PHARMACOLOGY OF CANCER THERAPY
Outline from PCTX 611, Power Point, Nov 1995

Copyright, Purdue Research Foundation, 1996

Overview

Brody, et. al 2nd ed. '94 Part VI
- Cytotoxic drugs
- Biological response modifiers
Tumor Biology
- Many sources
- Important source: Medical Oncology, 2nd Ed., Calabresi & Schein, et. al., McGraw Hill, 1993
- Shackney, et al. Chapter 4 in Calabresi & Schein, '93

Therapeutic Triangle

Patient - Pathogen Interaction

Patient Pathogen
- Defenses
- Location
- Circulation
Pathogen Patient
- Disease

Patient - Therapy

Patient Therapy
- Pharmacokinetic variables
- Recovery rates of tissues
Therapy Patient
- Toxicity
- Defense-enhancement

Therapy - Pathogen

Therapy Pathogen
- Effect on pathogen
  - none
  - kill
  - inhibit
  - stimulate
  - remove
Pathogen Therapy
- Resistance

Tumor Biology

The Cancer Rx Problem
Selectivity
Tumor Growth
Heterogeneity
Cell Kill Hypothesis
Resistance
Modalities

The Cancer Rx Problem

Selectivity
- Must kill ALL cancer cells
- Minimal harm to normal cells
- Differences mostly quantitative - based on rates
Change in sensitivity
- Cancer cells can change in sensitivity to drugs during therapy and /or
- Population of "sensitivities" can change

WHAT DOES IT MEAN TO SAY A PERSON IS CURED OF CANCER?

"CURE"
is when one kills ALL the cells

Importance of Growth Rate on Action of Cytotoxic Drugs

Do Tumors Grow at the Same Rate Throughout Their Life Span?

Clinical Tumor Growth Rate

Gompertzian Growth is Typical in Tumors

Doubling Time

Nt = number of cells (N0 = zero time)
k = fraction of cells dividing / unit time
Td = Doubling time (= Tc if cells are immortal)
Tc = Cell cycle time

Human Tumor Doubling Times (days)

Burkitt's lymphoma 2-5
Testicular cancer 21
Osteogenic sarcoma 34
Hodgkin's disease 38
Adenocarcinoma, colon 96
---------- " ---------, breast 129
---------- " ---------, lung 134

Impact of Growth Rate on Time to Detection

Heterogeneity in Tumors
from 3 types of sources

Pharmacokinetic
Kinetic
Genetic

Log Cell Kill
(after Skipper)

Each course of Rx kills SAME FRACTION of cells present. NUMBER changes
Example values of "Log Kill"
- Cell Fraction Fraction Log Log
  killed surviving surv kill
- 0.9 0.1 -1 1
- 0.99 0.01 -2 2
- 0.999 0.001 -3 3
- 0.999999999 0.000000001 -9 9

Log Kill & Host Survival

Heterogeneity -- Pharmacokinetic

Variation in Drug delivered to tumor cells
- Blood supply
- Necrosis
- Route

Heterogeneity -- Kinetic

Variation in Cell cycle time among cells
Cells not all in same cell cycle-phase

Drugs & The Cell Cycle

Growth Fraction is Important Determinant of Drug Effect

Growth Fraction Varies with Tumor

Thymidine labeling index (TLI)
- Expose cells to radio-isotopically labelled precursor, e.g., 1 h
- TLI = # labelled cells / total cells
Sample TLI
- Burkitt's lymphoma 0.29
- Squamous cell carcinoma (lung) 0.08
- Adenocarcinoma, colon 0.03
- Adenocarcinoma, breast 0.02

Kinetic Heterogeneity
Distribution of Tc Varies

With Kinetic Resistance, Response Decreases Immediately

Impact of Genetic Resistance Depends on When it Occurs

Impact of Resistance Depends on Its Origin

Resistance Types - Genetic

Defective activation
Increased inactivation
Altered nucleotide pools
Altered DNA repair
Altered target
Decreased accumulation
Decreased target
Gene amplification

