Estimating survival in advanced cancer: a comparison of estimates made by oncologists and patients.

PURPOSE: To compare estimates of expected survival time (EST) made by patients with advanced cancer and their oncologists. METHODS: At enrolment patients recorded their "understanding of how long you may have to live" in best-case, most-likely, and worst-case scenarios. Oncologists estimated survival time for each of their patients as the "median survival of a group of identical patients". We hypothesized that oncologists' estimates of EST would be unbiased (~ 50% longer or shorter than the observed survival time [OST]), imprecise (< 33% within 0.67 to 1.33 times OST), associated with OST, and more accurate than patients' estimates of their own survival. RESULTS: Twenty-six oncologists estimated EST for 179 patients. The median estimate of EST was 6.0 months, and the median OST was 6.2 months. Oncologists' estimates were unbiased (56% longer than OST), imprecise (27% within 0.67 to 1.33 times OST), and significantly associated with OST (HR 0.88, 95% CI 0.82 to 0.93, p < 0.01). Only 41 patients (23%) provided a numerical estimate of their survival with 107 patients (60%) responding "I don't know". The median estimate by patients for their most-likely scenario was 12 months. Patient estimates of their most-likely scenario were less precise (17% within 0.67 to 1.33 times OST) and more likely to overestimate survival (85% longer than OST) than oncologist estimates. CONCLUSION: Oncologists' estimates were unbiased and significantly associated with survival. Most patients with advanced cancer did not know their EST or overestimated their survival time compared to their oncologist, highlighting the need for improved prognosis communication training. Trial registration ACTRN1261300128871.

Author Correction: Chemotherapy with radiotherapy influences time-to-development of radiation-induced sarcomas: a multicenter study.

Since the publication of this paper, the authors noticed an error in Fig. 1. The X-axis on all the figure panels should read 'Time (years)', not 'Time (months)'. The corrected Fig. 1 is shown below.

Chemotherapy with radiotherapy influences time-to-development of radiation-induced sarcomas: a multicenter study.

BACKGROUND: An increasing number and proportion of cancer patients with apparently localised disease are treated with chemotherapy and radiation therapy in contemporary oncology practice. In a pilot study of radiation-induced sarcoma (RIS) patients, we demonstrated that chemotherapy was associated with a reduced time to development of RIS. We now present a multi-centre collaborative study to validate this association. METHODS: This was a retrospective cohort study of RIS cases across five large international sarcoma centres between 1 January 2000 to 31 December 2014. The primary endpoint was time to development of RIS. RESULTS: We identified 419 patients with RIS. Chemotherapy for the first malignancy was associated with a shorter time to RIS development (HR 1.37; 95% CI: 1.08-1.72; P=0.009). In the multi-variable model, older age (HR 2.11; 95% CI 1.83-2.43; P<0.001) and chemotherapy for the first malignancy (HR 1.61; 95% CI 1.26-2.05; P<0·001) were independently associated with a shorter time to RIS. Anthracyclines and alkylating agents significantly contribute to the effect. CONCLUSIONS: This study confirms an association between chemotherapy given for the first malignancy and a shorter time to development of RIS.

Outcomes of patients with non-melanoma solid tumours receiving self-funded pembrolizumab at Chris O'Brien Lifehouse.

BACKGROUND: Immunotherapy agents show anti-cancer activity in several solid cancers. Efficacy in non-melanoma solid tumours for non-approved indications is unknown. AIM: To evaluate patient and disease characteristics, rate and duration of response, and toxicity of self-funded pembrolizumab in patients with non-melanoma solid cancers. METHOD: Retrospective review describing outcomes and toxicity of self-funded pembrolizumab in patients with non-melanoma solid cancers treated at Chris O'Brien Lifehouse. RESULTS: From April 2015 to December 2015, 21 patients received or were planned to receive self-funded pembrolizumab. The median age was 50 years (16-76), 28 and 10% had an Eastern Cooperative Oncology Group performance status of 2, and 3-4 respectively. Sixty-two percent received at least two to four lines of prior drug treatment. Median follow-up was 3.0 months (range, 0.4-9.6). Fourteen (67%) patients requested pembrolizumab. Pembrolizumab was clinician offered for 7 (33%) patients. Patients who requested pembrolizumab had worse outcomes. Three patients died before receiving pembrolizumab. Of the 18 patients that received at least one dose, a partial response was observed in 3 (17%). Progressive disease occurred in 83%. Four patients received only one cycle of pembrolizumab and died after a median of 27 days (range 13-43). Immune-related adverse events of any grade occurred in 33%. No grade 3-4 events were observed. CONCLUSION: Pembrolizumab was well tolerated. Meaningful responses were observed in 17% of treated patients. Response continues after 5-6.5 months follow-up in 11% and >8 months of follow-up for the other responding patient. Financial impact to the patient can be substantial. Outcomes for 33% were poor with three patients dying prior to receiving therapy and four dying within weeks of receiving one dose. This highlights issues regarding the careful selection of patients, futility of anti-cancer therapy at the end-of-life and patients' perceived benefit of receiving this therapy.

