Topotecan for the treatment of recurrent or progressive central nervous system tumors - a pediatric oncology group phase II study.

Topotecan was studied as a 72 h infusion given every 3 weeks. Treatment began at a dose of 1.0 mg/m2/day and was increased to 1.25 mg/m2/day after the first 6 patients tolerated this higher dose without excessive toxicities. Eighty-eight evaluable children were accrued in 6 strata. There were no complete nor partial responses. Twenty subjects had stable disease (astrocytoma 5/11, malignant glioma 5/13, medulloblastoma 0/12, brain stem tumor 4/19, ependymoma 5/17, and miscellaneous histologies 1/16). Two patients (astrocytoma, ependymoma) completed the maximum 18 topotecan courses. The remaining 68 children developed progressive disease within 2 months. Myelosuppression was the main toxicity. Grade 4 leukopenia, neutropenia, anemia, and thrombocytopenia were observed in 18, 32, 5, and 23 participants, respectively. It was concluded that topotecan as given according to this schedule showed insufficient activity to promote it to frontline protocol usage.

Outpatient treatment of fever and neutropenia for low risk pediatric cancer patients.

BACKGROUND: Fever and neutropenia (F&N) is a common complication of cancer chemotherapy. It is conveniently managed by hospitalization and empiric administration of parenteral antibiotics. This study attempted to determine whether pediatric cancer patients with F&N identified as low risk for morbidity and mortality by clinical criteria at the time of presentation could be treated safely as outpatients. METHODS: Seventy-three episodes of F&N in 41 patients were studied prospectively over 2 years. Eligibility criteria included age > or =2 years, reliable caretakers, and residence within 1 hour of the hospital. Exclusion criteria included hemodynamic instability, dehydration, severe mucositis, pneumonia, leukemia/lymphoma induction therapy, bone marrow transplantation, or other serious comorbidity. Patients were evaluated, received a single dose of intravenous ceftazidime, and were observed for 3-16 hours. They were randomized to receive either oral ciprofloxacin or intravenous ceftazidime as outpatients. Patients were seen daily until they had been afebrile for at least 48 hours and had a rising absolute phagocyte count of >500 cells/microL. RESULTS: Sixty-three of 73 episodes (86%) were successfully managed on an outpatient basis. For 31 of 33 episodes in the ceftazidime arm, the patients remained outpatients, compared with 32 of 40 in the ciprofloxacin arm; this difference was not statistically significant. On average, patients remained febrile for 2.7 days and were treated for 4.7 days. Seventy-seven percent of episodes required no modification of initial antibiotic therapy. Of the 10 patients who were hospitalized, 4 had prolonged fever and 3 had emesis. Protracted neutropenia was associated with the need for hospitalization. There were no deaths, intensive care unit transfers, or serious complications. CONCLUSIONS: Carefully selected low risk children with fever and neutropenia can be treated safely as outpatients. Close daily medical scrutiny is required.

Economic and resource utilization analysis of outpatient management of fever and neutropenia in low-risk pediatric patients with cancer.

PURPOSE: To measure resource allocation in outpatient management of fever and neutropenia in low-risk pediatric patients with cancer and its impact on their families. PATIENTS AND METHODS: A prospective clinical trial was conducted. Eligible patients received a single dose of intravenous (IV) antibiotics and were observed for several hours in clinic. Patients were randomly assigned to continue either IV or oral antibiotics and were seen daily as outpatients. Charges were calculated based on the number of resources used and Medicare/Medicaid reimbursement schedules. A questionnaire was used to measure the impact of outpatient treatment on the family. RESULTS: Seventy-three episodes of fever and neutropenia were studied. The median duration of treatment was 4 days. Eighty-six percent of the episodes were managed without hospitalization. The median calculated charge was $1840. The median calculated charge for patients receiving oral antibiotics was $1544 and was significantly less than the $2039 median charge for outpatients treated with IV antibiotics. The estimated charge for comparable inpatient treatment was $4503. Nearly all families preferred outpatient care, and few reported a loss of work hours or increased child care expenses. CONCLUSIONS: Outpatient treatment of low-risk episodes of fever and neutropenia is substantially less costly than inpatient care and is preferred by most families.

