Chabner Chemotherapy Flashcards
Define the fractional cell kill hyopthesis
= a given drug concentration applied for a defined time period will kill a constant fraction of the tumor population, independent of the absolute number of cells but…
- regrowth of the tumor occurs during the drug-free interval between treatments
- objectives of using cycles of therapy was to progressively reduce the tumor burden to zero (or at least <1) through multiple fractions of cell kill
Limitations of fractional cell kill hypothesis in the real world
- assumed there is a uniform cell growth rate and uniform drug sensitivity:
a. many tumors become clinically apparent at the stage of decelerating growth–>tumor vascularity is NOT uniform and NOT adequate to provide oxygen and nutrients to the bulk of the tumor
b. large tumors contain a significant fraction of slowly dividing or non-dividing cells - assumed that the tumor population is biologically uniform
a. reality: there is a diverse population of tumor cells subjected to selective pressure of drug treatment–>the drug sensitive cells will die but the drug-resistant cells will survive - did not account for the existance of tumor stem cells (= non-dividing cells that possess multi-drug resistant and radiation-resistant properties); therefore, failure of therapy may be the result of the persistence of these stem cells after destruction of the more drug-sensitive populations
Rationale for combination chemotherapy protocols
- addresses intratumor heterogeneity within a given tumor wrt a. intrinsic resistance and b. phase of cell cycle
- addresses intertumor heterogeneity between patients with the same tumor type d/t genetic variation among tumor cells
- takes advantage of synergistic effects
- decreases rate of development of resistance
Explain the enhanced results that combination chemotherapy yields
independent drug actions result in statistically independent chances of achieving a certain log kill as long as there is no cross resistance (i.e. two chances for remission are better than one in the absence of cross-resistance); rarely synergistic effect
What are the considerations required to develop a multidrug/combination therapy protocol for the treatment of cancer
- responsiveness of the pathologic and molecular type of tumor to specific drugs (heterogeneous drug responses within a specific tumor d/t: a. clonal evolution and molecular diversity, b. diversity of resistance mechanisms and secondary driver mutations)
- MOA (biochemical mechanisms of cytotoxicity of each drug)
- drug cross-resistance patterns
- potential drug interactions affecting pharmacokinetics, toxicity, or response
Mechanisms of drug resistance (general)
- increased efflux (MDR1/PGP and multidrug resistance proteins)
- decreased drug uptake
- decreased drug activation
- increased drug target
- absent or mutated drug target
- enhanced DNA repair
- defective recognition of DNA adducts
- defective checkpoint function and apoptosis
- splice variants (hormonal therapy MOR)
- activation of cell survival pathway (hormonal therapy MOR)
- activation of an alternative pathway (molecularly targeted drugs MOR)
MDR1/PGP substrates
- vinca alkaloids
- anthracyclines
- taxanes
- actinomycin D
- epipodophyllotoxins
- small molecule inhibitors (e.g. tyrosine kinase inhibitors)
Mechanisms for reversing resistance via MDR1/PGP
- calcium-channel blockers
- steorid hormones
- cyclosporine analogues
multidrug resistance proteins (MRPs) substrates
- anthracyclines
- etoposide
- taxanes
- vinca alkaloids
- methotrexate
- 6-mercaptopurine
- captothecin
Considerations when developing a combined chemotherapy/RT protocol for cancer
- normal tissue at risk of toxicity
- sequence of administration (chemo then RT vs. RT then chemo)
- cumulative toxicity when administered concurrently
- synergistic interactions (i.e. some chemotherapy can potentiate the effects of RT)
- both chemo and RT are carcinogenic (secondary leukemias with use of alkylating agents; secondary malignancy in the RT field)
Veterinary examples of mechanisms of resistance to different cancer drug therapies
…
Process of identifying a druggable target
- statistical algorithm for highly suspect and recurrent mutations associated with specific subsets of cancer
- once a suspected driver mutation is found recurrently in tumor specimens, that target needs to be validated (should be essential to growth and survival and established by studies of cell lines in which the oncogene activity can be knocked out by siRNAs or by CRISPR
Process of identifying a lead compound
- assays for high throughput screening
