Oncology-Anti-Microtubule Agents

I. Core Concept:

Anti-microtubule agents, also known as "spindle poisons," are a class of cancer drugs that target tubulin, a protein forming microtubules crucial for cell division. By disrupting microtubule function, these agents prevent cancer cell proliferation. This makes tubulin a key target for anti-cancer drug development.

II. Key Players:

• Tubulin: The protein building block of microtubules. Mutations affecting tubulin can lead to drug resistance.
• Microtubules: Cellular structures essential for cell division and other vital processes. Disruption of microtubules leads to cell death.
• Taxanes (Paclitaxel, Docetaxel): A significant advancement in anti-cancer chemotherapy, showing considerable success in the 1990s. Current research focuses on improving their delivery and mitigating side effects.
• Vinca Alkaloids (Vincristine, Vinblastine, Vindesine, Vinorelbine): Another class of anti-microtubule agents. They differ in their specific mechanisms and clinical applications.

III. Mechanism of Action:

Anti-microtubule agents work by binding to tubulin, thereby:

• Destabilizing polymerized tubulin: Preventing the formation of functional microtubules (e.g., Vinca Alkaloids).
• Stabilizing polymerized tubulin: Preventing microtubule depolymerization, leading to dysfunctional microtubules (e.g., Taxanes).

IV. Clinical Significance

I. Microtubule-Targeting Agents: Mechanisms of Action

These drugs exert their anti-cancer effects by interfering with microtubule dynamics, essential for cell division and function.

• Paclitaxel (P) and Docetaxel (D): Microtubule Stabilizers: They bind to microtubules, preventing their depolymerization (disassembly). This leads to cell cycle arrest and ultimately apoptosis (programmed cell death). Additional mechanisms include anti-angiogenesis (blocking blood vessel formation to tumors), disruption of Ki-Ras function (a cancer-promoting protein), and apoptosis induction through bcl-2 phosphorylation.
• Estramustine phosphate (ep): Microtubule Destabilizer: Unlike P and D, ep binds to microtubule-associated proteins, promoting microtubule disassembly. This also disrupts cell division.

II. Drug Specifics

III. Key Concepts & Considerations:

• Dose and Schedule Dependency: The toxicity profiles of these drugs are significantly affected by the dosage and administration schedule. Weekly schedules are under investigation for both P and D.
• Resistance Mechanisms: Resistance to these drugs can develop through various mechanisms, including alterations in the expression or structure of β-tubulin (the protein that forms microtubules), overexpression of proteins like P-glycoprotein (Pgp), which pumps drugs out of cells, and mutations in genes like p53.
• Combination Therapy: These drugs are often used in combination with other chemotherapeutic agents (e.g., cisplatin, carboplatin, doxorubicin, etoposide) to enhance efficacy and overcome resistance.
• Pharmacokinetics: Understanding how each drug is metabolized (primarily hepatic for P and D) and its half-life is essential for determining appropriate dosage regimens and managing toxicity.

Oncology-Topoisomerase Inhibitors

I. Topoisomerases: The Basics

• Function: Topoisomerases are nuclear proteins crucial for managing DNA topology (shape and structure). They regulate DNA supercoiling, a process essential for replication, transcription, and recombination.
• Types: Eukaryotes possess two main types:
  • Topoisomerase I (Topo I): Cleaves and rejoins one strand of DNA, relaxing supercoils.
  • Topoisomerase II (Topo II): Cleaves both strands of DNA, allowing another DNA duplex to pass through the break.

II. Topoisomerase I Inhibitors

• Camptothecin (CPT) and Derivatives: CPT, derived from the Camptotheca acuminata tree, stabilizes the covalent complex between Topo I and DNA, preventing relegation and leading to DNA damage.
  • Limitations of CPT: High toxicity initially hindered its widespread use.
  • Licensed Derivatives: Irinotecan (CPT-11) and Topotecan are currently used clinically. They differ in administration schedules and toxicity profiles.
• Side Effects (common to both Irinotecan and Topotecan): Neutropenia, diarrhea (early or late onset), thrombocytopenia, anemia, alopecia, nausea, and vomiting.
• Clinical Pharmacology:
  • Absorption and Distribution: Both are administered intravenously (IV), and Topotecan can also be absorbed orally (30-50% bioavailability). Both distribute widely throughout the body, including the cerebrospinal fluid (CSF).
  • Metabolism and Excretion: Topotecan undergoes minimal metabolism and is primarily renally excreted. Its clearance is directly related to creatinine clearance. Irinotecan (CPT-11) is a prodrug; it's converted to the active form, SN-38, by carboxylesterases. SN-38 is eliminated via glucuronidation and biliary excretion. Liver dysfunction necessitates dose adjustments.