Immigrants' perceptions of the quality of their cancer care: an Australian comparative study, identifying potentially modifiable factors.

BACKGROUND: Recent data show a falling cancer mortality in the general population without a similar shift in immigrant outcomes, leading to a greater cancer burden and mortality for immigrants. Our aims were to compare perceived patterns of care in immigrants and native-born cancer patients. PATIENTS AND METHODS: This was a hospital-based sample of first-generation immigrants and Australian-born Anglo patients in the first year following diagnosis. It was restricted to Chinese, Arabic, or Greek speakers. Eligible participants, recruited via 16 oncology clinics, were over 18, with cancer (any type or stage), and having commenced treatment at least 1 month previously. Five hundred and seventy-one CALD patients (comprising 145 Arabic, 248 Chinese, and 178 Greek) and a control group of 274 Anglo-Australian patients participated. RESULTS: Immigrants had difficulty communicating with the doctor (73% versus 29%) and understanding the health system (38% versus 10%). Differences were found in 'difficulty knowing who to see' (P = 0.0002), 'length of time to confirm diagnosis' (P = 0.04), wanting more choice about a specialist and hospital (P < 0.0001); being offered the opportunity to see a counselor (P < 0.0001); and actually seeing one (P < 0.0001). There were no significant self-reported differences regarding how cancer was detected, time to see a health professional, or type first seen; however, immigrants reported difficulty knowing who to see. Previous studies showed differences in patterns of care according to socioeconomic status (SES) and educational level. Despite adjusting for age, sex, education, marital status, SES, time since diagnosis, and type of cancer, we did not find significant differences. Instead, we found that understanding of the health system and confidence understanding English were important factors. CONCLUSIONS: This study confirmed that immigrants with cancer perceive an inferior quality of cancer care. We highlight potentially modifiable factors including assistance in navigating the health system, translated information, and cultural competency training for health professionals.

Limb preservation surgery with extracorporeal irradiation in the management of malignant bone tumor: the oncological outcomes of 101 patients.

BACKGROUND: En bloc resection, extracorporeal irradiation (ECI) and reimplantation have been used selectively at our centers as part of limb preservation surgery of malignant bone tumors since 1996. We report the long-term oncological outcomes. PATIENTS AND METHODS: One hundred one patients were treated with ECI at two Australian centers between 1996 and 2011. A single dose of 50 Gy was delivered to the resected bone segments. The irradiated bones were reimplanted immediately as a biological graft. Patients were treated with chemotherapy as per standard protocol. The three main histological diagnoses were Ewing's sarcoma (35), osteosarcoma (37) and chondrosarcoma (20). There were nine patients with a range of different histologies. RESULTS: There was one local recurrence (2.86%) in Ewing's sarcoma and the 5-year cumulative overall survival was 81.9%. There was no local recurrence in osteosarcoma and five distant recurrences. The 5-year cumulative overall survival was 85.7%. The local recurrence rate was 20% (4 of 20) in chondrosarcoma, and the 5-year cumulative overall survival was 80.8%. Limb preservation was achieved in 97 patients. For the 64 patients with disease in the pelvis or lower limb, 53 (82.3%) could walk without aids at the time of last follow-up. CONCLUSIONS: This large series of ECI shows an excellent long-term local control. It is a good alternative reconstruction method in selected patients. The overall survival is comparable to other published series.

Using scenarios to explain life expectancy in advanced cancer: attitudes of people with a cancer experience.

