Studies on the antidiuretic effect of cyclophosphamide: vasopressin release and sodium excretion

Low-Dose Cyclophosphamide Associated With Hyponatremia and Hepatotoxicity

Cyclophosphamide (CY) is an alkylating agent often used as a chemotherapeutic agent, with increasing use as an immunosuppressant. Cyclophosphamide has many established adverse effects, including hyponatremia and limited reports of hepatotoxicity, particularly in high-dose treatment. A case of simultaneous hyponatremia and acute liver injury associated with the initiation of cyclophosphamide two weeks prior is discussed here. A 73-year-old male with acquired hemophilia A/factor VIII deficiency presented to the emergency department (ED) with four days of hip pain and was found to have jaundice and confusion. Laboratory evaluation demonstrated hyponatremia and an acute liver injury associated with his recent cyclophosphamide use. With the discontinuation of the offending agent and sodium correction, he made a full recovery. Cyclophosphamide-induced hyponatremia is likely secondary to the nephrogenic syndrome of inappropriate antidiuresis (NSIAD) and is most often associated with high-dose regimens. While the mechanism of hepatotoxicity requires further study, it is likely dose-dependent and related to excess levels of 4-hydroxycyclophosphamide (HCY). The management of cyclophosphamide-induced water toxicity and hepatotoxicity is centered around the discontinuation of medication, the correction of electrolyte abnormalities, and supportive treatment.

Pathophysiology of Drug-Induced Hyponatremia

Drug-induced hyponatremia caused by renal water retention is mainly due to syndrome of inappropriate antidiuresis (SIAD). SIAD can be grouped into syndrome of inappropriate antidiuretic hormone secretion (SIADH) and nephrogenic syndrome of inappropriate antidiuresis (NSIAD). The former is characterized by uncontrolled hypersecretion of arginine vasopressin (AVP), and the latter is produced by intrarenal activation for water reabsorption and characterized by suppressed plasma AVP levels. Desmopressin is useful for the treatment of diabetes insipidus because of its selective binding to vasopressin V2 receptor (V2R), but it can induce hyponatremia when prescribed for nocturnal polyuria in older patients. Oxytocin also acts as a V2R agonist and can produce hyponatremia when used to induce labor or abortion. In current clinical practice, psychotropic agents, anticancer chemotherapeutic agents, and thiazide diuretics are the major causes of drug-induced hyponatremia. Among these, vincristine and ifosfamide were associated with sustained plasma AVP levels and are thought to cause SIADH. However, others including antipsychotics, antidepressants, anticonvulsants, cyclophosphamide, and thiazide diuretics may induce hyponatremia by intrarenal mechanisms for aquaporin-2 (AQP2) upregulation, compatible with NSIAD. In these cases, plasma AVP levels are suppressed by negative feedback. In rat inner medullary collecting duct cells, haloperidol, sertraline, carbamazepine, and cyclophosphamide upregulated V2R mRNA and increased cAMP production in the absence of vasopressin. The resultant AQP2 upregulation was blocked by a V2R antagonist tolvaptan or protein kinase A (PKA) inhibitors, suggestive of the activation of V2R-cAMP-PKA signaling. Hydrochlorothiazide can also upregulate AQP2 in the collecting duct without vasopressin, either directly or via the prostaglandin E2 pathway. In brief, nephrogenic antidiuresis, or NSIAD, is the major mechanism for drug-induced hyponatremia. The associations between pharmacogenetic variants and drug-induced hyponatremia is an area of ongoing research.