Resistance Mechanisms - 1

Defective activation
- Cyclophosphamide requires metabolic activation
- Methotrexate conversion to more active MTX-polyglutamate in cells
Increased inactivation ( 2 of Brody mechs here)
- e.g., aldehyde dehydrogenase -- cyclophosphamide to inactive metabolite
- cell protective groups scavenge highly reactivte compounds -- sulfhydryl compounds -- glutathione, metallothionine

Resistance Mechanisms - 2

Altered nucleotide pools (not a Brody mech)
- Can occur with antimetabolites
Altered DNA repair
- Repair mechanisms increased, i.e., ability to remove cross-links
- Bleomycin and other DNA-directed drugs

Resistance Mechanisms - 3

Altered target
- Enzyme active, but less affinity for drug
- Methotrexate
  - Dihydrofolate reductase changes
Decreased target
- decreased topoisomerase II
- e.g., etoposide
Increased target -- gene amplification
- Methotrexate
  - Increase dihydrofolate reductase

Resistance Mechanisms - 4

Decreased Accumulation
- Decreased uptake
  - Methotrexate -- carrier protein decreases
  - Mechlorethamine -- decreased choline carrier
  - Melphalan / leucine transport
Increased Efflux

Resistance Due to
Decreased Accumulation,
i.e.,
Decreased Intracellular Concentration

Decreased transport into cell
Increased efflux
- Multidrug Resistance

Resistance due to Increased Efflux

usually alkaloids such as doxorubicin, etoposide, actinomycin D, vinca alkaloids
P-Glycoprotein (gP-170) in membrane, pumps drug out
Pump normally expressed in some cells, e.g., bone marrow stem cells
Some drugs can reverse resistance -- e.g., calcium channel blockers

Treatment Modalities

Adjuvant Therapy
Combination Therapy
Alternating Combination Therapy

Combination Therapy

Minimizes development of resistance
Same or greater cell kill with less toxicity
Drugs should have different limiting toxicities
Timing can be important
Example: MOPP for Hodgkins
- M: mechlorethamine; O: Vincristine (Oncovin); P: Procarbazine; P: Prednisolone

Toxicities Frequently Observed

Rapid growth rate tissues
- Bone marrow suppression
- GI toxicity (mucositis, stomatitis, etc.)
Other
- Anorexia and vomiting
- Hypersensitivity
See Brody, et. al., P 588-589 general and specific toxicities
See Calabresi et al, '93, Table 21-1

Toxicities Related to Specific Drugs

Renal
- Cisplatin, Methotrexate (high dose)
Hepatic
- Folic acid antagonists (e.g., methotrexate)
CNS
- Vincristine
Cardiac
- Doxorubicin

Toxicities -- Long Term

Types
- Oncogenesis
- Mutagenesis
- Teratogenesis
Typical Drugs
- Alkylating agents
- Antitumor antibiotics
Need for care in handling

Cytotoxic Drug Classes
After FORMER Textbook - Smith & Reynard

Cell cycle active agents (Antimetabolites)
Drugs that damage DNA
Antitumor antibiotics
Miscellaneous

Cytotoxic Drug Classes
Typical Categories

Akylating agents
Antimetabolites
Natural products
Miscellaneous agents

Cell Cycle-Active Agents

Methotrexate
Fluoropyrimidines
Cytarabine
Purine analogues (6-MP, 6-TG)
Hydroxyurea
Vinca alkaloids (vincristine, vinblastine)
Epipodophylotoxins
Taxol

Nucleic Acid Nomenclature

Base nucleoSIDE nucleoTIDE
Adenine (A) AdenoSINE AdenYLATE (AMP)
Guanine (G) GuanoSINE GuanYLATE (GMP)
Uracil (U) UriDINE UridYLATE (UMP)
Cytosine (C) CytiDINE CytidYLATE (CMP)
Thymine (T) DeoxythymiDINE DeoxythymidYLATE
(dTMP)
ETC.

