Microtubule inhibitors

microtubule inhibitors

1. ANTI-MICROTUBULE AGENTS

2. Microtubules are vital and dynamic cytoskeletal polymers that play a critical role in cell division, signaling, vesicle transport, shape, and polarity .

3. Microtubules are composed of 13 linear protofilaments of polymerized α/β-tubulin heterodimers arranged in parallel around a cylindrical axis . They are associated with regulatory proteins such as microtubule- associated proteins, tau, and motor proteins kinesin and dynein .

4. The specific biologic functions of microtubules are due to their unique polymerization dynamics. Tubulin polymerization is mediated by a nucleation-elongation mechanism. One end of the microtubules, termed the plus end , is kinetically more dynamic than the other end, termed the minus end

5. Microtubule dynamics are governed by two principal processes driven by guanosine 5- triphosphate (GTP) hydrolysis: TREAD- MILLING OR POLEWARD FLUX is the net growth at one end of the microtubule and the net shortening at the opposite end .

6. DYNAMIC INSTABILITY : process in which the microtubule ends switch spontaneously between states of slow sustained growth and rapid depolymerization .

7. Antimicrotubule agents These are tubulin-binding drugs that directly bind tubules, inhibitors of tubulin-associated scaffold kinases, or inhibitors of their associated mitotic motor proteins to, ultimately, disrupt microtubule dynamics.

8. They are broadly classified as microtubule stabilizing or microtubule destabilizing agents according to their effects on tubulin polymerization.

9. TAXANES Taxanes were the first-in-class microtubule stabilizing drugs . The prototype taxane is the natural product paclitaxel , originally known as Taxol and first derived from the bark of the Pacific Yew tree, Taxus brevifolia . Taxanes enhance stability of microtubules preventing the separation of chromosomes during metaphase.

10. Nab paclitaxel Cabazitaxel

11. Nanoparticle albumin-bound paclitaxel (nab-paclitaxel) is a formulation that avoids the solvent related side effects of non–water-soluble paclitaxel and docetaxel . Cabazitaxel overcomes docetaxel and paclitaxel’s susceptibility to the P-glycoprotein efflux pump. It is synthesized by adding two methoxy groups to the 10-deacetylbaccatin III, which results in the inhibition of the 5′-triphosphate–dependent efflux pump of P-glycoprotein

12. Mechanism of Action The taxanes bind to the interior surface of the microtubule lumen at binding sites completely distinct from those of exchangeable GTP, colchicine, podophyllotoxin , and the vinca alkaloids The taxanes profoundly alter the tubulin dissociation rate constants at both ends of the microtubule, suppressing treadmilling and dynamic instability .

13. Dose -dependent taxane β-tubular binding induces mitotic arrest at the G2/M transition and induces cell death.

14. Clinical Pharmacology Paclitaxel : D rug disposition is a biphasic process with values for alpha and beta half-lives averaging approximately 20 minutes and 6 hours, respectively 71% of an administered dose of paclitaxel is excreted in the stool via the enterohepatic circulation over 5 days The drug is metabolized by cytochrome P-450 mixed-function oxidases—specifically, the isoenzymes CYP2C8 and CYP3A4, which metabolize paclitaxel to hydroxylated 3′p-hydroxypaclitaxel (minor) and 6α- hydroxypaclitaxel (major), as well as dihydroxylated metabolites.

15. Nanoparticle Albumin-Bound Paclitaxel : Nab-paclitaxel is a solvent-free colloidal suspension made by homogenizing paclitaxel with 3% to 4% albumin under high pressure to form nanoparticles of ∼130 nm that disperse in plasma to ∼10 nm

16. Indications : M etastatic breast cancer F irst -line treatment of locally advanced or metastatic NSCLC-in combination with carboplatin. F irst -line treatment of metastatic pancreatic adenocarcinoma -- in combination with gemcitabine.