Advances and controversies in the management of childhood brain tumors.

Important studies have been reported in the past year on the biology and management of pediatric central nervous system tumors. Pediatric low-grade gliomas and adult gliomas do not share similar molecular genetic features. There may be a role for high-dose chemotherapy with bone marrow rescue in relapsed medulloblastoma and germinoma. Malignant gliomas in infants appear to be distinct entities compared with adult malignant gliomas. Further investigational studies are needed in ependymoma and diffuse pontine gliomas because current therapies have not resulted in improvements in outcomes.

Second malignancies after Ewing's sarcoma: radiation dose-dependency of secondary sarcomas.

BACKGROUND: An excess risk of second malignancies has been reported in survivors of Ewing's sarcoma. We examined a multiinstitutional data base to reevaluate the risk among survivors of Ewing's sarcoma and to identify possible causal factors. METHODS: Information was derived from a data base that included 266 survivors of Ewing's sarcoma. Cumulative incidence rates of second malignancies were calculated. Contributions of clinical features, type and dose of chemotherapy, and cumulative radiation dose to the risk of second malignancies were evaluated. RESULTS: After a median follow-up duration of 9.5 years (range, 3.0 to 30), 16 patients have developed second malignancies, which included 10 sarcomas (five osteosarcomas, three fibrosarcomas, and two malignant fibrous histiocytomas) and six other malignancies (acute myeloblastic leukemia, acute lymphoblastic leukemia, meningioma, bronchioalveolar carcinoma, basal cell carcinoma, and carcinoma-in-situ of the cervix). The median latency to the diagnosis of the second malignancy was 7.6 years (range, 3.5 to 25.7). The estimated cumulative incidence rates at 20 years for any second malignancy and for secondary sarcoma were 9.2% (SD = 2.7%) and 6.5% (SD = 2.4%), respectively. The cumulative incidence rate of secondary sarcoma was radiation dose-dependent (P = .002). No secondary sarcomas developed among patients who had received less than 48 Gy, while the absolute risk of secondary sarcoma was 130 cases per 10,000 person-years of observation among patients who had received > or = 60 Gy. CONCLUSION: The overall risk of second malignancies after Ewing's sarcomas is similar to that associated with treatment for other childhood cancers. The radiation dose-dependency of secondary sarcomas justifies modification in therapy to reduce radiation doses.

Methods to detect P-glycoprotein-associated multidrug resistance in patients' tumors: consensus recommendations.

Multidrug resistance (MDR), especially that associated with overexpression of MDR1 and its product, P-glycoprotein (Pgp), is thought to play a role in the outcome of therapy for some human tumors; however, a consensus conclusion has been difficult to reach, owing to the variable results published by different laboratories. Many factors appear to influence the detection of Pgp in clinical specimens, including its low and heterogeneous expression; conflicting definitions of detection end points; differences in methods of sample preparation, fixation, and analysis; use of immunological reagents with variable Pgp specificity and avidity and with different recognition epitopes; use of secondary reagents and chromogens; and differences in clinical end points. Also, mechanisms other than Pgp overexpression may contribute to clinical MDR. The combined effect of these factors is clearly important, especially among tumors with low expression of Pgp. Thus, a workshop was organized in Memphis, Tennessee, to promote the standardization of approaches to MDR1 and Pgp detection in clinical specimens. The 15 North American and European institutions that agreed to participate conducted three preworkshop trials with well-characterized MDR myeloma and carcinoma cell lines that expressed increasing amounts of Pgp. The intent was to establish standard materials and methods for a fourth trial, assays of Pgp and MDR1 in clinical specimens. The general conclusions emerging from these efforts led to a number of recommendations for future studies: (a) although detection of Pgp and MDR1 is at present likely to be more reliable in leukemias and lymphomas than in solid tumors, accurate measurement of low levels of Pgp expression under most conditions remains an elusive goal; (b) tissue-specific controls, antibody controls, and standardized MDR cell lines are essential for calibrating any detection method and for subsequent analyses of clinical samples; (c) use of two or more vendor-standardized anti-Pgp antibody reagents that recognize different epitopes improves the reliability of immunological detection of Pgp; (d) sample fixation and antigen preservation must be carefully controlled; (e) multiparameter analysis is useful in clinical assays of MDR1/Pgp expression; (f) immunostaining data are best reported as staining intensity and the percentage of positive cells; and (g) arbitrary minimal cutoff points for analysis compromise the reliability of conclusions. The recommendations made by workshop participants should enhance the quality of research on the role of Pgp in clinical MDR development and provide a paradigm for investigations of other drug resistance-associated proteins.