BASE PAIRS

RNA
Purine (G & A) - Pyrimidine (C & U)
- G - C pair (3 H-bonds)
- A - U pair (2 H-bonds)
DNA
Purine (G& A) - Pyrimidine (C & T)
- G - C pair (3 H-bonds)
- A - T pair (2 H-bonds)

Antimetabolites --
Group Characteristics

Resemble NORMAL substrates
Most inhibit DNA synthesis, some inhibit RNA synthesis and/or function
Bone Marrow cell replication is profoundly inhibited
GI Toxicity great with some
Highly cell cycle specific
- Also "phase specific", e.g., S or M phase

Antimetabolites --
Major Members

Folic Acid Analogs
- *Methotrexate
Pyrimidine analogs
- *5-FU
- Cytarabine
Purine analogs
- 6-Mercaptopurine (6-MP)
- 6-Thioguanine (6-TG)

Methotrexate -- Structure

Methotrexate
Clinical Uses

Broad range
Well established
- ALL of childhood
- Choriocarcinoma
- Cancers of breast, bladder, and head & neck
Useful in treatment of
- non-Hodgkin's lymphomas

Methotrexate (MTX) MOA

Folic Acid Analogue
Carrier transport into cell
Binds strongly to DHFR to deplete THF
- Decreases 1-carbon transfers in Purine synth.
- Decreases [1-C-THF]intracellular which decreases dUMP dTMP
- Therefore, decreases NUCLEIC ACID synthesis
Forms polyglutamyl-MTX which traps MTX inside cell & inhibits thymidylate synthase

Methotrexate MOA's

Leucovorin Rescue

Methotrexate -- Resistance

Impaired carrier mediated transport into the cell
DHFR
- Decrease affinity for MTX
- Increased concentration, due to
  - Gene amplification

MTX - Clinical Pharmacology
Critical Determinants

Critical pharmacologic determinants of action
- Extracellular concentration
  - 10-8M inhibit thymidylate synthetase
  - 10-7M inhibit de novo purine synthesis
  - 10-6M for 48 hours may cause death!
- Duration of drug exposure
  - To maximize number of cells entering S-phase
- Many schedules to optimize these for particular tumors

MTX - Clinical Pharmacology
Absorption - Distribution

Used PO, IM, IV
Elimination half-lives Tri-phasic
- Distribution -- 45 min
- Renal clearance -- 2-3.5 h
- "Third space fluids" -- 10 h
  - If prolonged exposure to >10-8M, severe toxicity may result
"Third space fluids", e.g., CSF
- Slow influx and efflux may be a problem

MTX - Clinical Pharmacology
Elimination

Elimination (>90%) via kidneys unchanged via filtration and secretion
Two poorly soluble metabolites eliminated in urine
- With high dose Rx, may precipitate
Weak organic acids, e.g., salicylates, decrease secretion
CAUTION in renally compromised patients

Methotrexate Toxicity
Dose Limiting

Myelosuppression
- Thrombocytopenia and Leukopenia
  - Nadirs 7-10 days after Rx, Recovery 14-21 days
GI toxicity
- Oral mucositis is early sign of GI toxicity
- If elimination prolonged, may be
  - Severe mucositis
  - Small bowel ulceration & bleeding
  - Diarrhea requires cessation to prevent perforation of gut

Methotrexate Toxicity
Nephrotoxicity

Conventional doses, infrequent toxicity
High doses, toxicity can be severe
- Precipitation of metabolites
- Alkalinize urine & hydrate patient to dilute & increase solubility of metabolites
- Leucovorin (folinic acid, N5-THF)
Reversed within 3 weeks spontaneously if exposure not prolonged

MTX Toxicity - Other

Immunosuppression
Hepatotoxicity
- With chronic administration as in ALL of childhood and psoriasis Rx
- Changes typical of portal fibrosis and cirrhosis.
Listlessness, anorexia, nausea, vomiting
Intrathecal administration
- Acute meningeal irritation with attendant signs, some severe

Fluoropyrimidines
structure

Fluoropyrimidines
Clinical Use of 5-FU

Single agent
- Palliative in advanced colorectal carcinoma
Combination
- Breast cancer
- Carcinomas of ovary, stomach, pancreas
Sequential MTX + 5-FU
- Head and neck cancer

Fluoropyrimidines - MOA
5-Fluorouracil & 5-Fluorodeoxyuridine

Activated by conversion to nucleotide
5-FU Incorporated into RNA
- Interfere with RNA processing - ALL types
- May be most important MOA in slowly growing tumors
Both drugs inhibit DNA synthesis
- Inhibition of Thymidylate synthase
- Most important MOA in rapidly growing tumors (?)