The Role of Vasopressin V2 Receptor in Drug-Induced Hyponatremia

Hyponatremia is frequently encountered in clinical practice and usually induced by renal water retention. Many medications are considered to be among the various causes of hyponatremia, because they either stimulate the release of arginine vasopressin (AVP) or potentiate its action in the kidney. Antidepressants, anticonvulsants, antipsychotics, diuretics, and cytotoxic agents are the major causes of drug-induced hyponatremia. However, studies addressing the potential of these drugs to increase AVP release from the posterior pituitary gland or enhance urine concentration through intrarenal mechanisms are lacking. We previously showed that in the absence of AVP, sertraline, carbamazepine, haloperidol, and cyclophosphamide each increased vasopressin V2 receptor (V2R) mRNA and aquaporin-2 (AQP2) protein and mRNA expression in primary cultured inner medullary collecting duct cells. The upregulation of AQP2 was blocked by the V2R antagonist tolvaptan or protein kinase A (PKA) inhibitors. These findings led us to conclude that the nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is the main mechanism of drug-induced hyponatremia. Previous studies have also shown that the V2R has a role in chlorpropamide-induced hyponatremia. Several other agents, including metformin and statins, have been found to induce antidiuresis and AQP2 upregulation through various V2R-independent pathways in animal experiments but are not associated with hyponatremia despite being frequently used clinically. In brief, drug-induced hyponatremia can be largely explained by AQP2 upregulation from V2R-cAMP-PKA signaling in the absence of AVP stimulation. This paper reviews the central and nephrogenic mechanisms of drug-induced hyponatremia and discusses the importance of the canonical pathway of AQP2 upregulation in drug-induced NSIAD.

Anticancer Medications and Sodium Dysmetabolism

Therapeutic advances have revolutionised cancer treatment over the last two decades, but despite improved survival and outcomes, adverse effects to anticancer therapy such as dyselectrolytaemias do occur and need to be managed appropriately. This review explores essential aspects of sodium homeostasis in cancer with a focus on alterations arising from anticancer medications. Sodium and water balance are tightly regulated by close interplay of stimuli arising from hypothalamic osmoreceptors, arterial and atrial baroreceptors and the renal juxtaglomerular apparatus. This delicate balance can be disrupted by cancer itself, as well as the medications used to treat it. Some of the conventional chemotherapeutics, such as alkylating agents and platinum-based drugs, can cause hyponatraemia and, on rare occasions, hypernatraemia. Other conventional agents such as vinca alkaloids, as well as newer targeted cancer therapies including small molecule inhibitors and monoclonal antibodies, can cause hyponatraemia, usually as a result of inappropriate antidiuretic hormone secretion. Hyponatraemia can also sometimes occur secondarily to drug-induced hypocortisolism or salt-wasting syndromes. Another atypical but distinct mechanism for hyponatraemia is via pituitary dysfunction induced by immune checkpoint inhibitors. Hypernatraemia is uncommon and occasionally ensues as a result of drug-induced nephrogenic diabetes insipidus. Identification of the aetiology and appropriate management of these conditions, in addition to averting treatment-related problems, can be lifesaving in critical situations.

Electrolyte Disorders Induced by Antineoplastic Drugs

The use of antineoplastic drugs has a central role in treatment of patients affected by cancer but is often associated with numerous electrolyte derangements which, in many cases, could represent life-threatening conditions. In fact, while several anti-cancer agents can interfere with kidney function leading to acute kidney injury, proteinuria, and hypertension, in many cases alterations of electrolyte tubular handling and water balance occur. This review summarizes the mechanisms underlying the disturbances of sodium, potassium, magnesium, calcium, and phosphate metabolism during anti-cancer treatment. Platinum compounds are associated with sodium, potassium, and magnesium derangements while alkylating agents and Vinca alkaloids with hyponatremia due to syndrome of inappropriate antidiuretic hormone secretion (SIADH). Novel anti-neoplastic agents, such as targeted therapies (monoclonal antibodies, tyrosine kinase inhibitors, immunomodulators, mammalian target of rapamycin), can induce SIADH-related hyponatremia and, less frequently, urinary sodium loss. The blockade of epidermal growth factor receptor (EGFR) by anti-EGFR antibodies can result in clinically significant magnesium and potassium losses. Finally, the tumor lysis syndrome is associated with hyperphosphatemia, hypocalcemia and hyperkalemia, all of which represent serious complications of chemotherapy. Thus, clinicians should be aware of these side effects of antineoplastic drugs, in order to set out preventive measures and start appropriate treatments.