17. Advantages Over Conventional : Improved pharmacokinetic profile with a larger volume of distribution and a high concentration of circulating, free drug . I mproved tumor accumulation by the enhanced permeability and retention (EPR) effect; and receptor-mediated transcytosis via an albumin-specific receptor (gp60) for endothelial transcytosis and binding of secreted protein acidic and rich in cysteine (SPARC) in the tumor interstitium E xtensive extravascular volume of distribution thus extensive tissue and extravascular protein distribution -- 33% higher drug concentration over CrEL -paclitaxel

18. Docetaxel : Semi synthetic analog - needles of Taxus baccata Terminal half-lives range from 11.1 to 18.5 hours . The docetaxel clearance is dependent on body surface area (BSA), hepatic function, and plasma α – 1 acid glycoprotein concentration. Plasma protein binding is high (greater than 80%), and binding is primarily to α1-acid glycoprotein , albumin, and lipoproteins. The hepatic cytochrome P -450 mixed-function oxidases, particularly isoforms CYP3A4 and CYP3A5, are principally involved in biotransformation.

19. The principal pharmacokinetic determinants of toxicity, particularly neutropenia, are drug exposure and the time that plasma concentrations exceed biologically relevant concentrations. The baseline level of α1-acid glycoprotein may be elevated as an acute phase reactant in advanced disease and is an independent predictor of response and a major prognostic factor of survival in patients with non–small-cell lung cancer treated with docetaxel chemotherapy.

20. Cabazitaxel : Cabazitaxel is a semisynthetic derivative of the natural taxoid 10-deacetylbaccatin III. It binds to and stabilizes the β-tubulin subunit, resulting in the inhibition of microtubule depolymerization and cell division, cell cycle arrest in the G2/M phase, and the inhibition of tumor cell proliferation . It is active against diverse cancer cell lines and tumor models that are sensitive and resistant to docetaxel , including prostate, mammary, melanoma, kidney, colon, pancreas, lung, gastric, and head and neck.

21. Cabazitaxel is a poor substrate for the membrane-associated, multidrug resistance P-glycoprotein efflux pump; therefore, is useful for treating docetaxel -refractory prostate cancer for which it gained FDA approval in 2010 . I t also penetrates the blood–brain barrier Pharmacokinetics of cabazitaxel is similar to docetaxel H owever, cabazitaxel has a larger volume of distribution and a longer terminal half-life (mean 77.3 hours versus 11.2 hours for docetaxel )

22. Tesetaxel : Tesetaxel (DJ-927, XRP6258) is a semisynthetic, orally bioavailable taxane currently in clinical trials in breast, gastric, and prostate cancer. Administration in phase I and II trials has not been associated with hypersensitivity and possibly less neurotoxicity compared to other taxanes . Dose -limiting toxicity has been neutropenia. Overall responses in phase II studies have been 50% and 38% in patients treated for first- and second-line breast cancer, respectively. A phase I/II study in advanced NSCLC showed an overall response rate of 5.6% . Tesetaxel activity is independent of P-glycoprotein expression .

23. Drug interactions Cisplatin paclitaxel (on a 24-hour schedule) more profound neutropenia due to 33 % reduction in the clearance of paclitaxel after cisplatin. Paclitaxel doxorubicin --Neutropenia and mucositis are more severe, due to 32 % reduction in the clearance rates of doxorubicin and doxorubicinol when doxorubicin is administered after paclitaxel. P aclitaxel on either a 3- or 24-hour schedule followed by carboplatin has been demonstrated to produce equivalent neutropenia and less thrombocytopenia as compared to carboplatin as a single agent.

24. Paclitaxel : The micelle-forming CrEL vehicle, is required for suspension and intravenous delivery of paclitaxel, causes its nonlinear pharmacokinetics and affects its therapeutic index. CrEL causes hypersensitivity reactions , with major reactions usually occurring within the first 10 minutes after the first treatment and resolving completely after stopping the treatment. All patients should be premedicated with steroids, diphenhydramine, and an H2 antagonist. Rechallenge can be done successfully after giving high doses of corticosteroids. Toxicity

25. Neuropathy : Peripheral neuropathy that presents in a symmetric stocking glove distribution, at first transient and then persistent . N eurologic examination - reveal axonal degeneration and demyelination . L oss of deep tendon reflexes occurs less commonly; however, autonomic and motor changes can occur. Severe neurotoxicity is uncommon when paclitaxel is given alone at doses below 200 mg/m2 on a 3- or 24-hour schedule every 3 weeks, or below 100 mg/m2 on a continuous weekly schedule.

