What are Microtubules?
Microtubules are a component of the cytoskeleton (scaffolding or skeleton of cell cytoplasm). These are cylindrical polymers of tubulin that can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm.
Microtubules are composed of tubulin molecules. Each tubulin molecule is a heterodimer consisting of α and β polypeptide subunits, each containing 450 amino acids. Tubulin molecules assemble to form linear profilaments, which arrange themselves in an imperfect helix and such 13 profilaments constitute microtubule.
Since microtubules are composed of heterodimers, they are polarised. Plus end terminates with β and α subunits are at minus end. The β-tubulin end is fast growing and the more dynamic end (+end), whereas the α-tubulin end is slow growing and the less dynamic end (−end).
What is Treadmilling?
Microtubules are very dynamic, growing and shrinking at either end. The rate of growth (or shrinkage) of the plus and minus ends of a microtubule is a function of the free tubulin-GTP concentration. The polymerization of a microtubule requires tubulin subunits bound to GTP.
At plus end, concentrations of tubulin-GTP are higher and growth occurs. At any instant one end of a filament (PLUS END) grows in length while the other end shrinks. This is due to the constant removal of the subunits from one end of the filament while protein subunits are constantly added at the other end.
There is a dynamic equilibrium between microtubule assembly and disassembly. This will help in forward movement of microtubules.
What are the functions of Microtubule?
Microtubule is principal components of the mitotic spindle apparatus that correctly separates, or segregates, the duplicate set of chromosomes into two daughter cells during cell division. It plays critical roles in many interphase functions such as maintaining cellular shape and serving as a scaffold for cellular organelles.
It is also involved in intracellular transport, secretion, neurotransmission, and in relaying signals between the cell surface receptors and the nucleus.( Paclitaxel induced neuropathy is probably due to dysfunctional microtubules in dorsal root ganglia, axons and Schwann cells).
Paclitaxel was discovered as part of a National Cancer Institute program in which extracts of thousands of plants were screened for anticancer activity. In 1963, a crude extract of the bark of the Pacific yew tree, Taxus brevifolia, was shown to have impressive activity in preclinical tumor models.
The production of a semisynthetic form of paclitaxel using the precursor 10-deacetyl-baccatin III, which is found in the needles of the European yew, Taxus baccata, has allowed large supplies to be produced.
Mechanism of Action
Paclitaxel binds to the interior surface of the microtubule lumen and profoundly alters the tubulin dissociation rate constants at both ends of the microtubule, suppressing treadmilling and dynamic instability, causing mitotic arrest.
However, in sharp contrast to the vinca alkaloids, they do not alter the association rate constants and the process of tubulin polymerization.
Mechanism of Resistance
Resistance to the drug may occur due to alterations in tubulin with causes decreased binding affinity for drug. Multidrug-resistant phenotype with increased expression of P170 glycoprotein results in enhanced drug efflux with decreased intracellular accumulation of drug.
Pharmacokinetics and Metabolism
It follows non-linear pharmacokinetics (absence of a dose–response relationship) due to which increasing the dose increases toxicity without an accompanying enhancement in efficacy. Longer infusions cause more neutropenia and neurotoxicity.
It is metabolized extensively by the hepatic P450 microsomal system. About 70%–80% of drug is excreted via fecal elimination and renal clearance is less than 10%. Terminal elimination half-life ranges from 9 to 50 hours.
It is poorly soluble and not orally bioavailable. Distributes widely to all body tissues, including third-space fluid collections such as ascites, but there is negligible penetration into the CNS. It undergoes extensive binding (90%) to plasma and cellular protein.
Each ml of paclitaxel solution contains 6 mg of paclitaxel and 527 mg polyoxyl 35 castor oil, dehydrated absolute alcohol. Should be diluted with NS or 5% D to a final concentration of 0.3 to 1.2 mg/ml.
- 30 mg/ 5ml vial
- 100 mg/16.66 ml vial
- 260 mg/43.33 ml vial
Why PVC free bottle is used for Paclitaxel Infusion?
DEHP (di(2-ethylhexyl)phthalate) a plastic-softening phthalate, has been used as a plasticiser in intravenous tubing and bags, catheters, nasogastric tubes, dialysis bags and tubing, and blood bags and transfusion tubing. It provides the PVC with desired mechanical properties flexibility, strength, suitability for use at a wide range of temperatures, resistance to kinking, optical clarity. When used in PVC plastic, DEHP is loosely chemically bonded to the plastic and readily leaches into blood and plasma.