PURPOSE: We sought the attitudes of people with a cancer experience to using best case, worst case, and typical scenarios for survival to explain life expectancy. METHODS: Oncology clinic attendees and Breast Cancer Network Australia (BCNA) members completed a survey describing two formats for explaining life expectancy to a hypothetical patient with advanced cancer-providing either three scenarios for survival or just the median survival time. RESULTS: Characteristics of the 505 respondents from outpatient clinics (n = 251) and BCNA (n = 254) were median age of 58 years, female 74 %, and breast primary 64 %. More respondents agreed that explaining three scenarios (vs. median survival) would make sense (93 vs. 75 %), be helpful (93 vs. 69 %), convey hope (68 vs. 44 %), and reassure (60 vs. 40 %), while fewer respondents agreed that explaining three scenarios (vs. median survival) would upset people (24 vs. 36 %); all p values < 0.001. Most respondents agreed that each scenario should be presented: best case 89 %, worst case 82 %, and typical 92 %. For information about their own prognosis, 88 % preferred all three scenarios and 5 % a single estimate of the median. Respondents with higher education were more likely to agree that presenting three scenarios would be helpful (95 vs. 90 %, p = 0.05). Respondents with breast cancer were more likely to agree that explaining three scenarios would upset people (31 vs. 13 %, p < 0.001). CONCLUSIONS: Most respondents judged presentation of best case, worst case, and typical scenarios preferable and more helpful and reassuring than presentation of just the median survival time when explaining life expectancy to patients with advanced cancer.

Impact of a cancer clinical trials web site on discussions about trial participation: a cluster randomized trial.

BACKGROUND: Cancer patients want access to reliable information about currently recruiting clinical trials. PATIENTS AND METHODS: Oncologists and their patients were randomly assigned to access a consumer-friendly cancer clinical trials web site [Australian Cancer Trials (ACT), www.australiancancertrials.gov.au] or to usual care in a cluster randomized controlled trial. The primary outcome, measured from audio recordings of oncologist-patient consultations, was the proportion of patients with whom participation in any clinical trial was discussed. Analysis was by intention-to-treat accounting for clustering and stratification. RESULTS: Thirty medical oncologists and 493 patients were recruited. Overall, 46% of consultations in the intervention group compared with 34% in the control group contained a discussion about clinical trials (P=0.08). The mean consultation length in both groups was 29 min (P=0.69). The proportion consenting to a trial was 10% in both groups (P=0.65). Patients' knowledge about randomized trials was lower in the intervention than the control group (mean score 3.0 versus 3.3, P=0.03) but decisional conflict scores were similar (mean score 42 versus 43, P=0.83). CONCLUSIONS: Good communication between patients and physicians is essential. Within this context, a web site such as Australian Cancer Trials may be an important tool to encourage discussion about clinical trial participation.

A case of uterine tumour resembling ovarian sex cord tumour responding to second-line, single agent anastrazole.

Uterine tumour resembling ovarian sex cord tumour (UTROSCT) are a histological variant of endometrial stromal sarcomas (ESS). There is no established medical management of metastatic UTROSCT or ESS, although there is evidence supporting the use of hormonal therapy. Given the success of aromatase inhibitors in breast cancer, their potential role in ESS and UTROSCT is of current interest. We report the first case of response to second-line, single agent anastrazole in a patient with metastatic UTROSCT.

An examination of the initial cancer consultation of medical and radiation oncologists using the Cancode interaction analysis system.

This study provides an analysis of the structure of the initial cancer consultation, the consultation styles of medical and radiation oncologists, and their effect on patient outcomes. One hundred and fifty-five cancer patients attending their first consultation with either a medical or radiation oncologist were audiotaped and the transcripts were analysed using the Cancode computer interaction analysis system. Findings revealed that medical oncologists allowed patients and their families more input into the consultation and were rated as warmer and more patient-centred compared with radiation oncologists. However, radiation oncologists spent a longer period discussing, and were more likely to bring up, social support issues with patients. Both medical and radiation oncologists varied their consultation style according to the patient's gender, age, anxiety levels, prognosis, and education. Patients seeing an oncologist who was rated as warmer and discussed a greater number of psychosocial issues had better psychological adjustment and reduced anxiety after consultation. These findings provide current evidence that may be used to inform improvements of communication skills training for oncologists and highlight the need for future communication research to separately consider oncologists from different disciplines.