5-FU: Incorporation into RNA

Impact of RNA(FU)

Alters synthesis and function of all species of RNA
On ribosomal function --
- Inhibits maturation of rRNA
- Interferes with formation & function of ribosomes
- causes miscoding during translation
Important mechanism of cytotoxicity
- Decreased protein synthesis
- Synthesis of abnormal proteins

5-FU: Inhibition of DNA Synthesis

THF-Derivatives Required for FdUMP Action on TS

5-FU: Activation to FdUMP & Incorporation into RNA

5FU: Relative Importance of Action on RNA vs. DNA

If Fluorodeoxyuridine (FUDR) administered, only affects DNA.
Inhibition of DNA synthesis only, if 5-FU activated directly to FdUMP, by thymidine phosphorylase/thymidine kinase pathway.
Otherwise, affects both RNA & DNA
Antitumor action correlates best with effect on Thymidylate Synthetase

5-FU: Biotransformation

Activation to active compounds
Inactivation to dihydrofluorouracil
- e.g., dihydrouracil dehydrogenase
  - rapid
  - high content in liver
  - Present in intestinal mucosa
  - Less in many GI tumor cells

Complexity of 5-FU Interactions

High doses of thymidine may enhance 5-FU toxicity by --
- increasing incorporation of FdUMP into DNA (Mechanism unknown)
- competing with 5-FU for inactivation, thus increasing the concentrationof 5-FU
Interactions with Methotrexate
Tumor differences in route of activation and, hence relative DNA / RNA action

MTX Enhancement of 5-FU Antitumor & Immunosuppressive Effects

Administer MTX first!
- Increases intracellular accumulation of 5-FU
- Increases intracellular pool of phosphoribosyl pyrophospate (PRPP) which increases formation of FUMP
  - Increases PRPP by inhibiting purine synthesis, i.e., by decreasing THF through action on DHFR.
  - Leucovorin may prevent the increase in PRPP. Illustrates complexity of interactions.

MTX - 5-FU Interactions

5-FU: Clinical Pharmacology

P.O. bioavailability: 1-15%
- Liver primary site of inactivation
Undetectable in plasma after 2 h
Administered --
- Once / week in bolus dose IV
- IV infusion daily for 5 days q 4-6 weeks
Produces effective concentrations --
- effusions, ascites, CSF

Fluoropyrimidines - Toxicity
Typical List of Major Toxicities

Dose limiting
- Bone marrow -- esp. with bolus admin.
  - Leukopenia & Thrombocytopenia
    - nadir 9-14 days after 5 days of Rx
    - recovery by day 21
- GI Toxicity -- esp. with infusion admin.
  - usually Stomatitis & Diarrhea 4-7 days after Rx

Fluoropyrimidine Toxicty: Effect of Route and Schedule

IV bolus
- myelosuppression is dominant
- Prolonged Rx, may cause megaloblastic anemia
Continuous IV Infusion
- Frequently produce, stomatitis, nausea, vomiting, and diarrhea
- Hepatotoxicity (elevated transaminases)
- myelosuppression less common

Fluropyrimidine Toxicity:
Effect of Peak 5-FU Concentration

Acute, reversible cerebellar syndrome
- somnolence, ataxia of trunk or extremities, unsteady gait, slurred speech, nystagmus
- Regimens that produce high peak concentrations in CSF

Fluoropyrimidine Toxicity -- Other

Hyperpigmentation of skin is frequent and may be accompanied by photosensitivity
Toxic effect of radiation to skin may be enhanced
Alopecia, acute and chronic conjunctivitis, and nail changes may be observed

Fluorodeoxyuridine (FUDR)

Substrate for thymidine kinase which converts it to FdUMP
Also converted to 5-FU
Primary use
- Patients with hepatic metastses by infusion into hepatic artery
- May have use in renal cell cancer via continuous infusion with adjustments for circadian rhythm

Cytosine Arabinoside
(Ara-C, Cytarabine) -- Structure

Cytarabine
Clinical Uses

Acute leukemias
- Frequently in combination with idarubacin or daunorubicin with or without 6-thioguanine
Non-Hodgkin's lymphomas
- as part of multi-agent regimens

