Cytoprotective Agents
Table 2 lists the protective agents that are currently available or under-going clinical evaluation. The cyto- protective agents currently in use are amifostine(Drug information on amifostine) (Ethyol), dexrazoxane (Zinecard), and mesna(Drug information on mesna) (Mesnex). Amifostine has the potential to protect a range of normal tissues against the toxicities of radiation and chemotherapy drugs that alter the structure and function of DNA (eg, platinum agents, alkylating agents). Dexrazoxane is specific for doxorubicin(Drug information on doxorubicin)-related cardiotoxicity, and mesna is specific for bladder toxicity from ifosfamide(Drug information on ifosfamide) or cyclophosphamide(Drug information on cyclophosphamide).
Amifostine
Amifostine, formerly known as WR-2721, is a naturally occurring thiol that can protect against cell damage by binding to the active species of alkylating agents or platinums or by scavenging free radicals. This drug arose from a classified nuclear warfare project sponsored by the US Army and was ultimately selected from more than 4,400 chemicals screened because of its superior radioprotective properties and safety profile.[22-24] Subsequently, amifostine was evaluated for its potential role in reducing the toxicity of chemotherapeutic drugs that alter the structure and function of DNA, such as alkylating agents and organoplatinum agents, as well as radiation therapy. Preclinical studies have demonstrated that amifostine can selectively protect a broad range of normal tissues, including bone marrow, gastric epithelium (including oral mucosa), heart, intestine, kidney, lungs, and salivary glands--but not neoplastic tissues--from the cytotoxic effects of chemotherapy and radiation.[25-27]
Mechanism of Action--Amifostine is a prodrug that is dephosphorylated by plasma membrane alkaline phosphatase to the free thiol WR-1065. It is postulated that WR-1065-mediated cell protection occurs through its binding to active species of platinums or alkylating agents, scavenging of oxygen-free radicals, and donation of hydrogen to DNA radicals.[22]
The mechanism by which amifostine selectively protects normal tissue is based on the higher concentration of free thiol achieved in normal organs than in tumors. Differences in the microenvironment of normal tissues and tumors result in differential uptake of the free thiol into normal tissues relative to tumor masses.[22] Tumors are relatively hypovascular, with resulting tissue hypoxia, anaerobic metabolism, and a low interstitial pH. The combined hypovascularity and low pH result in low rates of prodrug activation by alkaline phosphatase. In addition, the distribution of alkaline phosphatase in normal and malignant tissue differs; high concentrations of this enzyme are found in the capillaries and cell membranes of normal cells.
The selective protection of normal tissues results from reduced conversion of amifostine to the active protector WR-1065 and low uptake of WR-1065 by tumors.[28] Consequently, the steady-state concentration of the free thiol is as much as 100-fold greater in normal organs, such as bone marrow, kidney, and heart, than in tumor tissue (Figure 1).[23] Once the free thiol WR-1065 has entered a normal cell, it is available to detoxify the active species generated by alkylating agents, platinum agents, or radiation therapy (Figure 1).
In addition, because of the similarity of its propylamine structure to polyamine precursors, the free thiol may concentrate around DNA and confer preferential protection to this molecule.[29] Studies with Chinese hamster ovary cells have shown that WR-1065 protection is not related to fluctuating intracellular glutathione or cysteine(Drug information on cysteine) levels, but rather, may be related to inhibition of an inducible error-prone repair system or to the induction of phase II detoxification enzymes.[30]
In addition to protecting against the immediate effects of cytotoxic chemotherapy, sulfhydryl compounds such as amifostine may have important antimutagenic properties. Investigators have reported that amifostine protects against the formation of radiation-induced tumors in rodents.[31] In addition, whereas exposure to cisplatin(Drug information on cisplatin), bleomycin(Drug information on bleomycin), nitrogen mustard, and gamma- or neutron-irradiation can introduce mutations in the hypoxanthine-guanine phosphoribosyl transferase (hprt) gene locus of V79 Chinese hamster lung fibroblast cells, incubation of these cells with
WR-1065 (the active metabolite of amifostine) before administration of the cytotoxic agent significantly reduced mutation, resulting in calculated mutation protection factors of 7.1 for cis-platin, 2.8 for bleomycin, and 3.4 for nitrogen mustard. Such benefit could be significant, considering the potential for development of secondary malignancies from chemotherapy.[32,33]
Human pharmacokinetic studies of amifostine and its active metabolite WR-1065 demonstrate that more than 90% of the parent compound is cleared from the plasma within 6 minutes.[28] These data are extremely important in scheduling the administration of amifostine. Given its extremely short half-life, amifostine should be administered within 30 minutes of chemotherapy or radiation therapy, and several doses may be required to protect against cytotoxic drugs that have a long half-life and drugs that require a prolonged infusion time.