III. Topoisomerase II Inhibitors

• Etoposide and Teniposide: These drugs inhibit Topo II's ability to rejoin cleaved DNA, resulting in:
  • High levels of double-stranded DNA breaks.
  • Increased mutations.
  • Illegitimate recombination.
  • Apoptosis (programmed cell death).
• Pharmaceutical Properties: Poor water solubility necessitates formulation with excipients (e.g., polysorbate for etoposide, Cremophor EL for teniposide). Etoposide can be given orally or IV; teniposide is IV only.
• Clinical Use: Widely used in adult and pediatric cancers. Etoposide is more commonly used in first-line treatments, especially for small-cell lung cancer (SCLC) and germ cell tumors. Teniposide is not licensed in the UK.
• Toxicity: Similar toxicity profiles for both, including neutropenia, alopecia, mucositis, infusion-related blood pressure changes, and hypersensitivity reactions.
• Clinical Pharmacology:
  • Absorption: Etoposide absorption is non-linear, with reduced bioavailability at higher doses.
  • Protein Binding: Both are highly protein-bound; low albumin levels increase free drug concentration and toxicity.
  • Metabolism and Excretion: Both are extensively metabolized, with etoposide exhibiting faster elimination than teniposide. Etoposide clearance shows a linear relationship with creatinine clearance.

IV. Key Differences Summarized:

Oncology-Chemotherapy: Cisplatin and its Analogues -

I. Cisplatin

A. Mechanism of Action:

• Cisplatin directly binds to DNA, forming intra-strand and inter-strand cross-links.
• This inhibits DNA synthesis by altering the DNA template. It acts as a bifunctional alkylating agent.
• Cytotoxic effects are cell cycle-independent.
• Synergistic effects with anti-metabolites are observed (mechanism not fully understood, possibly due to DNA repair malfunction).

B. Side Effects:

• Highly emetogenic (causes vomiting).
• Dose-dependent nephrotoxicity (kidney damage).
• Peripheral neuropathy (nerve damage).
• Ototoxicity (hearing damage, tinnitus).
• Can cause anaemia (low red blood cell count).
• Relatively low toxicity to white blood cells and platelets.

C. Dosage:

• Standard dose limit: 100 mg/m² as a single daily dose.
• Higher doses used in clinical trials, often with neuroprotective agents.
• Alternative schedules exist (e.g., five daily injections of 20 mg/m² for teratoma).
• Dosage based on empirical body surface area calculations; no clear pharmacokinetic/pharmacodynamic relationship.
• Clearance is rapid initially, then slower due to plasma protein binding. Prolonged in renal insufficiency.

D. Clinical Indications:

• Major advancement in testicular cancer treatment (80%+ complete response in metastatic disease).
• Used in ovarian cancer, genitourinary tumors, squamous carcinomas (head and neck, non-small-cell lung cancer).
• Frequently used in combination with other cytotoxic agents for various solid cancers and pediatric tumors.

II. Carboplatin

A. Overview:

• Cisplatin analogue with reduced toxicity compared to cisplatin.
• Clinically viable alternative to cisplatin in most situations, except for germ cell tumors where cisplatin may be preferred.
• Cross-resistance with cisplatin: patients resistant to one will likely be resistant to the other.

B. Side Effects:

• Significant: Thrombocytopenia (low platelet count), leucopenia (low white blood cell count) – both worse around day 14.
• Less Significant: Renal toxicity, neurological toxicity, otological toxicity, nausea and vomiting (occasional), mild alopecia (hair loss), rare visual disturbances, allergy (2% incidence).

C. Dosage:

• Initial dosage based on body surface area led to variable thrombocytopenia.
• Pharmacokinetic-based dosing is the standard: Dose calculated to achieve a specific AUC (area under the curve).
• Commonly used formula: Dose (mg) ≈ AUC (mg/mL.min) × (GFR + 25)
  • AUC: Typically 4-7 mg/mL.min (varies with administration frequency, prior treatment, combination drugs).
  • GFR: Glomerular filtration rate (mL/min), ideally measured by isotope clearance (e.g., 51CrEDTA), 24-hour urinary creatinine clearance acceptable.

D. Activity:

• Less toxic substitute for cisplatin with similar indications.
• Increased thrombocytopenia may be a disadvantage in some combinations.
• Reduced non-hematological toxicity is advantageous in high-dose regimens with bone marrow/stem cell rescue.

E. Pharmacokinetic Interactions:

• Unlike cisplatin, carboplatin does not affect hepatic cytochrome P450 enzymes.
• Pharmacokinetic interactions with other drugs are rare.

III. Oxaliplatin

A. Overview:

• Platinum analogue differing chemically and possibly mechanistically from cisplatin and carboplatin.
• Broad spectrum of in vitro activity, differing from cisplatin/carboplatin.
• Extensively used in colorectal cancer (adjuvant and advanced disease).
• Broad-spectrum activity, used in other cancers (e.g., upper GI).

B. Dosage:

• Common regimens:
  • 85 mg/m² every 2 weeks as a 2-6 hour infusion.
  • 130 mg/m² every 3 weeks as a 2-6 hour infusion.
• Many other dosing regimens exist, including chronomodulated infusion with fluorouracil.