Cytarabine
Activation and Site of Action

Cytarabine
MOA

Activated after entering by carrier
- deoxycytidine kinase has higher affinity for natural substrate 2'-deoxycytidine, but still rapid activation
Ara-CTP competitive inhibitor of DNA polymerase -- decreases DNA synthesis
- S-Phase specific
Ara-CTP incorporated into DNA & RNA
- Abnormal DNA & RNA produced

Cytarabine
Resistance -- Increased Degradation

Increased degradative enzymes
- Cytidine deaminase
  - High in plasma, granulocytes, RBCs, mucosa of GI tract, liver, spleen and lungs
  - Higher than activating enzyme in human leukemia cells
- Deoxycytidine monophosphate deaminase
  - Higher than activating enzyme in tumor tissues
- Effectiveness depends on ratio of activating to degradative enzymes

Cytarabine Resistance "Swamping" & Activation

Increased pool of dCTP
- Decreases competitive inhibition
Decreased activity or deleting of activating enzyme -- deoxycytidine kinase
Changed DNA polymerase with less affinity for Ara-CTP

Cytarabine
Clinical Pharmacology

Poorly absorbed PO
Short half-life
- 2nd phase: 13.1 to 111 min
More than 90% in urine w/in 24h as ara-U (inactive). Rest as Ara-C
Adm as infusion or q12h as bolus because of short half-life and S-phase specificity

Cytarabine Toxicity

Primarily to rapidly proliferating tissues
- Bone marrow (granulocytopenia & thrombocytopenia
  - Nadirs 7-14 days
  - May recover by 21 days, but
    - depends on previous exposure to cytotoxic drugs
- Megaloblastic anemia is common
- GI tract
  - Nausea, vomiting, and diarrhea

5-Azacytidine (5-AZA)

Must be enzymatically activated
5-azaCTP incorp into RNA & DNA
- S-phase specific
Causes hypomethylation of DNA which results in altered gene expression and morphological differentiation of cells
Myelosuppression & severe nausea, vomiting, and diarrhea
May activate expressed genes

Purine Analogs

Anticancer
- 6-Mercaptopurine (6-MP)
- 6-Thioguanine (6-TG)
Immunosuppressant
- Azathioprine
Treat hyperuricemia
- Allopurinol
  - Inhibits xanthine oxidase

6-Mercaptopurine (6-MP)
Structure

6-Mercaptopurine
Clinical Indications

Hypoxanthine analog
Maintenance Rx for childhood ALL
Rx of Crohn's disease, ulcerative colitis, and autoimmune diseases

6-Mercaptopurine - MOA

Activation to 6-thioinosine-5'-phosphate (T-IMP)
T-IMP accumulates because is poor substrate, therefore, causes pseudo-feedback inhibition of purine biosynthesis

6-Mercaptopurine
Clinical Pharmacology

Given orally, but absorption highly variable
6-MP biotransformed by xanthine oxidase, therefore, interacts with allopurinol which inhibits XO

6-Mercaptopurine
Toxicity

Bone Marrow and GI mucosa sites of toxicity.
- myelosuppression nadir 7-10 days; recovery by 14 days
- Common, but not severe, mucositis, nausea, & vomiting
Hepatotoxicity in 1/3 of patients
- within 2weeks up 2 yrs after initiate Rx
- Reversible, but should stop Rx if signs

6-Thioguanine (6-TG)
Structure

6-Thioguanine
Clinical Indications

Acute nonlymphocytic leukemia in combination with other drugs for induction and maintenance

6-Thioguanine
MOA

Activated to 6-thioguanosine-5'-phosphate (6-thio GMP)
- by HGPRT (hypoxanthine-guanine-phosphoribosyltransferase)
6-Thio-GMP converted to ...GDP and ...GTP by guanylate kinase
6-Thio-GTP incorp into RNA and DNA
Faulty DNA causes cytotoxicity

6-Thioguanine
Clinical Pharmacology & Toxicity

PO absorption, variable & incomplete
IV, half-life 80-90 min
40% eliminated in urine, rest biotransformed
Myelosuppression major toxic effect
Anorexia, nausea, vomiting & mucositis may occur
Less hepatotoxicity than with 6-MP