- move the compound to in vitro testing to confirm identity, activity, target engagement, and demonstrate a dose-response relationship in a cell-based assay, demonstrate lack of toxicity for cells not expressing the target
- further chemical refinement/modification to increase potentcy and specificity and to incorporate favorable drug like properties
Information gained from in vitro screening process
- confirmation of hits
- definition of lead structures
- structural optimization
standard FDA/EMA approval process
https: //www.fda.gov/patients/drug-development-process/step-3-clinical-research
1. basic research
2. design and discovery
3. preclinical development (apply for Investigational New Drug status)
4. clinical development phase (1, 2, 3)
5. FDA filing and approval
accelerated FDA/EMA approval process
- basic research
- design and discovery
expedited phases (if data shows superior effectiveness or improved effect on serious outcomes for an unmet medical need) - preclinical development
- clinical development phases 1, 2, 3
- FDA fast track filing and approval
What should be included in an IRB consent form and the legalities associated with the consent form
https://www.hhs.gov/ohrp/regulations-and-policy/guidance/faq/informed-consent/index.html#:~:text=For%20the%20consent%20or%20parental,subject’s%20legally%20authorized%20representative%20or
Phase 1 clinical trial
- study participants
- length of study
- purpose
- outcome
- study participants: usually <30 (20-100 healthy and diseased volunteers); *but if new drug is intended for cancer patients, phase 1 will be in patients with that type of cancer
- length of study: several months
- purpose: safety and dosage
a. how the drug works in the body
b. side effects associated with increased dosage (finding a safe dose)
c. early information about efficacy to determine how best to administer it
d. determine how the drug should be given - outcome: 70% of drugs pass phase 1 clinical trials
Phase 2 clinical trial
- study participants
- length of study
- purpose
- outcome
- study participants: usually 100 or
Phase 3 clinical trial
- study participants
- length of study
- purpose
- outcome
aka pivotal studies
- study participants: 300-3,000
- length of study: 1-4 years
- purpose: efficacy and monitoring of adverse reactions
a. compare new treatment to current treatment to see which is better
b. provide the most safety data - outcome: 25-30% move to the next phase
Phase 4 clinical trial
- study participants
- purpose
carried out AFTER the drug has been approved by the FDA during the post-market safety monitoring phase
- study participants: several thousand volunteers who have the disease/condition
- purpose: safety and efficacy
Phase 1 clinical trial designs
- Fibonacci method of dose escalation: the previous dose is escalated 100%, 67%, 50%, 40%, and then 33% of the previous dose
- 3x3 cohort design
Factors influencing drug bioavailability
- properties of the drug
- route of administration
- dose
- integrity of GIT lining
- previous treatments with chemotherapeutics
Properties of the drug that influence bioavailability
- solubility (dissolution of the drug in the GIT)
- absorption (transport of dissolved drug)
- drug stability and release of drug from dosage form
- degree of first pass metabolism (GI bacteria and liver)
Factors influencing absorption
- molecular size
- lipid solubility
- presence of a transport system
- physiologic state of the intestines (determined by previous disease, previous treatments)
- vomiting post-administration
Bioavailability equation
F(route) = AUC(route) / AUC (IV)
Concentration equation
concentration (C) = mass/volume
Bioavailability
= the fraction of the amt of a dose administered via a certain route that reaches systemic circulation (AUCroute) compared to giving that same dose IV (which = 100% of dose going into plasma)
Potential problem of low bioavailability with oral dosing
Could possibly require administration of a toxic amt of that drug
ex. verdinexor
Volume of distribution equation
Vd = total mass absorbed/concentration in plasma
Volume of distribution
= the volume into which it appears that a certain dose of drug is distributed throughout the body
* tells us how much of a drug we need to get a desired plasma concentration (which is going to determine the AUC and efficacy of the drug)
Large Vd
= drug is distributed greatly into intravascular space, so it’s concentration in plasma will be low (appears the drug has distributed into a large volume)–>need to give a HIGHER dose of the drug
Small Vd
= drug is NOT distributed into a lot of other areas, so its concentration plasma remains high–>need to give a LOWER dose of the drug to achieve a certain plasma concentration (efficacy)