Pediatric Bone Marrow Transplantation: Intensive Care Management

Bone marrow transplantation (BMT) has assumed in creasing importance in the treatment of bone marrow failure, hematopoietic malignancies, congenital immuno deficiencies, and solid tumors. Children undergoing BMT are at high risk for infection and hemorrhage dur ing the period of aplasia. In addition, life-threatening complications of circulatory, pulmonary, gastrointesti nal, hepatic, and renal function are common and fre quently require intensive supportive care. This review provides an overview of pediatric BMT that focuses on management problems relevant to intensive care. Thor ough pretransplantation assessment of underlying organ dysfunction is mandatory before undertaking BMT. The complications associated with preconditioning regi mens that use total body irradiation and high doses of ablative chemotherapy are described. Finally, problems involving individual organs are discussed by systems. The challenge of improving the results of BMT in the treatment of childhood malignant and hematopoietic disorders depends, in large part, on successful preven tive measures and good management of complications that occur immediately before and within the first 100 days after transplantation. As BMT is indicated for treat ment of an increasing number of diseases, more patients will require the care of intensivists familiar with trans plantation-related complications.

Electrolyte disorders associated with the use of anticancer drugs

The use of anticancer drugs is beneficial for patients with malignancies but is frequently associated with the occurrence of electrolyte disorders, which can be hazardous and in many cases fatal. The review presents the electrolyte abnormalities that can occur with the use of anticancer drugs and provides the related mechanisms. Platinum-containing anticancer drugs induce hypomagnesemia, hypokalemia and hypocalcemia. Moreover, platinum-containing drugs are associated with hyponatremia, especially when combined with large volumes of hypotonic fluids aiming to prevent nephrotoxicity. Alkylating agents have been linked with the occurrence of hyponatremia [due to syndrome of inappropriate antidiuretic hormone secretion (SIADH)] and Fanconi's syndrome (hypophosphatemia, aminoaciduria, hypouricemia and/or glucosuria). Vinca alkaloids are associated with hyponatremia due to SIADH. Epidermal growth factor receptor monoclonal antibody inhibitors induce hypomagnesemia, hypokalemia and hypocalcemia. Other, monoclonal antibodies, such as cixutumumab, cause hyponatremia due to SIADH. Tyrosine kinase inhibitors are linked to hyponatremia and hypophosphatemia. Mammalian target of rapamycin inhibitors induce hyponatremia (due to aldosterone resistance), hypokalemia and hypophosphatemia. Other drugs such as immunomodulators or methotrexate have been also associated with hyponatremia. The administration of estrogens at high doses, streptozocin, azacitidine and suramin may induce hypophosphatemia. Finally, the drug-related tumor lysis syndrome is associated with hyperphosphatemia, hyperkalemia and hypocalcemia. The prevention of electrolyte derangements may lead to reduction of adverse events during the administration of anticancer drugs.

Cyclophosphamide-induced vasopressin-independent activation of aquaporin-2 in the rat kidney

Because cyclophosphamide-induced hyponatremia was reported to occur without changes in plasma vasopressin in a patient with central diabetes insipidus, we hypothesized that cyclophosphamide or its active metabolite, 4-hydroperoxycyclophosphamide (4-HC), may directly dysregulate the expression of water channels or sodium transporters in the kidney. To investigate whether intrarenal mechanisms for urinary concentration are activated in vivo and in vitro by treatment with cyclophosphamide and 4-HC, respectively, we used water-loaded male Sprague-Dawley rats, primary cultured inner medullary collecting duct (IMCD) cells, and IMCD suspensions prepared from male Sprague-Dawley rats. In cyclophosphamide-treated rats, significant increases in renal expression of aquaporin-2 (AQP2) and Na-K-2Cl cotransporter type 2 (NKCC2) were shown by immunoblot analysis and immunohistochemistry. Apical translocation of AQP2 was also demonstrated by quantitative immunocytochemistry. In both rat kidney and primary cultured IMCD cells, significant increases in AQP2 and vasopressin receptor type 2 (V2R) mRNA expression were demonstrated by real-time quantitative PCR analysis. Confocal laser-scanning microscopy revealed that apical translocation of AQP2 was remarkably increased when primary cultured IMCD cells were treated with 4-HC in the absence of vasopressin stimulation. Moreover, AQP2 upregulation and cAMP accumulation in response to 4-HC were significantly reduced by tolvaptan cotreatment in primary cultured IMCD cells and IMCD suspensions, respectively. We demonstrated that, in the rat kidney, cyclophosphamide may activate V2R and induce upregulation of AQP2 in the absence of vasopressin stimulation, suggesting the possibility of drug-induced nephrogenic syndrome of inappropriate antidiuresis (NSIAD).