Adenosine Analogs
in Clinical Trials

2'-Deoxycoformycin (DCF)
- Hairy cell leukemia (HCL), T-cell acute lymphoblastic leukemia, mycosis fungoides, immunosuppressant
- Inhibits adenosine deaminase
2-chlorodeoxyadenosine
- cytotoxic for lymphocytes -- B-cell lymphocytic leukemia (B-CLL) and HCL
Fludarabine
- B-CLL, cutaneous T-cell lymphoma

Hydroxyurea [Hydrea]

Acts as an antimetabolite
- Inhibits ribonucleotide reductase which is important in DNA synthesis
- Cell-cycle stage specific in S phase
Used for chronic myelocytic leukemia

Hydroxyurea [Hydrea]

Major Toxicity
- Leukopenia, thrombocytopenia, and megaloblastosis, but recovery is rapid
Other
- Anorexia, mild nausea, vomiting, and stomatitis
Administered orally

Vinca Alkaloids

Vincristine sulfate(Oncovin)
Vinblastine sulfate (Velban)
From periwinkle plant (Vinca rosea L)
Drug of choice for childhood leukemias in combination with prednisone
Used for lymphoreticular neoplasms, carcinomas, and sarcomas

Vinca Alkaloids
MOA

Uptake by energy dependent carrier
Bind to tubulin in microtubules to cause their dissolution
- Colchicine-like arrest in metaphase
- Altered intracellular transport
Contrast to Taxol which stabilizes tubules
No cross resistance between vincristine and vinblastine

Vinca Alkaloids
Toxicity

Severe vesicant
Neurotoxicity

Vinca Alkaloids
Toxicity Neurotoxicity

Especially with vincristine
- This surprise may be due to slower elimination than vinblastine which is more lipid soluble
Mild sensory neuropathy with sensory impairment and paresthesia: Keep Rx
Severe paresthesias, loss of reflexes, ataxia, and muscle wasting: stop Rx
Constipation and abdominal pain - take laxatives

Vinca Alkaloids
Toxicity -- Miscellaneous

Severe tissue vesicants
- Must be careful of IV equipment to avoid slough
Less hematologic effects than many other cytotoxic drugs
Inappropriate ADH secretion
- Cause accumulation of fluid

Vinca Alkaloids
Clinical Pharmacology

Eliminated by biotransformation and biliary excretion
Used IV
Typically given q1wk or q2wk

Etoposide
An Epipodophylotoxin

(also, VP-16, VePesid)
Relatively new
Activity in human small cell lung carcinoma
Included in combination therapies

Etoposide
MOA

Binds to Topoisomerase II -- may be cytotoxic effect
- topoisomerase II catalyzes topological alterations in chromosomes that allow DNA replication, transcription, and repair
Protein-linked single strand breaks in DNA
Inhibits nucleoside transport into cells
Inhibits RNA and DNA synthesis
Acts primarily in G2, but also S and M

Etoposide
Clinical Pharmacology

Highly lipophilic, but still low concentration in CSF
Over half is eliminated by biotransformation and biliary excretion
Reduce dose in liver dysfunction
Half-lives are 2.8 and 15.1 h
Given PO or IV
Give 3 doses, q1d or q2d

Etoposide
Toxicity

Predominant is hematologic, especially leukopenia
- Nadir 2nd week, recovery by 3rd
Nausea & vomiting mild
Hair loss common
Hypotension on IV injection so must give slowly

Taxol
PACLITAXEL

From bark of West yew (Taxus brevifolia)
- Old growth forests of NW
Active in drug refractory ovarian and breast carcinomas

Taxol
MOA

Inhibits cell replication, esp. in late G2 and M phases
Binds best to beta subunit of tubulin
- In presence of microtubule associated proteins can polymerize tubulin into stable microtubules
- Won't allow depolymerization of tubulin for normal function
- Some cells become dependent on Taxol!