Vd and dose equation
mass(absorbed) = Cp x Vd mass(absorbed) = dose required = target plasma concentration x Vd
Factors that influence Vd
- size (molecular weight)
- degree of plasma protein binding vs. extravascular protein binding
- charge
- patient status (hypoproteinemia, hypocalcemia, increased vascular permeability, pH)
Influence of size/molecular weight on Vd
high MW = stays in plasma = low Vd = requires a LOWER dose
low MW = goes into extravascular space = high Vd = requires HIGHER dose
Influence of degree of plasma protein binding
high plasma protein binding = low Vd
low plasma protein binding = high Vd
Influence of charge on Vd
lipophilic = moves easily to extravascular space = high Vd
hydrophilic (charged) = stays in plasma = low Vd
Influence of hypoproteinemia on Vd
hypoproteinemia/albuminemia–>decreased plasma protein binding–>more drug moves into extravascular space–>higher Vd–>low Cp (plasma concentration)–>decreased drug activity
Influence of hypocalcemia on Vd
hypocalcemia–>decreased binding of drug to calcium in the plasma–>increased movement to extravascular space–>higher Vd
Influence of increased vascular permeability on Vd
(e.g. septic shock, vasculitis, immune-mediated disease)–>more moves into extravascular space–>higher Vd
First pass metabolism
= metabolism of the drug BEFORE reaching systemic circulation (by the GI microbiota or by the liver)
Effect of high first pass metabolism
- LOW bioavailability (F)
- LOW plasma concentration
- longer Tmax
- longer Cmax
compared to drugs with low degree of first pass metabolism or ones administered IV
Chemotherapeutics that are prodrugs that must undergo first pass metabolism to be ACTIVATED by the liver
- Tanovea
2. cyclophosphamide
Possible outcomes of drug metabolism
- activation of the drug
- inactivation of the drug
- formation of a toxic compound vs. detoxification
- conversion of lipid soluble drugs to water soluble drugs–>more easily excreted
Phase 1 metabolism:
- purpose
- major location
- enzymes
- reactions
- purpose: to unmask or introduce polar groups onto a drug to make it more water soluble and to create targets for larger molecules (of phase II metabolism) to act
- major location: liver
- enzymes:
1. cytochrome p450 (most common CYP3A4, CYP2D6)
2. alcohol dehydrogenase - reactions of phase 1 metabolism
1. oxidation
2. hydrolysis
3. hydroxylation
example: drug + O2 + NADPH–>drug-O + H2O + NADP+
NADPH
- part of the pentose phosphate shunt/pathway; byproduct of the reaction: glucose-6-phosphate–>6-phosphogluconate
- reducing agent (can reduce other molecules)
reduction/oxidation OIL
OIL = oxygen = loss of electron reduction = gain of electron
Phase II metabolism
- purpose: to transfer small polar molecules onto a drug to make it more soluble
- enzymes = transferases (= conjugation enzymes that attach something to make a drug more soluble)
1. UDP-glucuronosyltransferase
2. N-acetyltransferase
3. glutathione-S-transferase
4. sulfotransferasae - reactions
1. glucuronidation - MOST COMMON
2. glutathione conjugation
3. acetylation
4. sulfation
First order kinetics
- increase plasma drug concentration–>increase rate of metabolism
- rate of metabolism is proportional to drug concentration = metabolizing a constant proportion of the drug per unit time
- half-life is constant
- happens with most drugs at most dosages
- first order elimination
First order elimination
= constant proportion of the drug is eliminated per time (t1/2 is constant)
Zero order kinetics
- increase plasma drug concentration–>NO increase in rate of drug metabolism
- rate of metabolism becomes independent of drug concentration
- rate of drug metabolism is constant (Vmax)
- can result in toxicity
- zero order elimination
Zero order elimination
= constant amt of drug is eliminated per time
Relationship of rate of metabolism and half life
t1/2 is inversely proportional to the rate of drug metabolism (i.e. faster drug metabolism = shorter t1/2)
- 1st order kinetics: the proportion of a drug metabolized over time is CONSTANT (Ke = 1st order elimination rate constant)
- 4.5 half lives to get rid of 95% of a drug
Equation for rate of elimination
rate of elimination = clearance x concentration
therefore, the rate of elimination of a drug is proportional to the drug’s
1. clearance (increased clearance = more blood is filtered of the drug per unit time)
2. concentration (higher concentration = faster elimination)
Pharmacokinetics
= the effect the drug has on the body 1. liberation (the process of release of a drug from its pharmaceutical formulation) 2. absorption 3. distribution 4. metabolism 5. elimination ADME