Diversity in renal function monitoring and dose modifications during treatment for childhood cancer: a call for standardization

Despite changes in survival and drug tolerability, nephrotoxicity remains an important complication of chemotherapy. To provide cutting-edge care for children with cancer oncologist must be familiar with their nephrotoxic potential. Careful monitoring of renal function during treatment is therefore indicated. Well-defined guidelines for this are lacking. We reviewed current DCOG protocols and showed that monitoring of renal function during treatment varies widely between protocols. In some protocols recommended renal function measures are inappropriate given the chemotherapy prescribed. Advices on dose modifications in case of renal dysfunction also vary, even with comparable regimens. These differences are unwanted and call for standardization.

The role of proximal nephron in cyclophosphamide-induced water retention: preliminary data

Cyclophosphamide is clinically useful in treating malignancy and rheumatologic disease, but has limitations in that it induces hyponatremia. The mechanisms by which cyclophosphamide induces water retention in the kidney have yet to be identified. This study was undertaken to test the hypothesis that cyclophosphamide may produce water retention via the proximal nephron, where aquaporin-1 (AQP1) and aquaporin-7 (AQP7) water channels participate in water absorption. To test this hypothesis, we gave a single dose of intraperitoneal cyclophosphamide to male Sprague-Dawley rats and treated rabbit proximal tubule cells (PTCs) with 4-hydroperoxycyclophosphamide (4-HC), an active metabolite of cyclophosphamide. In the short-term 3-day rat study, AQP1 protein expression was significantly increased in the whole kidney homogenates by cyclophosphamide administration at 48 (614 ± 194%, P < 0.005), and 96 (460 ± 46%, P < 0.05) mg/kg BW compared with vehicle-treated controls. Plasma sodium concentration was significantly decreased (143 ± 1 vs. 146 ± 1 mEq/L, P < 0.05) by cyclophosphamide 100 mg/kg BW in the long-term 6-day rat study. When primary cultured rabbit PTCs were treated with 4-HC for 24 hours, the protein expressions of AQP1 and AQP7 were increased in a dose-dependent manner. Quantitative polymerase chain reaction revealed no significant changes in the mRNA levels of AQP1 and AQP7 from cyclophosphamide-treated rat renal cortices. From these preliminary data, we conclude that the proximal nephron may be involved in cyclophosphamide-induced water retention via AQP1 and AQP7 water channels. Further studies are required to demonstrate intracellular mechanisms that affect the expression of AQP proteins.

Hyponatraemia induced by low-dose intravenous pulse cyclophosphamide

BACKGROUND

Cyclophosphamide is an alkylating agent and was traditionally known to potentiate the renal action of vasopressin. Although low-dose intravenous pulse cyclophosphamide therapy is being used extensively in the treatment of malignant and rheumatological diseases, there have been only a few case reports of cyclophosphamide-induced hyponatraemia.

METHODS

Clinical data were retrospectively analysed from 84 patients (42 lupus nephritis; 42 non-Hodgkin's lymphoma; a total of 112 treatment episodes) admitted for intravenous pulse cyclophosphamide (500-750 mg/m(2)) therapy. In all patients, half-isotonic saline was used for prophylactic hydration. Cyclophosphamide-induced hyponatraemia was defined as serum sodium concentration <135 mEq/L at 24 hours after the therapy in patients whose basal serum sodium concentrations were normal.