P-glycoprotein expression at diagnosis may not be a primary mechanism of therapeutic failure in childhood rhabdomyosarcoma.

PURPOSE: To evaluate the prognostic significance of tumor cell P-glycoprotein (Pgp) expression at diagnosis in children with rhabdomyosarcoma. PATIENTS AND METHODS: A panel of three anti-Pgp monoclonal antibodies (mAb) (C219, C494, and JSB-1) that recognize different Pgp epitopes was used to measure Pgp expression in rhabdomyosarcoma specimens obtained at diagnosis from 76 patients treated at St Jude Children's Research Hospital from 1969 to 1991. Two separate experiments using different immunohistochemical methods (immune alkaline phosphatase and immunoperoxidase) were performed to evaluate Pgp expression. The immunostaining was graded using a semiquantitative scale corresponding to the percentage of tumor cells with detectable staining. The influence of Pgp expression on outcome was assessed by the Kaplan-Meier method and Cox regression analysis with stepwise selection. The relationship between Pgp expression and clinical features was assessed using the Mantel-Haenszel method. RESULTS: Pgp expression at diagnosis did not predict worse overall survival or progression-free survival when tested in either experiment with C219, C494, or JSB-1 separately. No association was shown between Pgp expression and clinical features (clinical group, primary site, or histology) or response. However, in the immune alkaline phosphatase experiment, patients whose tumors had more than 10% tumor cell staining with all three mAbs had a significantly higher rate of estimated 5-year survival (78% +/- 10%) than did all other patients (38% +/- 8%; P = .025). In this instance, Pgp expression had independent prognostic value after adjusting for clinical group. CONCLUSION: We found no strong association between Pgp expression at diagnosis and clinical features or extent of disease in pediatric rhabdomyosarcoma. Depending on the criteria used to define it, high Pgp expression at diagnosis does not predict poor outcome. Although a large prospective study is needed to provide definitive conclusions, our findings suggest that Pgp-mediated multidrug resistance may not be a primary mechanism of therapeutic failure in rhabdomyosarcoma.

Cerebrospinal fluid pharmacokinetics and penetration of continuous infusion topotecan in children with central nervous system tumors.

The purpose of this study was to describe the cerebrospinal fluid (CSF) penetration of topotecan in humans, to generate a pharmacokinetic model to simultaneously describe topotecan lactone and total concentrations in the plasma and CSF, and to characterize the CSF and plasma pharmacokinetics of topotecan administered as a continuous infusion (CI). Plasma and CSF samples were collected from 17 patients receiving 5.5 or 7.5 mg/m2 per day as a 24-h CI (5 patients, 7 courses), or 0.5 to 1.25 mg/m2 per day as a 72-h CI (12 patients, 12 courses). CSF samples were obtained from either a ventricular reservoir (VR) or a lumbar puncture (LP). Topotecan lactone and total (lactone plus hydroxy acid) concentrations were determined by HPLC and fluorescence detection. Using MAP-Bayesian modelling, a three-compartment model was fitted simultaneously to topotecan lactone and total concentrations in the plasma and CSF. The penetration of topotecan into the CSF was determined from the ratio of the CSF to the plasma area under the concentration-time curve. The median CSF ventricular lactone concentrations, obtained prior to the end of infusion (EOI), were 0.86, 1.4, 0.73, 5.3, and 4.6 ng/ml for patients receiving 0.5, 1.0, 1.25, 5.5, and 7.5 mg/m2 per day, respectively. EOI CSF lumbar lactone concentrations measured in three patients were 0.44, 1.1, and 1.7 ng/ml for topotecan doses of 1.0, 5.5, and 7.5 mg/m2 per day, respectively. In two patients receiving 1.25 mg/m2 per day, EOI CSF concentrations were obtained simultaneously from a VR and LP; the lumbar lactone concentrations were 30% and 49% lower than the ventricular concentrations. During a 24-h and a 72-h CI, the median CSF penetration of topotecan lactone was 0.29 (range 0.10 to 0.59) and 0.42 (range 0.11 to 0.86), respectively. A three-compartment model adequately described topotecan lactone and total concentrations in the plasma and CSF. Topotecan was therefore found to significantly penetrate into the CSF in humans. The pharmacokinetic model presented may be useful in the design of clinical studies of topotecan to treat CNS tumors.