Clinical trials with amifostine were initiated in the 1980s. Early phase I trials were designed to determine dose-limiting side effects and the maximum tolerated dose of amifostine.[22,34] A maximum tolerated dose was not reached, but an acceptable tolerated dose, 740 mg/m², was established for use in phase II studies.[35] Subsequent clinical trials utilizing vigorous hydration and improved antiemetics determined that 910 mg/m² was the maximum tolerated dose of this drug.[36]
Initial clinical trials focused on the protective effects of amifostine against alkylating agent-induced hematologic toxicity and platinum-induced nephrotoxicity and neurotoxicity. In one such trial, amifostine, 740 mg/m², was administered before cisplatin was given at doses that escalated from 50 to 150 mg/m². Transient nephrotoxicity was observed in 40% of patients treated with 150 mg/m² of cisplatin plus amifostine but in none of the patients given 120 mg/m² of cisplatin plus amifostine.[34]
Compared with published clinical trials in which cisplatin alone was used, trials that added amifostine to cisplatin-based regimens appeared to offer protection against cisplatin-induced nephrotoxicity and neurotoxicity.[37,38] In addition, a controlled trial demonstrated that amifostine pretreatment decreased both the degree and duration of neutropenia associated with cyclophosphamide.[39]
More recently, a randomized, phase III study designed to evaluate the protective effects of amifostine in patients receiving cyclophosphamide and cis-platin was completed. A total of 242 patients with advanced ovarian cancer were randomized to receive six cycles of cyclophosphamide (1,000 mg/m²) and cisplatin (100 mg/m²), either alone or preceded by amifostine (910 mg/m²). Pretreatment with amifostine reduced cumulative treatment-related toxicities; specifically, the amifostine-treated patients had significantly fewer episodes of grade 4 neutropenia associated with fever or infection requiring hospitalization and antibiotics, as well as a reduction in cumulative neurotoxicity and nephrotoxicity associated with cisplatin.
Tumor response rates were equivalent in the two treatment arms, as documented at the time of second-look surgery. Median survival was comparable (31 months for both amifostine plus chemotherapy and chemotherapy alone), thus demonstrating a selective protective effect of amifostine against myelotoxicity, nephrotoxicity, and neurotoxicity with full preservation of tumor response.[40]
The preliminary results of a phase I study of patients with advanced malignancy treated with amifostine and escalating doses of paclitaxel(Drug information on paclitaxel) administered as a 3-hour infusion suggests that, in the presence of amifostine pretreatment, paclitaxel can be escalated beyond previously neurotoxic doses. Patients are currently receiving 310 mg/m² and thus far have not experienced significant neurotoxicity or arthralgias.[41]
The ability of amifostine to reduce treatment-limiting neurologic toxicity associated with both cisplatin and paclitaxel has important medical benefits in view of the increasing use of this combination in the treatment of ovarian cancer and other solid tumors. A planned randomized clinical trial will confirm the cytoprotective effects of amifostine against paclitaxel-induced toxicity.
Extensive preclinical studies showed that amifostine protects normal jejunum, colon, lung, and bone marrow tissues against the acute and late toxicities of radiation therapy. The radioprotective effects of this drug have now been demonstrated in a number of clinical studies.[25,35,42]
In a randomized trial, Liu et al compared the use of amifostine plus radiotherapy with radiotherapy alone in the treatment of rectal cancer.[42] They randomized 100 patients with inoperable, unresectable, or recurrent rectal carcinoma to receive equivalent doses of radiation with or without amifostine pretreatment. The incidence of moderate or severe late toxicities (alterations in bladder and GI mucosa) was significantly lower in the amifostine-plus- radiation arm than in the radiation-alone arm (0% vs 14%; P = .03). With a median follow-up of 2 years (range, 13 to 30 months), median survival is 15 months for the amifostine-plus-radiation arm, compared with 12.6 months for the radiation-alone arm.