26. Neutropenia –frequent The onset is usually on days 8 to 11, and recovery is generally complete by days 15 to 21 with an every 3 weeks dosing regimen. Neutropenia is noncumulative , and the duration of severe neutropenia,even in heavily pretreated patients is usually brief. Severity of neutropenia is related to the duration of exposure. The most common cardiac rhythm disturbance- a transient sinus bradycardia , Routine cardiac monitoring during paclitaxel therapy is not necessary but is advisable for patients who may not be able to tolerate bradyarrhythmias .

27. Vomiting and diarrhea, are uncommon. Extravasation of large volumes can cause moderate soft tissue injury . Paclitaxel also induces reversible alopecia of the scalp in a dose-related fashion. Nail disorders - ridging, nail bed pigmentation, onychorrhexis , and onycholysis . These side effects have been reported more commonly with dose-intensified paclitaxel regimens.

28. Nab- paclitaxel : S teroid premedications are not necessary as no h ypersensitivity reactions have been observed. The main dose-limiting toxicities are neutropenia and sensory neuropathy. Other toxicities include alopecia , diarrhea, nausea and vomiting, elevations in liver enzymes, arthralgia, myalgia, and asthenia.

29. Docetaxel : Neutropenia is the main toxicity of docetaxel . When docetaxel is administered on an every 3 weeks schedule, the onset of neutropenia is usually noted on day 8, with complete resolution by days 15 to 21. FDA black box warnings include increased toxicity in patients with abnormal liver function and, in select NSCLC patients that received prior platinum, severe hypersensitivity reactions and severe fluid retention despite dexamethasone at-home premedication.

30. Hypersensitivity reactions Fluid retention syndrome : It is characterized by edema, weight gain, and third-space fluid collection. Fluid retention is cumulative and is due to increased capillary permeability. Prophylactic treatment with corticosteroids has been demonstrated to reduce the incidence of fluid retention. Aggressive and early treatment with diuretics has been successfully used to manage fluid retention .

31. Skin toxicity occurs in as many as 50% to 75% of patients; however, premedication reduce the overall incidence of this effect . Other cutaneous effects include palmar–plantar erythro - dysesthesia and onychodystrophy . Docetaxel produces neurotoxicity , which is qualitatively similar to that of paclitaxel Asthenia has been a prominent complaint in patients who have been treated with large cumulative doses. Stomatitis appears to occur more frequently. Other reported toxicities of note include necrotizing enterocolitis , interstitial pneumonitis, and organizing pneumonia .

32. Cabazitaxel Cabazitaxel was approved by the FDA in June 2010 to treat metastatic castration-resistant prostate cancer in those who had received prior chemotherapy. Cabazitaxel was associated with more grade 3 or 4 neutropenia (82%) than mitoxantrone (58%). Side effects reported in more than 20% of patients treated with cabazitaxel included myelosuppression , diarrhea, nausea, vomiting, constipation, abdominal pain, or asthenia. FDA black box warnings are similar to those for docetaxel .

33. VINCA ALKALOIDS

34. Natural compunds --vinblastine (VBL) and vincristine (VCR), were isolated from the leaves of the periwinkle plant Catharanthus roseus G. Don . Semisynthetic analogs -- vinorelbine (VRL), vindesine (VDS), and vinflunine (VFL).

35. Mechanism of action The vinca alkaloids depolymerize micro- tubules and destroy mitotic spindles . At low but clinically relevant concentrations, VBL does not depolymerize spindle microtubules, yet it powerfully blocks mitosis It occur as a result of the suppression of microtubule dynamics rather than microtubule depolymerization .

36. This group of compounds binds to the β subunit of tubulin dimers at a distinct region called the vinca -binding domain. In mitotic spindles, the slowing of the growth and shortening or treadmilling dynamics of the microtubules block mitotic progression. Disruption of the normal mitotic spindle assembly leads to delayed cell cycle progression with chromosomes stuck at the spindle poles and unable to pass from metaphase into anaphase, which eventually induces to apoptosis.

37. Clinical Pharmacology The vinca alkaloids are usually administered intravenously as a brief infusion. The vinca alkaloids have large volumes of distribution, high clearance rates, and long terminal half-lives that reflect the high magnitude and avidity of drug binding in peripheral tissues. VCR has the longest terminal half-life and the lowest clearance rate; VBL has the shortest terminal half-life and the highest clearance rate; and VDS has intermediate characteristics. The longest half-life and lowest clearance rate of VCR may account for its greater propensity to induce neurotoxicity.