Castor oil (Cremophor EL), dramatically increase the rate of DEHP extraction from PVC bags. This leaching of DEHP increases the risk of certain adverse health outcomes and can damage the liver, kidneys, lungs, and reproductive system, It may cause complete infertility in females and reduced fertility in males. Seminiferous tubule damage with adverse effects on sperm numbers, motility, and morphology.
To prevent these complications, paclitaxel should be given in glass bottle or polyethylene containers. Use polyvinyl chloride (PVC) free IV set i.e. use polyethylene lined IV set with 0.2 microm filter. These precautions are needed to avoid leaching of plasticiser DEHP ( Di Ethyl Hexyl Phthalate) from PVC infusion bags or sets.
- In Ovarian cancer, with platinum for first-line treatment of advanced carcinoma of the ovary or single agent for subsequent therapy of advanced carcinoma of the ovary
- Breast cancer as a single agent for adjuvant treatment of node-positive breast cancer administered sequentially to standard doxorubicin-containing combination chemotherapy, single agent for treatment of breast cancer after failure of combination chemotherapy for metastatic disease or relapse within 6 months of adjuvant chemotherapy
- Non-small cell lung cancer, with cisplatin in non-small cell lung cancer in patients who are not candidates for potentially curative surgery and/or radiation therapy
- AIDS-related Kaposi sarcoma, single agent for second-line treatment
- Small cell lung cancer,
- head and neck cancer,
- esophageal cancer,
- bladder cancer,
- treatment of unknown primary.
- Metabolism of paclitaxel may be affected by inhibitors and/or activators of the liver cytochrome P450 CYP3A4 and CYP2C8 enzyme systems.
- Phenytoin, phenobarbital—Accelerate the metabolism of paclitaxel resulting in lower plasma levels of drug
- Administration of cisplatin prior to paclitaxel leads to greater myelosuppression than administrating it after. In patients receiving cisplatin prior to paclitaxel there is 33 % decrease in paclitaxel clearance.
- Paclitaxel reduces the clearance of doxorubicin causing more profound myelosuppression.
- Cyclophosphamide—Myelosuppression is greater when cyclophosphamide is administered before paclitaxel.
Dose should be reduced if serum bilirubin is more than 1.6 times upper limit of normal (ULN) and liver enzymes more than 10 times ULN. No dose adjustments necessary in renal impairment.
It is the principal toxicity of paclitaxel.(20%)
- Major hypersensitivity reactions occur in 3% of patients with effective prophylaxis.
- Dexamethasone, 20 mg orally or intravenously, 12 and 6 hours before treatment; an H1-receptor antagonist (e.g., diphenhydramine, 50 mg intravenously) 30 minutes before treatment; and an H2-receptor antagonist (e.g., cimetidine, 300 mg; famotidine, 20 mg; or ranitidine, 150 mg intravenously) 30 minutes before treatment.
- A single dose of a corticosteroid (dexamethasone, 20 mg intravenously) administered 30 minutes before treatment also appears to confer effective prophylaxis against major hypersensitivity reactions
Peripheral sensory neuropathy
- At cumulative doses in excess of 1,400 mg/m2 / more than 250 mg/m2 infused over ≥24 hours .
- With lower doses of paclitaxel (135-200 mg/m2) , neuropathy is less common.
- Partially reversible.
- Neuroprotective agents: Amifostine; Glutamate; Nerve Growth Factor; Insulin-like Growth Factor; Acetyl-L-carnitine; oral vitamin E at a daily dose of 600 mg/day
- Treatment : Amitryptyline, Acetyl-L-carnitine, Pregabalin, gabapentin combined with nonsteroidal anti-inflammatory drugs such as ibuprofen, can provide some relief to painful paclitaxel neuropathy.
- Transient myalgias
- Transient sinus bradycardia, can be observed in up to 30% of patients.
- Onycholysis. Mainly observed in those receiving 6 courses on the weekly schedule. Not seen with the every-3-week schedule
It is a semisynthetic taxane prepared with a precursor extracted from european yew plants. Docetaxel is slightly more water soluble than paclitaxel and a more potent antimicrotubule agent in vitro, reduction in peripheral neuropathies as compared to paclitaxel.
Mechanism of Action
- Binds to free tubulin and promotes assembly of tubulin into stable microtubules while simultaneously inhibiting their disassembly.
- Stabilization of microtubules results in loss of normal function of microtubules with resultant inhibition of mitosis.