In vitro studies have shown that amifostine pretreatment protects normal bone marrow progenitor cells from the cytotoxicity of the marrow-purging agent perfosfamide or mafosfamide (the active metabolite of cyclophosphamide) without protecting breast cancer cells or leukemia cells. On this basis, Shpall et al conducted a small randomized trial in patients with breast cancer undergoing high-dose chemotherapy followed by autologous bone marrow transplantation. Patients' bone marrows were purged by perfosfamide in the ex vivo setting, with or without preincubation with amifostine. When given before marrow purging, amifostine significantly shortened the time to engraftment, from 36 to 26 days (P = .042), decreased the number of platelet transfusions (P less than .017), and lessened the mean number of days patients required antibiotics (P less than .012).[43]
In a study conducted by Gorin, Douay, et al, amifostine enhanced the antitumor effect of mafosfamide against leukemia cells while simultaneously protecting bone marrow progenitor cells. This resulted in a sixfold increase in the therapeutic index.[44]
Clinical Use--Amifostine is generally well tolerated, but transient side effects include nausea, vomiting, hypotension, sneezing, a warm or flushed feeling, mild somnolence, a metallic taste during infusion, and occasional allergic reactions.[22,25] Emesis associated with amifostine is clearly dose-related and can be severe.
Clinically, the most significant adverse effect associated with amifostine is transient hypotension. Decreases in systolic blood pressure of more than 20 mm Hg that last longer than 5 minutes and symptomatic hypotension occur in fewer than 5% of patients. The median time to onset of hypotension is 14 minutes. The mechanism by which amifostine causes hypotension is unclear.
Transient hypocalcemia has been reported rarely. It is due to inhibition of parathyroid hormone secretion and direct inhibition of bone resorption.[45]
Amifostine is infused intravenously over a 15-minute period 30 minutes before chemotherapy or radiotherapy. Antihypertensive drugs should be withheld for 24 hours prior to amifostine therapy. Before amifostine administration, patients should be hydrated and treated with antiemetics (dexamethasone, 20 mg IV, and a serotonin antagonist) and IV fluids.
Administration of amifostine requires close patient monitoring. Blood pressure should be measured every 5 minutes during the 15-minute infusion. In the event of a significant drop in blood pressure (eg, less than 20 mm Hg) or the occurrence of symptoms associated with low blood pressure, the amifostine infusion should be interrupted. The patient should receive saline and be placed in the Trendelenburg position.
Clinicians must be aware of the importance of hydration and be familiar with the use of appropriate antiemetics before administering amifostine. In a recent trial of paclitaxel with amifostine cytoprotection in patients with advanced malignancies, pretreatment with dexamethasone(Drug information on dexamethasone) (20 mg), ondansetron(Drug information on ondansetron) (Zofran) (.15 mg/kg IV), cimetidine, diphenhydramine(Drug information on diphenhydramine), and lorazepam(Drug information on lorazepam) resulted in no interruption of amifostine infusion, no hypotension, and no significant nausea or vomiting in 42 cycles of therapy.[41]
In clinical trials involving drugs with relatively long half-lives (eg, carboplatin(Drug information on carboplatin) [Paraplatin]), administration of more than one dose of amifostine resulted in an increase in the maximally tolerated dose of carboplatin without additional myelosuppressive effects.[46]
Dexrazoxane
Preclinical studies suggest that the mechanism of antitumor activity of anthracyclines differs from the mechanism of cardiotoxicity, the dose-limiting toxicity of these agents. Cardiotoxicity is characterized by diffuse myocardial injury that can lead to chronic cardiomyopathy.[47,48] Two theories have been proposed to explain how anthracyclines alter mitochondrial membrane function, which leads to cardiac dysfunction. Both postulated biochemical mechanisms involve the production, through an iron-dependent process, of compounds with powerful oxidizing abilities. Cardiac myocytes are thought to be especially susceptible to this free-radical-mediated damage because they have lower levels of superoxide dismutase and catalase than other tissues.[47]
These findings led to the development of dexrazoxane (ICRF-187; [+]1,2-bis-[3,5 dioxopiperazinyl-1-yl] propane), a cyclized analog of ethylenediaminetetraacetic acid (EDTA) that could prevent toxicity by chelation of iron.[49] Distribution studies, conducted in animals with carbon-14-labeled dexrazoxane, demonstrated a lack of selective distribution to cardiac tissue, with the highest concentration in the kidney and liver.[48] In the myocardium, dexrazoxane undergoes hydrolytic ring-opening and chelates iron, reducing the amount available for the formation of oxygen radical-forming iron-doxorubicin complexes.[49] Furthermore, differences in the uptake and metabolism of dexrazoxane by cancer cells compared with myocardial cells may partially explain the differential protective effects of this agent.