Multidrug resistance in pediatric oncology.

Cancer survival among children and adolescents has improved markedly due to evolution of multimodal treatment that incorporates combination chemotherapy, radiation therapy and/or surgery. However, 20-30% of children with malignancies will succumb to their disease or complications associated with their disease or treatment. A major limiting factor to improvement in survival among these patients is the occurrence of intrinsic and/or acquired resistance to our treatment interventions, chemotherapy and radiotherapy. Among these mechanisms, multidrug resistance, the focus of this review, is a well-documented phenomenon whose biochemistry, pharmacology and molecular biology has been extensively studied. A role for multidrug resistance in chemoresistance and therapeutic failure in childhood malignancies is suggested by the observation of clinical resistance to treatment regimes containing agents that are known substrates of multidrug resistance mechanisms. With the current results from studies in rhabdomyosarcoma, neuroblastoma, osteosarcoma, Ewing's sarcoma, leukemia and retinoblastoma, the role of multidrug resistance is still unclear. Earlier studies attempted to define a role for P-glycoprotein-mediated multidrug resistance; however, a limited number of reports suggest that the multidrug-associated resistance protein may play an active role in neuroblastoma. Further studies will be necessary using standardized and uniform approaches for the analyses of these mechanisms. Clinical trials directed toward reversal of multidrug resistance are premature since the exact role of P-glycoprotein is controversial in pediatric malignancies, the role of other mechanisms of multidrug resistance must be assessed and selective inhibitors of multidrug resistance have yet to be developed.

Low-grade astrocytoma with neuraxis dissemination at diagnosis.

Little is known about low-grade astrocytoma with neuraxis dissemination at diagnosis. A review of medical records identified this phenomenon in eight of 150 pediatric patients evaluated between 1985 and 1994 for histologically confirmed low-grade astrocytoma. These patients (five male and three female) ranged in age from 5 months to 20 years (median 8 years). Symptoms of neuraxis disease were minimal or absent. Primary tumor sites were the hypothalamus in four cases, brainstem/spinal cord in three, and temporal lobe in one. Patterns of dissemination (evaluated by computerized tomography and/or magnetic resonance imaging techniques) appeared to be related to the primary site: hypothalamic tumors metastasized along the ventricular cerebrospinal fluid pathways, and tumors in other locations disseminated along subarachnoid pathways. Following initial treatment with chemotherapy (in three), partial resection (in one), radiation therapy (in three), and chemotherapy plus irradiation (in one), four patients required salvage therapy for progressive or recurrent disease. Seven of the eight patients are alive with stable or progressive disease 6 to 105 months postdiagnosis (median 15 months). Low-grade astrocytoma with initial neuraxis dissemination is responsive to chemotherapy and radiation, a proportion showing periods of stable disease. The optimum therapy or combination of therapies remains unclear.

Malignant melanocytic lesions of unknown primary site in children and adolescents.

BACKGROUND: Metastatic melanoma with an unidentified primary site represents 4% of all newly diagnosed cases of malignant melanoma in adults. Little is known of the incidence and clinicopathologic features of this clinical entity in the pediatric population. METHODS: We reviewed all previously diagnosed cases of malignant melanoma in children and adolescents (< 21 years) who were treated at our institution and identified three patients who presented with metastatic melanocytic lesions with an unidentified primary site. RESULTS: This clinical presentation accounted for 9% of all malignant melanocytic lesions treated at our center over a 20-year period. The clinicopathologic features were similar to those seen in adults. Two patients died of progressive disease within two years of presentation; the third is alive and disease-free 18 years post-diagnosis. CONCLUSIONS: Although uncommon in the pediatric population, malignant melanoma should be considered in the differential diagnosis of poorly differentiated disseminated malignancy that involves lymph nodes or viscera with no identifiable primary tumor.

