agent for

Answer 19

many solid tumors
drug requires non-enzymatic conversion to active aquo form, aquo form produces intrastrand cross-links
some tumor cells more sensitive in G1 than in S phase

Question 20

Cisplatin SEs

Answer 20

dose-limiting nephrotoxicity
severe N/V
minimal bone marrow toxicity
peripheral toxicity
ototoxicity

Question 21

Drug resistance

Answer 21

increased expression of DNA repair enzymes
increased intracellular concentration of non-protein thiols, especially glutathione - free thiols have extremely high reactivity toward electrophilic intermediates
increased expression of cellular glutathione S-transferase (GST) - GST catalyzes reaction of glutathione with alkylating agents