38. Vincristine : Plasma clearance is slow, and terminal half-lives that range from 23 to 85 hours VCR is metabolized and excreted primarily by the hepatobiliary system. VCR metabolism is mediated principally by hepatic cytochrome P-450 CYP3A5. Vinblastine : The clinical pharmacology of VBL is similar to that of VCR. VBL binding to plasma proteins and formed elements of blood is extensive . Distribution is rapid, and terminal half-lives range from 20 to 24 hours. VBL disposition is principally through the hepatobiliary system with excretion in feces (approximately 95%); however, fecal excretion of the parent compound is low, indicating that hepatic metabolism is extensive .

39. Vinorelbine : After intravenous administration, there is a rapid decay of VRL concentrations followed by a much slower elimination phase (terminal half-life, 18 to 49 hours) . High Plasma protein binding, principally to α1-acid glycoprotein, albumin, and lipoproteins, and drug binding to platelets is extensive . VRL is widely distributed due to its lipophilicity , and high concentrations are found in virtually all tissues, except the central nervous system . T he liver is the principal excretory organ, and up to 80% of VRL is excreted in the feces, The cytochrome P-450 CYP3A isoenzyme appears to be principally involved in biotransformation.

40. Vinflunine VFL is a novel semisynthetic microtubule inhibitor with a fluorinated catharanthine moiety, which translates into lower affinity for the vinca binding site on tubulin and, therefore, different quantitative effects on microtubule dynamics. The low affinity for tubulin may be responsible for its reduced clinical neurotoxicity. Despite this lower affinity, it is more active in vivo than other vinca alkaloids, and resistance develops more slowly. VFL is still under clinical development.

41. Drug interactions Methotrexate : Increased accumulation of the methotrexate within the cell due to decreased drug efflux. L- Aspariginase : decreases hepatic clearance of vinca alkaloids, so increased vinca -related toxicity . Mitomycin C : The combined use of mitomycin C and the vinca alkaloids has been associated with acute dyspnea and bronchospasm. The onset of these pulmonary toxicities has ranged from within minutes to hours after treatment with the vinca alkaloids, or up to 2 weeks after mitomycin C

42. Phenytoin : decreased plasma concentrations of the drug—decreased seizure threshold. Because involvement of the cytochrome P-450 CYP3A isoenzyme in vinca alkaloid metabolism, administration of the vinca alkaloids with erythromycin and other inhibitors of CYP3A may lead to severe toxicity H2 receptor antagonists- decreased hepatic clearance.

43. TOXICITY Vinblastine and Vinorelbine : Neutropenia is the principal dose-limiting toxicity . Thrombocytopenia and anemia occur less commonly. The onset of neutropenia is usually day 7 to 11, with recovery by day 14 to 21, and can be potentiated by hepatic dysfunction . Mucositis occurs more frequently with VBL than with VRL and is least common with VCR. Nausea , vomiting, diarrhea , and pancreatitis also occur to a lesser extent.

44. VINCRISTINE - Neurotoxicity P eripheral , symmetric mixed sensory motor and autonomic polyneuropathy . Mechanism : Interference with axonal microtubule function Toxic manifestations include constipation, abdominal cramps, paralytic ileus, urinary retention, orthostatic hypotension, and hypertension Early symmetric sensory impairment and paresthesias can progress to neuritic pain and loss of deep tendon reflexes with continued treatment, which may be followed by foot drop, wrist drop, motor dysfunction, ataxia, and paralysis.

45. Cranial nerves are rarely affected because the uptake of VCR into the central nervous system is low. Severe neurotoxicity occurs infrequently with VBL and VDS . Mild -to-moderate peripheral neuropathy, principally characterized by sensory effects, occurs in 7% to 31% of patients, and constipation and other autonomic effects are noted in 30% of patients, whereas severe toxicity occurs in 2% to 3% Patients with delayed biliary excretion or hepatic dysfunction, and those with antecedent neurologic disorders, such as Charcot-Marie-Tooth disease, hereditary and sensory neuropathy type 1, and Guillain-Barré syndrome, are predisposed to neurotoxicity.