- Antiangiogenic effects
- In vitro analyses demonstrate docetaxel to be 100-fold more potent than paclitaxel in bcl-2 phosphorylation and apoptotic cell death.
Mechanism of Resistance
- Alterations in tubulin with decreased affinity for drug.
- Multidrug-resistant (MDR-1) phenotype with increased expression of P170 glycoprotein. Results in enhanced drug efflux with decreased intracellular accumulation of drug.
- Cross-resistant to other products, including vinca alkaloids, anthracyclines.
- Extensive binding (>90%) to plasma and cellular proteins.
- Extensively metabolized by the hepatic P450 microsomal system.
- About 75% of drug is excreted via fecal elimination.
- Renal clearance is less than 10% . Terminal half-lives ranging from 11.1 to 18.5 hours have been reported.
- Use with caution in patients with abnormal liver function. Patients with abnormal liver function are at significantly higher risk for toxicity, including treatment-related mortality.
Doses or Concentrations Available
- Docetaxel injection concentrate: 20 mg( 1 ml) , 80 mg(4 ml) , 120 mg(6 ml) .Stored below 25 degrees
- Solvent for Docetaxel injection concentrate: 5 ml vial for 20 mg, 6 ml for 80 mg, 9 ml for 120 mg.
- We have to bring the concentrate at room temperature by allowing it to stand at room temperature for 5 minutes after taking from refrigerator. Then reconstitute with solvent. Inject this premix into 250 ml NS or 5% Dextrose. Mix thoroughly, solution should be clear and there should be no precipitation.
- FDA-approved for the treatment of metastatic or recurrent breast cancer after failure of prior anthracycline based chemotherapy .
- For adjuvant treatment of patients with node positive breast cancer.
Metastatic or locally advanced gastric adenocarcinoma
- Adjuvant CT :DCF ( 75 mg/m2 every 3 weeks)
- FDA-approved for use in combination with cisplatin and 5-fluorouracil for induction treatment of patients with inoperable, locally advanced disease. 75 mg/m2 every 3 weeks.
- FDA-approved in combination with prednisone for androgen-independent (hormone-refractory) metastatic prostate cancer.
Non–small cell lung cancer
- Locally advanced or metastatic disease after failure of prior platinum-based chemotherapy, 75 mg/m 2 IV every 3 weeks or 35–40 mg/m 2 IV weekly for 3 weeks with 1 week rest.
Non–small cell lung cancer
- non resectable 75 mg/m 2 IV every 3 weeks in combination with cisplatin in patients who have not received prior chemotherapy.
Recurrent ovarian cancer
- platinum sensitive ( Docetaxel + carboplatin ) ;
- platinum resistant ( Docetaxel single agent).
- Neutropenia is the main toxicity of docetaxel.
- Grade 4 neutropenia with ANC < 500/mm3 occurs 100 % with 100 mg/m2 and 75-80% with 60-75 mg/m2).
- When docetaxel is administered on an every 3-week schedule, the onset of neutropenia is usually noted on day 8 with complete resolution by days 15 to 21.
- Neutropenia is significantly less when low doses are administered weekly.
- Closely monitor CBCs, and docetaxel therapy should not be given to patients with neutrophil counts of <1,500 cells/mm 3 .
- Thrombocytopenia and anemia are also observed
- The solvent used for Docetaxel is poly-oxy-ethylated surfactant, polysorbate-80 which causes alteration of membrane fluidity, increased capillary permeability
- Fluid retention is cumulative and Incidence increases with total doses >400 mg/m 2 . Occurs in about 50% of patients.
- Prophylactic Dexamethasone 8 mg BD for 3-5 days beginning one day prior to docetaxel infusion. Aggressive and early treatment with diuretics has been successfully used to manage fluid retention.
- This syndrome resolves slowly once docetaxel therapy is stopped, with complete resolution occurring several months after treatment in patients who experience severe toxicity.
- Hypersensitivity reactions with generalized skin rash, erythema, hypotension,dyspnea, and/or bronchospasm.
- The most popular prophylactic regimen for prevention of hypersensitivity reaction and fluid retention syndrome is dexamethasone 8 mg orally twice daily for 3 or 5 days starting 1 or 2 days before docetaxel, with or without H1– and H2-receptor antagonists given 30 minutes before docetaxel. Overall incidence decreased to less than 3%.
- Maculopapular skin rash and dry, itchy skin. Most commonly affect forearms and hands. Brown discoloration of fingernails may occur. Observed in up to 50% of patients usually within 1 week after therapy.
- Other cutaneous effects include palmar-plantar erythrodysesthesia.