Embryonal malignancies of unknown primary origin in children.

BACKGROUND: Little is known about embryonal malignancies of unknown primary origin in children. All such cases referred to a pediatric cancer center over a 30-year period were reviewed to delineate their clinical and prognostic features and to develop recommendations for evaluation and therapy. METHODS: Seventeen patients with embryonal malignancies of unknown primary origin were identified by reviewing the institutional solid tumor database. The medical records, imaging studies, laboratory records, and pathology reports for each patient were reviewed and tabulated. RESULTS: Pathology review confirmed the original histologic diagnosis in 15 of the cases. Final diagnoses after review were rhabdomyosarcoma (n = 9), neuroblastoma (n = 7), and Ewing's sarcoma (n = 1). At initial evaluation, an extensive array of radiologic studies revealed multiple abnormalities in 16 cases. Bone marrow sampling and urinary catecholamine analysis were valuable diagnostic tools. A primary site of origin was identified in five patients: by interval assessments during treatment in two cases, and at autopsy in three. Although the median survival was only 6 months, 3 patients are alive at 12, 15 and 17 years after diagnosis. CONCLUSIONS: An exhaustive search for a primary lesion appears unwarranted in children with disseminated embryonal malignancies. Tumor sampling should be adequate for the pathologic work-up to determine the definitive diagnosis. Although the chances of cure are slight, tumor specific therapy may improve the interval of disease control, the quality of life, and the possibility of long term survival.

High-activity 125I interstitial irradiation in the treatment of pediatric central nervous system tumors: a pilot study.

Malignant pediatric tumors of the central nervous system (CNS) have a poor prognosis, with local failure rates as high as 50%. In an attempt to improve local tumor control, we used stereotactic interstitial therapy with 125I implants in patients with recurrent/secondary or newly diagnosed CNS malignancies. Catheters were placed using computed tomography (CT) guidance; computerized dosimetry was completed with the aid of orthogonal films. Implants delivered 1,000 cGy/day to the tumor periphery (0.5 cm beyond the boundary of enhancement on CT scans), to a total dose of 60 Gy. Hyperfractionated external beam irradiation (HEBI), started 2-4 weeks after removal of implants, delivered total doses of 66-70.4 Gy in 110-cGy fractions twice daily to a 3-cm margin around the implant volume. Eight of the 11 patients with newly diagnosed tumors also received 48.4 Gy HEBI to the craniospinal axis. Tumor regression was noted at 2 months after implantation in the 4 patients treated for recurrent/secondary tumors; local progression was subsequently documented in 2 cases at 6 and 20 months after implantation, while a third patient died 6 months after implantation with no evidence of local recurrence. The remaining recurrent/secondary tumor patient has no evidence of active recurrence 15 months after implantation. Local control was maintained in 9 of the 11 patients treated for primary tumors for a median of 27 months (range 15 to 48+ months). The two local failures occurred at 5 and 7 months after implantation. Six patients are alive without evidence of progressive disease (median = 23 months after implantation). There were no severe acute toxicities, but 7 patients later developed histologically confirmed tumor necrosis. Quality of life assessment (QLA) following initial primary therapy with implantation was evaluated utilizing an established criteria and found to be excellent with only one child showing marked QLA score decrease which was related to neurosurgical intervention for radiation-induced necrosis and dysfunctional family social situation. This small series suggests that stereotactic 125I implantation followed by HEBI merits further evaluation in selected children with supratentorial malignant lesions.

Cross-resistance to antitumor diarylsulfonylureas and collateral sensitivity to mitochondrial toxins in a human cell line selected for resistance to the antitumor agent N-(5-indanylsulfonyl)-N'-(4-chlorophenyl)urea.