Enhancing treatment decision-making: pilot study of a treatment decision aid in stage IV non-small cell lung cancer.

We developed a decision aid (DA) for patients with metastatic non-small cell lung cancer (NSCLC), to better inform patients of their prognosis and treatment options, and facilitate involvement in decision-making. In a pilot study, 20 patients with metastatic NSCLC attending outpatient clinics at a major cancer centre, who had already made a treatment decision, reviewed acceptability of the DA. The median age of the patients was 61 years (range 37-77 years), 35% were male, 20% had a university education, and most (75%) had English as a first language. Most had received chemotherapy, with 65% currently on treatment. Patients were not anxious at baseline and had clear understanding of the goals and toxicity of chemotherapy in advanced NSCLC. After reviewing the DA, patients' anxiety decreased slightly (P=0.04) and knowledge scores improved by 25% (P<0.001). Most improvements in understanding were of prognosis with and without chemotherapy, although patients still believed advanced NSCLC to be curable. Patients rated the DA highly with respect to information clarity, usefulness and were positive about its use in practice, although 40% found the prognostic information slightly upsetting. The DA for advanced NSCLC is feasible, acceptable to patients and improves understanding of advanced NSCLC without increasing patient anxiety.

Prostaglandin F2-alpha receptor regulation in human uterine myocytes.

Investigations of the modulation of prostaglandin F(2alpha) receptor (FP) expression in primary cultures of human uterine myocytes showed that FP mRNA expression was reduced by progesterone, unaltered by cAMP (8-bromo cAMP or forskolin), but increased by the PKA antagonist H89. Interleukin (IL)-1beta, tumour necrosis factor-alpha and oxytocin increased FP mRNA expression and IL-6 and prostaglandin E(2) reduced FP mRNA expression. The changes in FP protein levels were similar to the mRNA responses. We found that the IL-1beta-induced increase in FP expression was mediated at least in part via protein kinase C (PKC), but was independent of mitogen-activated protein kinase, phospholipase C and PI3 kinase. Since IL-1beta activates NFkappaB, AP-1 and C/EBP, we over-expressed these transcription factors alone and in combination and found that only NFkappaB alone increased FP mRNA expression. Finally, we found that the IL-1beta-induced increase in FP expression was unaffected by progesterone and/or cAMP, but was accentuated by H89. These data suggest that the pregnancy-induced down-regulation in myometrial FP expression is mediated by progesterone and cAMP and that the increase with labour is induced by inflammatory cytokine activation of PKC and NFkappaB.

Ecto-adenosine triphosphatase deficiency in cultured human T and null leukemic lymphocytes. A biochemical basis for thymidine sensitivity.

Cultured leukemic T and null lymphocytes are highly sensitive to growth inhibition by thymidine, as well as the other deoxynucleosides, deoxyguanosine and deoxyadenosine. By contrast, Epstein-Barr virus-transformed B lymphocytes are relatively resistant to deoxynucleosides. Growth inhibition is associated with the development of high deoxyribotriphosphate pools after exposure to the respective deoxynucleotides. We show that malignant T and null lymphocytes are deficient in ecto-ATPase activity. We show this cell surface enzyme to be of broad specificity, capable of degrading both ribotriphosphates and deoxyribotriphosphates. High levels of this ecto-enzyme are found in deoxynucleoside-resistant, Epstein-Barr virus-transformed B lymphocytes. Ecto-ATPase deficiency may represent a mechanism for increased sensitivity to deoxynucleoside growth inhibition.

Methotrexate cytotoxicity in cultured human leukemic cells studied by flow cytometry.