Drugs That Damage DNA

Alkylating agents
Nitrosoureas
Platinum compounds
Antibiotics
- Anthracyclines
  - Damage DNA, but S&R has Antibiotics as separate major category

Alkylating Agents

Developed from nitrogen mustard war gases of WW I which were highly reactive vesicants
First chemicals used for cancer Rx
Not cell cycle specific, but still more active in dividing tissues
"Radiomimetic" -- action on DNA resembles radiation

Alkylating Agents
General MOA

Strong electrophilic chemical reactions through formation of carbonium ion intermediates
Intermediates react with nucleophiles to form covalent bonds, e.g.,
- phosphate, amino, sulfhydryl, hydroxyl, carboxyl, and imidazole groups

Alkylating Agents
Consequences of MOA

Bi-functional agents
DNA-DNA strand and DNA-Protein cross-links
- especially by reacting with 7-Nitrogen of guanine
Misreading of genetic code
- Alkylation of guanine may shift electon configuration so it forms GT rather than GC pairs
DNA Chain breaks - labilized imidazole

Alkylating Agents
General Toxicities of Group

More toxic to bone marrow and gut than to liver and kidney, etc.
Infertility to both males and females
Mutagenic
Carcinogenic

Alkylating Agents
Antitumor Activity

Wide spectrum
Lymphoreticular tissue tumors
Limited activity against sarcomas

Alkylating Agents
Tumor Resistance

Develops slowly & may require several genetic / biochemical changes
Increased intracellular nucleophiles
- metallothionein, glutathione
Increased DNA repair systems
Reduced drug accumulation
Resistance not necessarily uniform across group

Nitrogen Mustards

Examples
- Mechlorethamine
- Cyclophosphamide (Cytoxan)
- Melphalan
- Chlorambucil

Nitrogen Mustards
Structures

Nitrogen Mustards
Major differences

Pharmacokinetic
- Mechlorethamine
  - highly unstable
  - must be given IV
  - half-life in minutes
- Cyclophosphamide
  - can be given orally
  - must be activated in liver
    - e.g., melphalan / leucine transport; mechlorecthamine / choline transport

Nitrogen Mustards
Similar toxicities

Especially toxic to lymphocytes & bone marrow cells
Within hours of Rx dose
- mitosis stopped
- formed elements disintegrate
Immunosuppression is profound
GI toxicity less than some other groups

Mechlorethamine
(Nitrogen Mustard) Structure

Mechlorethamine HCl
Nitrogen mustard, Mustargen

Major use in '90s is Hodgkin's Disease
Severe vesicant
First chemical agent to be used successfully in clinical oncology
Must give IV
Toxicity
- Myelosuppression
- More GI toxicity including nausea and vomiting than most alkylating agents

Cyclosphosphamide
Structure

Cyclosphosphamide
Clinical Applications

Most widely used alkylating agent, in part due to availability of oral route
Active lymphoproliferative diseases, e.g.,
- Hodgkin's disease
- Chronic lymphocytic leukemia
Significant activity vs
- multiple myeloma
- ovarian, breast, small cell lung carcinoma
Many combinations

Cyclophosphamide
MOA

Hepatic cytochrome P-450 system, enzymes phosphatase and phosphamidase are primary activators (hydrolyze P-N bond) to intermediate, aldophosphamide, which nonenzymatically breaks down to --

Phosphoramide mustard (bifunctional)
Acrolein

Cyclophosphamide
Clinical Pharmacology

Oral bioavailability = 90-100%
Not vesicant
IV injection no local irritation
Half-life --
- cyclophosphamide -- 3-10 h
- aldophosphamide -- 1.6 h
- phosphoramide mustard -- 8.7 h
Most metabolized-- < 14% unchanged in urine

Cyclophosphamide
Toxicity

Bone marrow suppression most imp.
- leukopenia
- thrombocytopenia
Nausea and vomiting said to be rare
Other toxicity similar to group except
- Hemorrhagic cystitis

Cyclophosphamide
Sterile necrotizing hemorrhagic cystitis

Acrolein is probable cause
Associated with chronic administration
Cause for stopping Rx
Minimize
- high water intake and take in AM
Rx
- Bladder irrigationwith thiol compounds
- Systemic Rx with N-acetylcysteine and mesna

Cyclophosphamide
Mesna for Px & Rx of Cystitis

Used prophylactically and therapeutically
Mesna (2-mercaptoethansulfonate, Mesonex)
injectable as inert disulfide which is reduced in renal tubules to mesna
Mesna binds to and detoxifies urotoxc metabolites of cyclophosphamide and Ifosfamide