RESULTS

After the low-dose intravenous pulse cyclophosphamide, serum sodium concentration significantly decreased from 139.9 +/- 3.5 to 137.9 +/- 5.1 mEq/L (P < 0.001). Cyclophosphamide-induced hyponatraemia occurred in 15 treatment episodes (13.4%) from 12 patients (14.3%). Patients with hyponatraemia were significantly older than those without hyponatraemia (57.3 +/- 14.7 vs. 40.0 +/- 17.0 years, P < 0.01). Hyponatraemia was associated with male sex on univariate analysis (P < 0.05), but not on multivariate analysis. No factors were found to independently predict the occurrence of cyclophosphamide-induced hyponatraemia when multivariate analysis was performed including parameters age, sex, underlying disease, presence or absence of comorbidities associated with hyponatraemia, presence or absence of concurrent medications associated with hyponatraemia and dose of cyclophosphamide.

CONCLUSIONS

Hyponatraemia occurring after low-dose intravenous pulse cyclophosphamide is not rare, especially when hypotonic solutions are adopted for hydration protocol. Thus, the use of hypotonic fluids should be avoided when using cyclophosphamide. Instead, isotonic solutions should be used if a forced diuresis is required.

Causes and mechanisms of acid-base and electrolyte abnormalities in cancer patients

Patients with cancer frequently exhibit acid-base and electrolyte disturbances that complicate their management and prolong their hospitalization. The mechanisms encountered for these abnormalities are multifactorial in origin. Both the underlying disease and the therapeutic interventions can contribute to the development of these disturbances. An understanding of the mechanisms involved in their pathogenesis is of paramount importance for their prevention and treatment in cancer patients. This article briefly reviews the causes and the pathophysiology of acid-base and electrolyte abnormalities observed in cancer patients.

Metabolism and transport of oxazaphosphorines and the clinical implications

The oxazaphosphorines including cyclophosphamide (CPA), ifosfamide (IFO), and trofosfamide represent an important group of therapeutic agents due to their substantial antitumor and immuno-modulating activity. CPA is widely used as an anticancer drug, an immunosuppressant, and for the mobilization of hematopoetic progenitor cells from the bone marrow into peripheral blood prior to bone marrow transplantation for aplastic anemia, leukemia, and other malignancies. New oxazaphosphorines derivatives have been developed in an attempt to improve selectivity and response with reduced toxicity. These derivatives include mafosfamide (NSC 345842), glufosfamide (D19575, beta-D-glucosylisophosphoramide mustard), NSC 612567 (aldophosphamide perhydrothiazine), and NSC 613060 (aldophosphamide thiazolidine). This review highlights the metabolism and transport of these oxazaphosphorines (mainly CPA and IFO, as these two oxazaphosphorine drugs are the most widely used alkylating agents) and the clinical implications. Both CPA and IFO are prodrugs that require activation by hepatic cytochrome P450 (CYP)-catalyzed 4-hydroxylation, yielding cytotoxic nitrogen mustards capable of reacting with DNA molecules to form crosslinks and lead to cell apoptosis and/or necrosis. Such prodrug activation can be enhanced within tumor cells by the CYP-based gene directed-enzyme prodrug therapy (GDEPT) approach. However, those newly synthesized oxazaphosphorine derivatives such as glufosfamide, NSC 612567 and NSC 613060, do not need hepatic activation. They are activated through other enzymatic and/or non-enzymatic pathways. For example, both NSC 612567 and NSC 613060 can be activated by plain phosphodiesterase (PDEs) in plasma and other tissues or by the high-affinity nuclear 3'-5' exonucleases associated with DNA polymerases, such as DNA polymerases and epsilon. The alternative CYP-catalyzed inactivation pathway by N-dechloroethylation generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde (CAA). Various aldehyde dehydrogenases (ALDHs) and glutathione S-transferases (GSTs) are involved in the detoxification of oxazaphosphorine metabolites. The metabolism of oxazaphosphorines is auto-inducible, with the activation of the orphan nuclear receptor pregnane X receptor (PXR) being the major mechanism. Oxazaphosphorine metabolism is affected by a number of factors associated with the drugs (e.g., dosage, route of administration, chirality, and drug combination) and patients (e.g., age, gender, renal and hepatic function). Several drug transporters, such as breast cancer resistance protein (BCRP), multidrug resistance associated proteins (MRP1, MRP2, and MRP4) are involved in the active uptake and efflux of parental oxazaphosphorines, their cytotoxic mustards and conjugates in hepatocytes and tumor cells. Oxazaphosphorine metabolism and transport have a major impact on pharmacokinetic variability, pharmacokinetic-pharmacodynamic relationship, toxicity, resistance, and drug interactions since the drug-metabolizing enzymes and drug transporters involved are key determinants of the pharmacokinetics and pharmacodynamics of oxazaphosphorines. A better understanding of the factors that affect the metabolism and transport of oxazaphosphorines is important for their optional use in cancer chemotherapy.