46. Vesicants To decrease the risk of phlebitis, the vein should be adequately flushed after treatment .

47. Suspected extravasation : T reatment should be discontinued. Aspiration of any residual drug remaining in the tissues should be attempted. Prompt application of heat ( not ice) for 1 hour four times daily for 3 to 5 days can limit tissue damage. Hyaluronidase , 150 to 1,500 U (15 U/mL in 6 mL 0.9% sodium chloride solution) subcutaneously, through six clockwise injections in a circumferential manner using a 25-gauge needle (changing the needle with each new injection) into the surrounding tissues may minimize discomfort and latent cellulitis. Surgical consultation.

48. Mild and reversible alopecia occurs in approximately 10% and 20% of patients treated with VLR and VCR, respectively. Acute cardiac ischemia , chest pains without evidence of ischemia, fever, Raynaud syndrome, hand–foot syndrome, and pulmonary and liver toxicity ( transaminitis and hyperbilirubinemia ) All of the vinca alkaloids can cause a syndrome of inappropriate secretion of antidiuretic hormone (SIADH), and patients who are receiving intensive hydration are particularly prone to severe hyponatremia secondary to SIADH

49. MICROTUBULE ANTAGONISTS Estramustine Epothilones Maytansinoids and Auristatins

50. Estramustine Estramustine is a conjugate of nor-nitrogen mustard linked to 17β-estradiol by a carbamate ester bridge. Estramustine binds to β-tubulin at a site distinct from the col- chicine and vinca alkaloid binding sites. This agent depolymerizes microtubules and microfilaments, binds to and disrupts MAPs, and inhibits cell growth at high concentrations, resulting in mitotic arrest and apoptosis in tumor cells.

51. Pharmacology The selective actions of estramustine phosphate is dependent on the expression of the estramustine -binding protein (EMBP) . The disposition of estramustine is principally by rapid oxidative metabolism of the parent compound to estromustine . Estromustine concentrations in plasma are maximal within 2 to 4 hours after oral administration, and the mean elimination half-life of estromustine is 14 hours. Estromustine and estramustine are principally excreted in the feces, with only small amounts of conjugated estrone and estradiol detected in the urine (less than 1%).

52. Toxicity : Nausea and vomiting are the principal toxicities encountered . Myelosuppression is rare . Common estrogenic side effects include gynecomastia , nipple tenderness, and fluid retention. Thromboembolic complications may occur in up to 10% of patients.

53. Epothilones The epothilones are macrolide compounds that were initially isolated from the mycobacterium Sorangium cellulosum . They exert their cytotoxic effects by promoting tubulin polymerization and inducing mitotic arrest . In general, the epothilones are more potent than the taxanes . In contrast to the taxanes and vinca alkaloids, overexpression of the efflux protein P-glycoprotein minimally affects the cytotoxicity of epothilones .

54. Natural compounds : epothilone B ( patupilone ; EPO906) S emisynthetic epothilone compounds : aza -epothilone B ( ixabepilone ; BMS-247550) , epothilone D ( deoxyepothilone B, KOS-862 ) Fully synthetic analog : sagopilone (ZK-EPO )

55. Ixabepilone Ixabepilone has been evaluated in several schedules using a cremophor -based formulation It is FDA approved for the treatment of patients with breast cancer . It is active in breast cancer previously treated with paclitaxel or docetaxel . The principal toxicities observed include neutropenia and peripheral neuropathy . Elimination is by hydrolysis.

56. Maytansinoids and Auristatins : DM1,DM4, MMAE,MMAF Maytansinoids and auristatins are unrelated, although both are tubulin - binding agents of the vinca binding site and inhibit tubulin polymerization . Maytansinoids are derived from maytansine which is a macrolide isolated from maytenus plant They are 100- to 1,000- fold more cytotoxic that most cancer chemotherapeutics .

57. Ado- trastuzumab Emtansine (T-DM1) Drug maytansinoid-1 (DM1) is the chemotherapeutic delivered using a thioether linker. It is FDA approved for patients with HER2- positive metastatic breast cancer previously treated with trastuzumab and taxane chemotherapy . In the international phase III study, there was a 3.2-month improved progression-free survival among patients that received T-DM1 compared to those receiving standard treatment with capecitabine and lapatinib . The most common side effects in the trial were thrombocytopenia, transient transaminitis , as well as nausea, fatigue, myalgias and arthralgias .