Area under the curve (AUC)
= total drug exposure; concentration (y) x time (x)
Biologic half-life (terminal half-life)
= time required for plasma concentration to decrease by 50%
Cmax
maximum drug concentration after administration
Tmax
time to peak drug concentration after administration (time to Cmax)
Chemotherapies to dose reduce with liver dysfunction
- doxorubicin
2. vincristine
Chemotherapies to dose reduce with renal dysfunction
- carboplatin
- cyclophosphamide
- melphalan
- methotrexate (reduce dose in proportion to creatinine clearance)
Chemotherapies metabolized by the liver
- cyclophosphamide (activated - so reduced efffectiveness with liver dysfunction)
- cytarabine (degraded by cytidine/deoxycyctidine deaminases in the liver to ara-U)
- gemcitabine (deaminated to uracil metabolite difluorodeoxyuradine)
Synonymous genetic variants
located in the open reading frame; do NOT alter the amino acid sequence of the translated protein
Non-synonymous variants
result in a change to the amino acid encoded; generally found in the open reading frame of a gene:
- missense mutation
- nonsense mutation
- insertion/deletion (aka indels)
- splice site
Missense mutation
single base change; causes a codon to encode a different amino acid
Nonsense mutation
single base change; introduces a premature stop codon into the reading frame
Insertion/deletion
add or delete entire codon from the reading frame or alter downstream codons
Broad categories of the effects of genetic variants on drug efficacy and toxicity
- altered drug metabolism
- increased/decreased clearance
- increased/decreased activation
- increased/decreased inactivation - altered drug target
- structural change–>cannot bind effectively
- increased drug target - altered cellular pathways
- decreased p53–>decreased apoptosis
- decreased DNA repair enzymes–>cannot sense DNA damage–>cannot perform apoptosis - altered drug transport
- cannot bring drug into or keep drug in cell–>decreased effect
- cannot get drug out of cell–>increased toxicity
Features of the blood brain barrier
- thick basal lamina of the blood vessels
- lack of intracellular fenestrations within capillary walls
- lack of pinocytotic vesicles
- low hydraulic conductance
- low ionic permeability
- high electrical resistance
- active efflux proteins along luminal surface of capillary endothelial cells (include p-glycoprotein, multidrug resistance-associated proteins (MRPs), breast cancer resistance proteins (BCRP)
Implications of the BBB on drug delivery
inability for hydrophilic/water-soluble non-electrolytes to easily cross without a membrane transport protein
Features of molecules that are able to cross the BBB
- lipid soluble
- molecular weight <200
- neutral charge at physiologic pH
- not highly bound to plasma protein
List of antimetabolites
- methotrexate
- 5-FU
- cytidine analogs
a. cytosine arabinoside (ara-C)
b. gemcitabine - purine analogs
a. hydroxyurea
Methotrexate class
antimetabolite (antifolate)
Methotrexate cell cycle specificity
S phase
MTX MOA
- inhibit DHFR–>decreased reduced folates–>decrease purine synthesis and thymidine synthesis
- inhibit TS–>decrease thymidine–>decrease DNA synthesis
- inhibit AICART–>decrease IMP–>decrease purine–>decrease DNA and RNA synthesis
MTX mechanisms of uptake
- reduced folate carrier (RFC) system
- folate receptor (FR) system
- proton coupled transporter
MTX active compound
MTX polyglutamate (PG)
MTX inactivation
7-hydroxylation in the liver to inactive 7-OH-MTX form–>excreted in bile
MTX elimination
renal, intact
MTX drug interactions
- toxicity to normal tissues rescued by leucovorin calcium
- L-asparaginase blocks toxicity and antitumor effect
- pretreatment with MTX increases 5-FU and cytosar nt formation
- NSAIDs decrease renal clearance and increase toxicity
MTX toxicities
- myelosuppression
- mucositis, GI denudation
- renal tubular obstruction and injury
- hepatotoxicity (enzyme elevations)
- pneumonitis
- hypersensitivity reactions
- neurotoxicity (sz, altered mental status)
MTX mechanisms of resistance
- decreased uptake
- mutated reduced folate carrier (RFC1) - altered target
- increased TS
- mutated FPGS
+ many more
5-FU class
antimetabolite (fluoropyrimidine)
5FU cell cycle specificity
S phase (specifically increases at G1/S interface)
5FU MOA
- 5-FdUMP inhibits TS–>depletion of thymine–>inhibits DNA synthesis
- 5-FUTP incorporates into RNA–>stops RNA synthesis
- incorporation of fluorouridine and Urd nucleotides into DNA–>DNA repair strand breaks–>apoptosis
Rate limiting step of 5FU breakdown
dihydropyrimidine dehydrogenase (DPD)