Diarylsulfonylurea (DSU) antitumor agents represent a new class of oncolytic compounds with an unknown, potentially novel, mechanism of action. At high concentrations of several of these agents, cytotoxicity appears to be a consequence of uncoupling of mitochondria. However, the mechanism of action at pharmacologically achievable concentrations is unknown. To further study these agents a subline of human colon carcinoma, GC3/c1, was selected for resistance to N-(5-indanylsulfonyl)-N'-(4-chlorophenyl)urea (ISCU) (Sulofenur). This clone (designated LYC5) was stably resistant for 2 years in the absence of selection pressure and was characterized for cross-resistance to other antitumor DSU and therapeutically used oncolytic agents. LYC5 was cross-resistant to six of seven DSU analogues examined when cells were exposed to drugs for 7 days. However, the degree of resistance was inversely related to the potency of the individual DSU against the parental GC3/c1 clone. Consequently, against LYC5 cells there was a relatively narrow range for concentrations inhibiting colony formation by 50% (4-fold), compared with that in GC3/c1 cells (12-fold range). With a single exception, each DSU examined caused uncoupling of oxidative phosphorylation in isolated mitochondria at 50 microM, and data suggest that cytotoxicity in LYC5 cells may be a consequence of mitochondrial impairment. In contrast, LYC5 cells were collaterally sensitive to the mitochondrial toxins rotenone, antimycin, and oligomycin, by 11.4-, 7.2-, and 36.9-fold respectively. LYC5 cells were also collaterally sensitive to vincristine (7.7-fold), Actinomycin D (5.9-fold), and rhodamine-123 (10.5-fold), agents associated with P-glycoprotein (Pgp)-mediated multidrug resistance (MDR). LYC5 cells were slightly more sensitive to Melphalan and doxorubicin (2.8- and 2.3-fold, respectively) but not to cisplatin or dideazatetrahydrofolic acid. Collateral sensitivity to vincristine and Actinomycin D was consistent with decreased Pgp levels in LYC5 cells. Immunohistochemical staining and Western blotting with anti-Pgp antibodies indicated an 8-fold reduction in Pgp levels in LYC5 cells, relative to expression in parental GC3/c1 cells. Consequently, association of mitochondrial toxins with resistance in MDR KB8-5 cells was examined in the presence or absence of the MDR-reversing agent verapamil. KB8-5 cells had equal or greater sensitivity, compared with parental KB3-1 cells, to rotenone, antimycin, and oligomycin and also to each DSU analogue examined. In addition, verapamil tended to have a protective effect against these mitochondrial toxins.(ABSTRACT TRUNCATED AT 400 WORDS)

Phase I study of topotecan for pediatric patients with malignant solid tumors.

PURPOSE: To determine the dose-limiting toxicity and potential efficacy of topotecan in pediatric patients with refractory malignant solid tumors. PATIENTS AND METHODS: In this phase I clinical trial, 27 patients received topotecan 0.75-1.9 mg/m2 by continuous intravenous infusion daily for 3 days. Fifty-three treatment courses were given to these patients. RESULTS: Myelosuppression was the dose-limiting toxicity at levels of 1.3 to 1.9 mg/m2 for 3 days, requiring significant support with transfused packed RBCs and platelets. Myelosuppression was variable in severity at the 1.0-mg/m2 dosage level; thus, additional patients were treated with this dosage, followed by human recombinant granulocyte-colony stimulating factor (G-CSF). Other toxicities were not significant. One patient with neuroblastoma had a complete response that lasted for 8 months. Stable disease activity was recorded for other patients with neuroblastoma, rhabdomyosarcoma, and islet cell carcinoma. Pharmacokinetic studies showed that topotecan plasma concentrations ranged from 1.6 to 7.5 ng/mL during infusions of 1.0 mg/m2/d, and that there was a biphasic plasma distribution with a mean terminal half-life of 2.9 +2- 1.0 hours. CONCLUSION: Topotecan is a promising anticancer agent that deserves phase II testing in pediatric solid tumors. We recommend that pediatric phase II topotecan trials use 1.0 mg/m2/d for 3 days as a constant intravenous infusion, followed by G-CSF for 14 days, and that these treatment courses be repeated every 21 days.

Pharmacological characterization of N-substituted phenoxazines directed toward reversing Vinca alkaloid resistance in multidrug-resistant cancer cells.