58. Monomethyl auristatin E (MMAE) Brentuximab Vedotin -- Monomethyl auristatin E (MMAE) is linked to a monoclonal antibody targeted against CD30. Dose -limiting toxicities include thrombocytopenia, hyperglycemia, diarrhea, and vomiting, and the most common side effects includes peripheral neuropathy (42%), nausea (35%), and fatigue (34%) . The FDA black box warning includes contraindicated use with bleomycin due to increased pulmonary toxicity and the risk of John Cunningham (JC) virus–induced progressive multifocal leukoencephalopathy .

59. The linker is a peptide-based substrate for cathepsin -B and thereby designed to detect the lysosome/endosome compartment for drug release.

60. MITOTIC MOTOR PROTEIN INHIBITORS Aurora Kinase : Aurora kinases are serine/threonine kinases crucial for mitosis in their recruitment of mitotic motor proteins for spindle formation. They are particularly overexpressed in high growth rate tumors. Aurora A and B kinases are expressed globally throughout all tissues, and Aurora C kinase is expressed in testes and participates in meiosis. Aurora A kinase is expressed and frequently amplified in many epithelial tumors and implicated in the microtubule-targeted agent-resistant phenotype . MLN-8237 is in clinical development for treatment- related neuroendocrine prostate cancer. The main dose-limiting toxicity of these agents is neutropenia.

61. Pololike kinases (PLKs) Pololike kinases (PLKs) are serine or threonine kinases crucial for cell cycle process. Overexpression of PLKs has been shown to be related to histologic grading and poor prognosis in several types of cancer. BI -2536 and ON01910 are PLK inhibitors in early clinical development .

62. Kinesin Spindle Protein Inhibitor Ispinesib : Kinesin spindle protein (KSP; also known as EG5) is a kinesin motor protein required to establish mitotic-spindle bipolarity . SB -715992 ( ispinesib ) is a small-molecule inhibitor of KSP ATPase and has been evaluated in two different schedules. The dose-limiting toxicity is neutropenia. Ispinesib was found to be inactive in phase 2 studies evaluating efficacy in patients with castration-resistant and largely docetaxel -resistant prostate cancer, advanced renal cancer, and head and neck cancer .

63. MECHANISMS OF RESISTANCE Drug resistance is often due to ( 1) factors that reduce the ability of drugs to reach their cellular target (e.g., activation of detoxification pathways and decreased drug accumulation); ( 2) modifications in the drug target ( 3) events downstream of the target (e.g., decreased sensitivity to, or defective, apoptotic signals). Many tubulin binding agents are substrates for multidrug transporters such as P-glycoprotein and the multidrug resistance gene (MDR1)

64. The MDR1-encoded gene product MDR1 (ABC subfamily B1; ABCB1) and MDR2 (ABC subfamily ABCB4) are the best-characterized ABC transporters thought to confer drug resistance to taxanes . Mutations of tubulin isotype genes, gene amplifications, and isotype switching have also been reported in taxane -resistant cell lines. In patients, levels of class III β-tubulin have been shown to correlate with response—those with high RNA levels have poor response—and immunohistochemical stains can correlate and may be predictive . As opposed to taxanes , resistance to vinca alkaloids has been associated with decreased class II β-tubulin expression.

65. MAPs are important structural and regulatory components of microtubules that act in concert to remodel the microtubule network by stabilizing or destabilizing microtubules during mitosis or cytokinesis. Alterations in the activity and/or balance of stabilizing or destabilizing MAPs can profoundly affect microtubule function . The overexpression of stathmin , a destabilizing protein , has been reported to decrease sensitivity to paclitaxel and vinblastine . MAP -tau, a stabilizing protein, was a prognostic factor; however, it was not predictive for benefit from paclitaxel-based chemotherapy .

66. C ertain tumor subtypes may be sensitive to the taxane dosing schedule. In two randomized trials of low-dose, weekly paclitaxel, the luminal breast cancer subtype was found to have a better outcome compared with the control arm. This suggests that not only the drug, but also the schedule may influence the response to therapy and that genomic approaches may reveal these insights.