Methotrexate (MTX)(2 x 10(-8) M) inhibited DNA synthesis in CCRF-CEM cells, causing cells to accumulate in early S phase while cellular RNA content and cell size continued to increase. Two-parameter flow cytometric analysis of DNA and RNA showed these cells to be unbalanced with excessive RNA relative to DNA content. Fifty % of cells remained viable after a 96-hr exposure to 2 x 10(-8) M MTX. In contrast, 10(-4) M MTX inhibited cell cycle progression of cells in both G1 and S phases and also prevented the development of unbalanced growth. In this instance, cell viability was reduced to 10% after 96 hr of drug exposure. The relative contribution of inhibition of thymidylate and purine biosynthesis to MTX cytotoxicity was investigated by addition of exogenous thymidine (10(-5) M) or hypoxanthine (10(-4) M). Thymidine reduced the cytotoxicity and inhibition of DNA synthesis caused by both doses of MTX and prevented classical unbalanced growth with 2 x 10(-8) m MTX; treatment with 10(-4) M MTX resulted in a form of unbalanced growth where cells had a relative excess of DNA compared with RNA content. The addition of hypoxanthine enhanced the classical unbalanced growth pattern seen with 2 x 10(-8) M MTX but was accompanied by a partial reduction of both the MTX-induced cytotoxicity and the inhibition of DNA synthesis (to an extent similar to that seen with exogenous thymidine). Potentiation of cell killing was observed with the addition of hypoxanthine to cells treated with 10(-4) M MTX. Complete rescue from MTX cytotoxicity at both concentrations was found only when both thymidine and hypoxanthine were present. These finding suggest that MTX cytotoxicity is associated with inhibition of DNA synthesis resulting from the disturbance of both thymidylate and purine biosynthesis.

Flow cytometric enumeration of drug-resistant tumor cells.

In an attempt to elucidate the role of somatic mutation in the development of resistance to cancer chemotherapy, an assay was sought to measure the frequency of hypoxanthine-guanine phosphoribosyltransferase (HPRT) mutants in human tumors. Based on the same principle as [3H]thymidine/autoradiography, a method was developed to identify cell proliferation using the thymidine analog 5-bromo-2'-deoxyuridine (BrdUrd). BrdUrd incorporation into DNA was measured following the immunofluorescent staining of fixed cells using a monoclonal antibody highly specific for this nucleoside analog. The human leukemic cell line, CCRF-CEM, was used to investigate the conditions necessary for the stringent selection of HPRT- mutants using 6-thioguanine (6TG). The appropriate 6TG exposure necessary to inhibit BrdUrd incorporation in wild-type cells, while allowing proliferation of spontaneous HPRT- mutants, was greater than or equal to 30 microM 6TG for 72 h (10 microM BrdUrd added 24 h prior to harvest). BrdUrd did not affect the growth of HPRT- mutants in the presence of 6TG. BrdUrd-labeled 6TG-resistant cells were enumerated flow cytometrically using fluorescent microspheres as an internal standard and the nonparametric, Kolmogorov-Smirnov test was used for independent statistical analysis of the subpopulations of fluorescent, 6TG-resistant cells. Evidence that CCRF-CEM cells which incorporated BrdUrd in the presence of 6TG were, in fact, HPRT- mutants was sought. It was demonstrated that spontaneous 6TG-resistant cells from the CCRF-CEM population were reduced by growth in medium containing aminopterin. The mutant frequency in the CCRF-CEM cell line was found to be 4.28 x 10(-5) +/- 2.04 x 10(-5) using the BrdUrd/flow cytometric technique.

Mechanism of deoxycytidine rescue of thymidine toxicity in human T-leukemic lymphocytes.

Cultured malignant human lymphocytes are highly sensitive to growth inhibition by thymidine (50% inhibitory dose congruent to 10(-5) M). Growth inhibition reflects sustained elevation of the deoxythymidine 5'-triphosphate pool associated with secondary elevation of the deoxyguanosine 5'-triphosphate pool and reduction in the deoxycytidine 5'-triphosphate (dCTP) pool. Deoxycytidine was capable of partially reversing thymidine growth inhibition at a concentration of 0.5 microM, and growth recovery was virtually complete at 8 microM. The dCTP pool remained depressed until growth inhibition reversal by deoxycytidine was complete, and at a higher concentration of deoxycytidine the dCTP rose above control levels, but the deoxythymidine 5'-triphosphate and deoxyguanosine 5'-triphosphate pools remained elevated. These results support the view that thymidine growth inhibition induces a critical deficiency of dCTP via allosteric inhibition of ribonucleotide reductase rather than inhibiting DNA replication directly by elevated deoxythymidine 5'-triphosphate or deoxyguanosine 5'-triphosphate pools.

Deoxyuridine metabolism in cultured human lymphoblasts treated with methotrexate.