Previously we reported the synthesis and partial characterization of 21 N10-substituted phenoxazines in reversing Vinca alkaloid resistance. Here we report on a subset of these compounds; we have compared their activities in increasing Vinca alkaloid accumulation and reversing drug resistance in KB-ChR8-5 and GC3/c1 (human colon carcinoma) cell lines. Results demonstrated that 1) N-substituted phenoxazines increase accumulation of vinblastine; 2) within this series, there is little correlation or ranking of activity between the two cell lines when Vinca alkaloid accumulation is compared at equal concentrations of modulator; 3) N-substituted phenoxazines demonstrate both quantitative and qualitative differences, compared with verapamil, a standard modulator; and 4) the series includes at least two compounds, 10-[3'-[N-bis(hydroxyethyl)amino]propyl]phenoxazine and 10-(N-piperidinoacetyl)phenoxazine, which increase Vinca alkaloid accumulation but do not significantly inhibit efflux. Additionally, certain of these multidrug resistance modulators significantly enhance accumulation (8-50-fold) of Vinca alkaloids in cell lines with very low or undetectable P-glycoprotein levels, where verapamil has little activity. It is concluded that at least part of the activity of some of these N-substituted phenoxazine modulators may be mediated through a P-glycoprotein-independent mechanism.

Renal transport of 2'-deoxytubercidin in mice.

Previous results [J. F. Kuttesch, Jr. and J. A. Nelson, Cancer Chemother, Pharmac. 8, 221 (1982)] from this laboratory indicate that mechanisms exist for renal secretion of 2'-deoxyadenosine and possibly for reabsorption of adenosine in humans and in mice. Since significant metabolism of these purine nucleosides occurs even in the presence of adenosine deaminase inhibitors, the renal handling of a compound which is not significantly metabolized by the deaminase or by kinases was studied. Unlike 2'-deoxyadenosine itself, the 2'-deoxyadenosine analog, [4-amino-7-(2'-deoxy-beta-D-erythro-pentofuranosyl)-pyrrolo-(2,3-d)pyrimidine; 2'-deoxytubercidin], is not significantly metabolized by mammalian tissues. In mice, the renal plasma clearance of 2'-deoxytubercidin exceeded that of inulin by about 3-fold. Also, mouse kidney slices concentratively accumulated 2'-deoxytubercidin by a saturable and metabolically dependent process. The uptake by mouse kidney slices was inhibited by classical substrates for the organic cation secretory system (tetraethylammonium, choline and N1-methylnicotinamide) but was not markedly inhibited by classical substrates for the organic anion secretory system (p-aminohippurate, phenol red and probenecid). Since 2'-deoxytubercidin inhibited the active, concentrative uptake of [14C]tetraethylammonium, but failed to inhibit the uptake of p-[14C]aminohippurate by mouse kidney slices, it is concluded that 2'-deoxytubercidin may be secreted by the organic cation system. Additional studies are required, however, to unequivocally establish the relationships between 2'-deoxytubercidin, 2'-deoxyadenosine and tetraethylammonium renal secretory mechanisms.

Renal handling of 2'-deoxyadenosine and adenosine in humans and mice.

In a child lacking adenosine deaminase and in patients treated with deoxycoformycin (a potent inhibitor of the enzyme), apparent renal secretion of 2'-deoxyadenosine (dAdo) and reabsorption of adenosine (Ado) were observed. The renal clearance of dAdo in humans was approximately five-fold that of creatinine, whereas the renal clearance of Ado was only one-fifth that of creatinine. In mice treated with deoxycoformycin, a similar paradigm was observed. Specifically, plasma levels of Ado and dAdo were elevated to detectable levels and apparent renal secretion and reabsorption of these purine nucleosides became manifest. Thus, the mouse may serve as a suitable model to study the renal handling of these two compounds. The active renal secretion of dAdo may occur because the compound has not been appreciably synthesized by mouse kidney in situ, and 'ion-trapping' of dAdo in acid urine could not explain the net secretion. The differential transport of these similar purine nucleosides suggests a very selective transport system in mammalian kidney. Although carrier-mediated, facilitated diffusion of purine nucleosides across cell membranes is a well-known ph enomenon, the present data indicate the existence of (an) active transport system(s) for the transepithelial secretion of dAdo, and possibly for the reabsorption of Ado.