Central gustatory lesions and learned taste aversions: unconditioned stimuli

The efficacy of two different unconditioned stimuli (US) in producing conditioned taste aversion (CTA) was tested in rats after bilateral ibotenic acid (IBO) lesions of the gustatory nucleus of thalamus (TTAx) and the medial and lateral parabrachial nuclei (mPBNx, lPBNx). An initial study determined an equivalent dose for the two USs, LiCl and cyclophosphamide (CY), using non-lesioned rats. Subsequently, using a separate set of lesioned animals and their sham controls (SHAM), injections of CY were paired 3 times with one of two taste stimuli (CSs), 0.1 M NaCl for half the rats in each group, 0.2 M sucrose for the other half. After these conditioning trials, the CS was presented twice more without the US, first in a 1-bottle test, then in a 2-bottle choice with water. The acquisition and test trials had 2 intervening water-only days to assure complete rehydration. Two weeks later, the same rats were tested again for acquisition of a CTA using LiCl as the US and the opposite CS as that used during the CY trials. The SHAM and TTAx groups learned to avoid consuming the taste associated with either CY or LiCl treatment. The two PBNx groups failed to learn an aversion regardless of the US.

Water intoxication induced by low-dose cyclophosphamide in two patients with systemic lupus erythematosus

Cyclophosphamide (CY) is an alkylating agent used to treat a variety of autoimmune disorders. Water intoxication is a well-known complication of high-dose intravenous (i.v.) CY, but is rare in patients treated with low dose i.v. CY. We describe two patients with lupus nephritis and water intoxication following low dose i.v. CY. The first patient was treated with oral prednisolone and azathioprine for eight weeks with inadequate response and persistent renal inflammatory activity. Eight hours after the first i.v. CY pulse she had a grand mal seizure. The second patient had WHO class III lupus nephritis, and after a single i.v. CY pulse developed vomiting, diarrhoea and grand mal seizures. They were both fluid-restricted and their serum sodium levels returned to normal. In conclusion, even at low doses i.v. CY may induce hyponatremia related to inappropriate antidiuretic hormone secretion. This potentially life-threatening complication of i.v. CY could be minimized by avoidance of overhydration following pulse i.v. CY.

Fluid and electrolyte abnormalities

Patients with cancer are at risk for developing a variety of fluid and electrolyte disturbances caused by the disease process or by complications from therapy. An understanding of the pathophysiology of these potential abnormalities allows the clinician to manage patients expectantly and to avoid severe metabolic disarray by correcting imbalances promptly.

Pulse intravenous cyclophosphamide therapy for dermatologic disorders

Cyclophosphamide is a potent immunosuppressive agent that has an important role in the treatment of autoimmune, neoplastic, granulomatous, and neutrophilic disorders. Pulse intravenous cyclophosphamide has been shown to be efficacious for several dermatologic disorders, particularly pemphigus vulgaris, with a low incidence of toxicity reported. As reported earlier, studies performed on the use of pulse intravenous cyclophosphamide in the treatment of a variety of dermatology-related diseases strongly suggest that the toxicities frequently noted with the use of oral cyclophosphamide therapy may be significantly less common with pulse intravenous administration of cyclophosphamide. The short follow-up period of patients treated with this modality so far, however, requires constant vigilance for the development of side effects, particularly secondary malignancy. At this time, pulse intravenous cyclophosphamide is a promising treatment modality with an acceptable risk profile for moderate-to-severe dermatologic diseases recalcitrant to standard therapy. Prospective comparative trials are needed to assess further the efficacy and toxicity of this therapy.