Ifosfamide

Structural analog of cyclophosphamide
Must be activated in liver, but activation is slower than with cyclophosphamide
More toxic to bladder than cyclophosphamide, so not first line drug

Dacarbazine
DTIC, DTIC-Dome, DIC

Must be activated --
- by hepatic cytochrome P-450 dependent N-demethylation
- Alkylating agent and 5-aminoimidazole-4-carboxamide (AIC) release from activated moiety in target cells.
- Effects on RNA and protein synthesis inhibited more than DNA synthesis
Best when used in combo
Toxicity like other alkylating agents

Nitrosoureas

Methylnitrosoureas
- Streptozotocin
2-Chloroethylnitrosoureas
- Carmustine (BCNU)
- Lomustine (CCNU)
- Semustine (Methyl-CCNU)
Lipophilic
Chemically unstable

Nitrosoureas
Clinical Indications

Streptozotocin -- Pancreas
- functional and nonfunctional islet cell tumors
Carmustine (BCNU)
- Hodgkin's disease, non-Hodgkin's lymphomas, multiple myeloma
- Brain tumors -- primary and metastatic

Nitrosoureas
MOA

Enter cells by diffusion
Alkylation
Carbamoylation
Inhibition of repair of x-ray-induced DNA strand breaks
Interference with synthesis and processing of ribosomal and nucleoplasmic RNA
Inhibition of DNA polymerase II

Carmustine (BCNU)
Toxicity

Delayed myelosuppression
- leukocyte & thrombocyte nadir 4-6 WEEKS
- Bone marrow toxicity, cumulative and dose limiting
Hepatotoxicity in upto 26% of patients
- Increased alkaline phosphatase & transaminases
Pulmonary fibrosis (may be subclinical in all patients)

Carmustine (BCNU)
Toxicity - 2

Renal in many at high doses (>300 mg/m2)
- Radiation nephritis-like
- glomerulosclerosis, tubular loss, interstitial fibrosis, thickening of basement membranes
- Cumulative dose >1200 mg/m2 should be carefully monitored
Mutagenic & carcinogenic
- Acute myelocytic leukemia

Platinum Compounds

Cisplatin (cis-DDP)
Clinical Indications

Effective in --
- Testicular tumors
- Ovarian carcinoma
- Bladder tumors
- Head and Neck
- Breast cancer
- Lung cancer
- refractory non-Hodgkins lymphomas
Radiosensitizer -- optimal use of this ???

Cisplatin (cis-DDP)
MOA

Enters cells by passive diffusion
Aquated Cl- moieties react with N7 of guanine and other nucleophiles
Intrastrand (rapidly) and Interstrand (slowly) crosslinks in DNA
DNA - protein crosslinks
Cell-cycle nonspecific, but may be most active in G1

Cisplatin
Clinical Pharmacology

Non-protein bound cisplatin (10%) is active form in blood
Half-lives
- 25-49 min (distribution) and 59-73 h (elim)
Not cross blood-brain barrier
Highest concentrations in KIDNEY, head (not brain), liver, intestines
Urinary excretion initially rapid (toxic!)

Cisplatin
Toxicity

Nephrotoxicity is dose-limiting
Intractable, almost universal, nausea and vomiting
Anaphylactic-like reactions
Neurotoxicity
Myelosuppression
- mild compared to alkylating agents
- thrombocytopenia (by day 7-9)
- granulocytopenia (by day 17-19)

Cisplatin
Nephrotoxicity

Dose related , cumulative, & irreversible
Primarily affects proximal tubuls
- tubular degeneration & loss of brush border
- necrosis and mineralization of tubular epithelial cells
Minimize
- Hypersalination - forced infusion of hypertonic saline
- Pre-Rx with thiol compounds may help (diethyldthiocarbamate (DDTC) or thiol sulfate)

Carboplatin

Organic analog of cisplatin
Used for ovarian tumors
Same MOA and cross-resistant with cisplatin
More bone marrow toxicity and less nephrotoxicity than cisplatin

Antitumor Antibiotics

Anthracyclines
- Doxorubicin
- Daunorubicin
- Mitoxantrone
Bleomycin
Dactinomycin (actinomycin D)
Mitomycin C
Plycamycin (Mithramycin)