- Alopecia occurs in up to 80% of patients.
- Mucositis and/or diarrhea seen in 40% of patients. Mild to moderate nausea and vomiting, usually of brief duration.
- Peripheral neuropathy is less commonly observed with docetaxel than with paclitaxel.
- Generalized fatigue and asthenia are common, occurring in 60%–70% of patients. Arthralgias and myalgias also observed.
- Reversible elevations in serum transaminases, alkaline phosphatase, and bilirubin.
- Phlebitis and/or swelling can be seen at the injection site
- In traditional paclitaxel, cramophore is used as a solvent which makes the particles bigger, so that absorption of the drug is very low and hypersensitivity reactions are very common.
- The first drug made by nanotechnology in the field of oncology is nanoxel (plant alkaloid), in which traditional paclitaxel is fragmented into micro-molecules(mean particle size 80 nm).
- The formulation is water soluble and obviates the need for CrEL.
- Fonseca et al have prepared nanoparticles of paclitaxel by using the interfacial deposition method, in which they added an organic solution of PLGA (Poly lactic co glycolic acid) and paclitaxel in acetone to an aqueous poloxamer 188 solution under magnetic stirring at room temperature, followed by washing and harvesting of nanoparticles by ultracentrifugation.
- Studies indicate that drugs delivered in nanoparticle carriers result in an extended retention of the drug in the tumors as nanoparticles can avoid rapid clearance by the reticuloendothelial system and can referentially accumulate in solid tumors; diminution in tumor growth, and prolonged survival of the test subject.
- The multidrug resistance phenotype, mediated by p-glycoprotein in the tumor cells, can also be overcome by nanoparticle drug delivery. This is important because acquired resistance to paclitaxel has been reported.
- Treatment of breast cancer after failure of combination chemotherapy for metastatic disease or relapse w/in 6 mth of adjuvant chemotherapy.
- Adjuvant treatment of node positive breast cancer administered sequentially to standard doxorubicin containing combination chemotherapy.
- Advanced ovarian cancer : First line ( in combination with cisplatin ) and subsequent therapies.
- NSCLC in pt. who are not candidates for surgery/ RT : in combination with cisplatin
- Paclitaxel solution for nanoparticle: 1.5ml (30 mg), 5 ml( 100 mg ), 15 ml( 300 mg)
- Concentrate of excipients for nanoparticle paclitaxel : 1.5 ml, 5 ml, 15 ml.
- 220 mg/m2 IV over 1 hr 3 wkly.
- Should not be given in patients with baseline neutrophil counts of <1,500 cells/mm3.
Nanoparticle Albumin Bound Paclitaxel
- Standard formulation paclitaxel requires the use of solvents, such as Cremophor-EL, polyoxyethylated castor oil derivative.
- Cremophor EL (CrEL) is a formulation vehicle used for various poorly-water soluble drugs, including the anticancer agent paclitaxel .
- Nanoparticle albumin-bound paclitaxel (nab-paclitaxel) is a novel solvent-free formulation of paclitaxel.
- It is the first of a new class of anticancer agents that incorporate albumin particle technology and exploit the unique properties of albumin, a natural carrier of lipophilic molecules in humans.
- nab-Paclitaxel binds to gp60, the albumin receptor on endothelial cells, which in turn activates caveolin-1 and the formation of caveolae.
Mechanism of Action
- High-affinity binding to microtubules enhances tubulin Normal dynamic process of microtubule network is inhibited, leading to inhibition of mitosis and cell division.
- Cell cycle–specific, active in the mitosis (M) phase of the cell cycle.
Role of SPARC Protein
Since secreted protein acidic rich in cysteine (SPARC) SPARC is overexpressed in many solid tumors the nab-paclitaxel leads to a 33% increase in intratumoral concentrations and a 50% higher dose of paclitaxel delivered compared with a conventional paclitaxel infusion.
- Step 1: injection of nab-paclitaxel into blood vessels.
- Step 2: suspended in the blood, nab-paclitaxel dissociates into individual particles.
- Step 3: because it is associated with albumin, nab-paclitaxel binds to gp60 receptors present on the endothelial cells of tumour blood vessels.
- Step 4: binding of albumin to gp60 activates caveolin-1 which creates vesicles (caveolae) in the endothelial cell wall; these caveolae fill with nab-paclitaxel and migrate across the cytoplasm.
- Steps 5&6: caveolae deposit their contents into the interstitium of the tumour, where nab-paclitaxel binds to SPARC.