High-dose chemotherapy regimens for solid tumors

High-dose chemotherapy with blood progenitor cell transplantation is increasingly recognized as a potentially valuable treatment for breast cancer, germ cell cancer, ovarian cancer and other solid tumors. A variety of cytotoxic drugs, particularly alkylating agents, have been investigated either alone or in combinations. Current, predominantly small, phase I and phase II clinical trials to not adequately compare the efficacy of these regiments and patterns of dose-limiting extramedullary toxicity are emerging. Busulfan, carmustine (BCNU) and mitomycin C cause veno-occlusive disease (VOD) of the liver in some patients and the latter two agents also cause interstitial pneumonitis. Cisplatin and ifosfamide only allow minor dose escalation before renal failure becomes prohibitive. Cyclophosphamide, thiotepa, melphalan and etoposide allow substantial dose escalation above standard and are mainly associated with mucositis. Moderate dose escalations of mitoxantrone and carboplatin are possible, limited by cardiotoxicity and neurotoxicity, respectively. Advances in supportive care have abolished bone marrow suppression as the dose-limiting toxicity in chemotherapy. Severe and potentially fatal extramedullary toxicity following high-dose chemotherapy can only be avoided by administering agents with predictable toxicity patterns and by carefully considering their clinical pharmacology.

Clinical pharmacokinetics of cyclophosphamide

Cyclophosphamide has been in clinical use for the treatment of malignant disease for over 30 years. It remains one of the most useful anticancer agents, and is also widely used for its immunosuppressive properties. Cyclophosphamide is inactive until it undergoes hepatic transformation to form 4-hydroxycyclophosphamide, which then breaks down to form the ultimate alkylating agent, phosphoramide mustard. Sensitive and specific methods are now available for the measurement of cyclophosphamide, its metabolites and its stereoisomers in plasma and urine. The pharmacokinetics of cyclophosphamide have been understood for many years; those of the cytotoxic metabolites have been described more recently. The pharmacokinetics are not significantly altered in the presence of hepatic or renal insufficiency. As activity resides exclusively in the metabolites, whose pharmacokinetics are not predicted by those of the parent compound, correlations between cyclophosphamide pharmacokinetics and pharmacodynamics have not been demonstrated. Cyclophosphamide is used in doses that range from 1.5 to 60 mg/kg/day. A steep dose-response curve exists, and reductions in dose can lead to unfavourable outcomes. Myelosuppression is the dose-limiting toxicity, although in the setting of bone marrow transplantation, escalation beyond that dosage range is limited by cardiac toxicity. Longer term complications of cyclophosphamide therapy include infertility and an increased incidence of second malignancies. Cellular sensitivity to cyclophosphamide is a function of cellular thiol concentration, metabolism by aldehyde dehydrogenases to form inactive metabolites, and the ability of DNA to repair alkylated nucleotides. Whether alteration of these cellular functions will lead to further improvements in clinical outcomes is an area of active investigation.

The effect of cyclophosphamide on arginine vasopressin and the atrial natriuretic factor

The effect of cyclophosphamide (CTX) on the renal handling of water and sodium and on plasma concentrations of vasopressin (AVP) and of the atrial natriuretic factor (ANF) was studied in the rabbit. When compared to controls, animals receiving CTX (100 mg/kg IV) showed an increase in the diuresis and natriuresis. In addition, ANF plasma concentrations increased by 84% between 4 and 8 hours after CTX, while AVP plasma concentrations remained unchanged. It is concluded that in the rabbit CTX has a diuretic and a natriuretic effect that may, at least in part, be related to the increase in ANF plasma concentrations.