Preclinical Studies--Substantial evidence from animal models supports the role of dexrazoxane in protecting against anthracycline-induced cardiomyopathy but not against other anthracycline-induced toxic effects.[48,50]
Clinical Studies--Dexrazoxane was initially evaluated as an antitumor agent. Its dose-limiting toxicities, observed in phase I trials, included transient leukopenia and moderate thrombocytopenia. Limited phase II studies showed only minimal antitumor activity.[48] On the basis of the preclinical data, dexrazoxane was evaluated as a cardioprotective agent against anthracycline-induced cardiotoxicity.
In a study conducted by Speyer et al,[51] 150 patients with metastatic breast cancer were randomized to receive chemotherapy with fluorouracil(Drug information on fluorouracil) (500 mg/m²), cyclophosphamide (500 mg/m²), and doxorubicin (50 mg/m²), either alone or preceded by dexrazoxane (1,000 mg/m² administered intravenously over a 15-minute period 30 minutes before chemotherapy). Patients in the dexrazoxane group received more cycles than control patients (median, 11 vs 9 cycles; P less than .01), as well as higher cumulative doses of doxorubicin (median, 500 vs 441 mg/m²; P less than .05). The incidence of clinical congestive heart failure was significantly lower in the dexrazoxane group than in the control group (2 vs 20 patients, respectively; P less than .0001). There was also a significant difference (P less than .000001) in the number of patients who could not complete the study because of a decrease in resting left-ventricular ejection fraction (5 patients in the dexrazoxane group vs 32 patients in the control group).
There was little difference in noncardiac toxicities between the two treatment groups. The addition of dexrazoxane to the chemotherapeutic regimen caused, after two cycles of therapy, a more pronounced suppression of white blood cell count (2.3 ×109/L, vs 2.6 ×109/L in controls; P less than .05) and platelet count (187 ×109/L, vs 226 ×109/L in controls; P = NS).
Complete responses were seen in 7 patients (9%) in the dexrazoxane group, as compared with 5 (7%) in the control group, and partial responses were noted in 21 patients (28%) in the dexrazoxane group and 25 (34%) in the control group. In addition, median time to progression of disease was 9.4 months in the group receiving chemotherapy alone, as opposed to 10.1 months in those receiving chemotherapy plus dexrazoxane. Median overall survival was 18.3 months in the dexrazoxane group vs 16.7 months in the control group. A higher percentage in the dexrazoxane group than in the control group were taken off the study because of disease progression (68% vs 31%).[51]
Clinical Use--Marketing authorization for dexrazoxane was granted "for the reduction of the incidence and severity of cardiomyopathy associated with doxorubicin administration in women with metastatic breast cancer who have received a cumulative doxorubicin dose of 300 mg/m² and who would benefit from continuing therapy with doxorubicin." The dose of dexrazoxane is based on the dose of doxorubicin. The recommended dosage ratio of dexrazoxane to doxorubicin is 10:1 (500 mg/m² of dexrazoxane; 50 mg/m² of doxorubicin).
Dexrazoxane is administered as a slow IV push or rapid IV infusion 30 minutes before doxorubicin administration. It is recommended that dexrazoxane not be initiated at the same time as chemotherapy, because in one study, a lower overall response rate was observed in patients who received chemotherapy with dexrazoxane than in those treated with chemotherapy alone (41% vs 50%).[52]
Dexrazoxane therapy is associated with moderate myelosuppression affecting white blood cell and platelet counts.[48]
Mesna
The sulfhydryl compound, mesna (2-mercaptoethane sulfate, sodium salt), was developed as a prophylactic agent to prevent ifosfamide- and cyclophosphamide-induced hemorrhagic cystitis.[53] The major source of urothelial toxicity is believed to be urinary excretion of acrolein, a metabolite of these chemotherapeutic agents. Mesna was designed to function in the urinary tract to detoxify urotoxic metabolites.
Following IV administration, mesna rapidly undergoes oxidation to mesna disulfide. In the urinary tract, the sulfhydryl groups of mesna react with the terminal methyl group of acrolein, forming a nontoxic thioether. The presence of mesna also inhibits spontaneous breakdown of cyclophosphamide to acrolein in the urine. Recent studies show, however, that mesna is only partially effective in protecting renal tubules from ifosfamide toxicity. The incomplete protection results in tubulotoxicity in the absence of urotoxicity.[54]
Mesna is generally administered intravenously, usually on a fractionated dosing schedule. One schedule is a loading dose, equivalent to 20% of the ifosfamide dose, given 15 minutes before ifosfamide administration and followed by two similar doses 4 and 8 hours after ifosfamide administration. Mesna doses as high as 60% to 120% of cyclophosphamide have been used and given at similar intervals. Since mesna is hydrophilic, it does not penetrate cells and thus does not interfere with the antitumor activity of the chemotherapeutic agents.