C. Limitation:

• Cumulative dosage limited by the development of peripheral neuropathy (usually reversible upon drug withdrawal).

IV. Summary Table:

Oncology-Anti-Tumor Antibiotics

I. Anthracyclines (Doxorubicin, Daunorubicin, Epirubicin, Idarubicin)

A. Mechanism of Action:

• Multiple Effects: Anthracyclines affect cell surfaces, signal transduction (activating protein kinase C), and, most importantly, DNA topoisomerase II (topo II). The exact contribution of each effect to cytotoxicity remains unclear.
• Free Radical Generation: Reduction to highly reactive compounds and free radical production contribute to their anti-cancer activity, but also cause cardiotoxicity (due to lower antioxidant defenses in the heart).
• Topoisomerase II Inhibition: Anthracyclines bind to the DNA-topo II complex, preventing DNA strand rejoining after temporary breaks, leading to DNA damage and cell death.

B. Drug Resistance:

• MDR1 Gene/P-glycoprotein (Pgp): This efflux pump, encoded by the MDR1 gene, actively removes anthracyclines from cells, limiting their effectiveness. Manipulating Pgp expression has had limited clinical success.
• MRP (Multidrug Resistance-associated Protein): Another efflux pump contributing to anthracycline resistance.

C. Pharmacokinetics & Metabolism:

• Rapid Initial Decline: After IV administration, plasma levels fall rapidly due to tissue distribution and DNA binding.
• Slow Elimination: Subsequent metabolism and elimination result in slow plasma concentration decline over several days.
• Liver Function: Dose reduction is recommended for patients with abnormal liver function to minimize toxicity.

D. Clinical Use:

• Doxorubicin & Epirubicin: IV administration for breast cancer, sarcoma, and hematological malignancies.
• Daunorubicin & Idarubicin: Primarily used in acute leukemia (idarubicin can be oral).

E. Toxicity:

• Acute Toxicities (5-10 days post-treatment): Myelosuppression (bone marrow suppression), mucositis (mouth sores), and alopecia (hair loss - reversible). Extravasation (leakage from IV site) is severe and lacks effective treatment.
• Cumulative Cardiotoxicity: Dose-dependent heart failure due to free radical accumulation. Risk is <5% below 450mg/m² doxorubicin, significantly increasing at higher doses. Pre-existing heart disease and radiation therapy increase risk. Liposomal doxorubicin reduces cardiotoxicity. Epirubicin, daunorubicin, and idarubicin are less cardiotoxic than doxorubicin.

II. Other Anti-Tumor Antibiotics

A. Mitoxantrone:

• Mechanism: Binds to DNA, interacts with topo II, but produces fewer free radicals than anthracyclines. Also a Pgp substrate.
• Clinical Use: Less cardiotoxic alternative to doxorubicin in advanced breast cancer, NHL, and non-lymphocytic leukemia, but less effective.

B. Dactinomycin (Actinomycin-D):

• Mechanism: Strong DNA intercalator, inhibiting RNA and protein synthesis. Pgp substrate.
• Clinical Use: Highly active against childhood cancers.

C. Mitomycin (MMC):

• Mechanism: Active against various solid tumors; also used as a radiosensitizer.
• Clinical Use: Used in combination chemotherapy for breast cancer, NSCLC, GI cancer, and as a radiosensitizer in anal cancer.
• Toxicity: Delayed and cumulative myelosuppression (especially thrombocytopenia). Administered every 6 weeks due to this toxicity. Uncommon side effects include hemolytic-uremic syndrome, pulmonary fibrosis, and cardiac complications.

III. Key Differences & Summary Table

Oncology-Alkylating Agents

I. Introduction

Alkylating agents are antiproliferative cytotoxic drugs used in cancer chemotherapy. They function by covalently binding to DNA via alkyl groups, primarily causing cross-linking. This leads to cell cycle arrest (G1-S transition), followed by either DNA repair or apoptosis (programmed cell death).

II. Clinical Use

Alkylating agents are widely used to treat:

• Leukemia
• Lymphoma
• A range of solid tumors

III. Mechanisms of Resistance

Resistance to alkylating agents is complex and varies depending on the specific agent. Key mechanisms include:

• Enhanced DNA repair: Increased expression of enzymes like O6-alkyltransferase (especially relevant for nitrosoureas).
• Increased detoxification: Elevated levels of:
  • Glutathione
  • Metallothionein
  • Glutathione-S-transferase

IV. Examples of Alkylating Agents

This section details key characteristics, uses, and toxicities of specific alkylating agents. Pay close attention to the differences in their mechanisms, side effects, and clinical applications.

Important Note: Both cyclophosphamide and ifosfamide are pro-drugs activated by hepatic cytochrome P450 to form nitrogen mustards. Remember the differences in their toxicity profiles and how those toxicities are managed clinically.