- Step 7: accumulation of nab-paclitaxel via SPARC at tumour cell membranes.
- Step 8: diffusion of paclitaxel into the intracellular compartment and subsequent induction of cell death.
Advantages over Conventional Paclitaxel
- Phase III trial compared nab- and Cremphor-EL-paclitaxel in patients with metastatic breast cancer.
- Patients treated with nab-paclitaxel experienced a higher response, longer time to tumor progression and, in patients receiving second-line or greater therapy, a longer median survival.
- Patients treated with nab-paclitaxel had a significantly lower rate of severe neutropenia and a higher rate of sensory neuropathy.
- Moreover,the nab-paclitaxel being solvent free, the infusion time is much shorter compared to paclitaxel with Cremophor EL.
- Supplied as a white to yellow, sterile, lyophilized powder for reconstitution with 20 mL of 0.9% Sodium Chloride prior to intravenous infusion.
- Each single-use vial contains 100 mg of paclitaxel and approximately 900 mg of human albumin.
- Each milliliter (mL) of reconstituted suspension contains 5 mg paclitaxel.
- Metabolized extensively by the hepatic P450 microsomal system.
- About 20% of drug is excreted via fecal elimination.
- Renal clearance is relatively minor with less than 1% of drug cleared via the kidneys.
- The clearance of abraxane is 43% greater than paclitaxel,and the volume of distribution is about 50% higher than paclitaxel.
- Terminal elimination half-life is on the order of 27 hours.
Indications and Dosage
- FDA-approved for the treatment of breast cancer after failure of combination chemotherapy for metastatic disease or relapse within six months of adjuvant chemotherapy.
- Recommended dose is 260 mg/m 2 IV on day 1 every 21 days. An alternative regimen is a weekly schedule of 125 mg/m 2 IV on days 1, 8, and 15 every 28 days.
- Non small cell lung cancer as a single agent or in combination with other cytotoxic agents.
- Contraindicated in patients with baseline neutrophil counts of <1,500 cells/mm .
- Use with caution in patients with abnormal liver function, as patients with abnormal liver function may be at higher risk for toxicity.
- Closely monitor infusion site for infiltration during drug administration as injection site reactions have been observed.
- Myelosuppression with dose-limiting neutropenia and anemia.
- Thrombocytopenia relatively uncommon.
- Neurotoxicity mainly in the form of sensory neuropathy with numbness and paresthesias. Dose-dependent effect. In contrast to paclitaxel, abraxane mediated neuropathy appears to be more readily reversible.
- Ocular and visual disturbances seen in 13% of patients with severe cases seen in 1%.
- Asthenia, fatigue, and weakness.
- AlopeciaNausea/vomiting, diarrhea, and mucositis
- Cardiac toxicity with chest pain, supraventricular tachycardia, hypertension,pulmonary embolus, peripheral edema, and rare cases of cardiac arrest.
- Transient elevations in serum transaminases, bilirubin, and alkaline phosphatase.
- Semi-synthetic taxane prepared with a precursor extracted from yew needles.
Indication and Dose
- FDA-approved for the treatment of patients with hormone-refractory metastatic prostate cancer previously treated with a docetaxel-containing regimen.
- Recommended dose is 25 mg/m 2 as a one-hour infusion every three weeks in combination with oral prednisone 10 mg administered daily throughout cabazitaxel treatment.
- Myelosuppression with dose-limiting neutropenia. Thrombocytopenia and anemia are also observed.
- Hypersensitivity reaction
- Diarrhea, nausea/vomiting, constipation, abdominal pain and loss of appetite
- Fatigue and asthenia.
- Neurotoxicity, mainly in the form of peripheral neuropathy, dizziness,and headache.
- Myalgias and arthralgias
- Topoisomerase Inhibitors – Topotecan, Irinotecan, Etoposide, Teniposide
- Platinum Agents – Cisplatin, Carboplatin, Oxaliplatin, Newer Platins
- Proteasome Inhibitors – Bortezomib, Carfilzomib
- Anticancer Antimetabolites Drugs
- Drugs for CML Treatment – Imatinib, Nilotinib, Dasatinib, Bosutinib, Ponatinib
- Antiangiogenic AntiVEGF Monoclonal Antibodies – Bevacizumab, Ziv-Aflibercept, Ramucirumab
- Bone Targeted Therapies – Bisphophonates, Denosumab
- Hematopoietic Growth Factors – G-CSF, GM-CSF, Erythropoietin, Darbopoietin
- Alkylating Agents