The deoxyuridine suppression test and labeled deoxyuridine incorporation studies assume a stable level of deoxyuridine phosphorylase (thymidine:orthophosphate deoxyribosyltransferase, EC 2.4.2.4) activity. We report a large increase in deoxyuridine phosphorylase activity in three of five cultured lymphoblast lines treated with 10(-7) M methotrexate. In two of these lines, a parallel increase in tritiated deoxyuridine incorporation into RNA was seen following methotrexate treatment. The high basal deoxyuridine phosphorylase activity in another lymphoblast line was associated with 80% incorporation of tritiated deoxyuridine into RNA in untreated cells. Since methotrexate-induced changes in deoxyuridine phosphorylase activity were time dependent, the reliability of the deoxyuridine suppression test and labeled deoxyuridine incorporation into DNA as measures of thymidylate synthetase (EC 2.7.4.6) inhibition would also vary with time. Moreover, increases in deoxyuridine phosphorylase activity in methotrexate-treated cells may influence the metabolism of fluorouracil, a drug frequently used in combined treatment regimens.

Methotrexate rescue by 5-methyltetrahydrofolate or 5-formyltetrahydrofolate in lymphoblast cell lines.

The rescue of lymphocytes from methotrexate (MTX) growth inhibition by 5-methyltetrahydrofolate (5-methyl-THF) and 5-formyltetrahydrofolate (5-formyl-THF) has been studied. Rescue by 5-methyl-THF is selective for cells with high levels of homocysteine:5-methyl-THF methyl-transferase (methyltransferase). At MTX concentrations which inhibited growth greater than or equal to 85% in both leukemic T-lymphocytes (CCRF-CEM) and Epstein-Barr-transformed B-lymphocytes (LAZ-007), 5 micro M 5-formyl-THF rescued more effectively than did 5-methyl-THF, in either the presence or absence of the methyltransferase inhibitor, nitrous oxide. At less inhibitory MTX concentrations, both reduced folates rescued equally, except when methyltransferase was inhibited by nitrous oxide in which case 5-formyl-THF was clearly superior. In the absence of nitrous oxide, both cell lines contained approximately equal amounts of methyltransferase. Some apparent differences in the rescue of these cell lines with 5-methyl-THF were attributable to their different sensitivity to MTX. When metabolism of reduced folates was severely impaired by MTX and nitrous oxide, lymphocytes were rescued with 5-[methyl-14C]methyl-THF, and the uptake of 14C into DNA was measured. In corporation was very low, indicating that cellular oxidation of 5-methyl-THF to 5,10-methylene-tetrahydrofolate is minimal even under forcing conditions. MTX selectively in vivo will be influenced by the level of methyltransferase in tumor and normal tissues.

Critical modulation by thymidine and hypoxanthine of sequential methotrexate-5-fluorouracil synergism in murine L1210 cells.

Treatment of murine L1210 cells with methotrexate (MTX) followed by 5-fluorouracil (FUra) produced synergistic cytotoxicity, but only in media containing serum with low concentrations of hypoxanthine, such as horse serum and dialyzed fetal calf serum. Addition of hypoxanthine (1 to 10 microM) during drug exposure reduced the synergism of sequential MTX (1 to 100 microM)-FUra (30 to 300 microM) treatment. The reduction of synergy by hypoxanthine varied with the MTX concentration, higher hypoxanthine concentrations being required to prevent synergy at higher MTX concentrations. The cytotoxicity produced by sequential MTX (10 microM)-FUra (30 to 300 microM) treatment was also reduced if thymidine was added to the regrowth media following drug exposure. The rescue by thymidine was concentration dependent, but as little as 0.5 microM thymidine was sufficient to substantially reduce the synergistic cytotoxicity. These results indicate that both hypoxanthine and thymidine are critical determinants of sequential MTX-FUra synergy and call into question the relevance of experiments in low-thymidine- and low-hypoxanthine-containing media to the clinical situation, where plasma hypoxanthine and thymidine concentrations are often greater than 1 and 0.5 microM, respectively.

Purine modulation of methotrexate cytotoxicity in mammalian cell lines.

The modulation of MTX cytotoxicity by purines has been studied in a number of mammalian cell lines. In each case, it was found that exogenous purines (guanosine, deoxyguanosine, adenosine, deoxyadenosine, and hypoxanthine) both reduced and potentiated MTX cytotoxicity depending on the MTX concentration. At low MTX concentrations (less than 6 X 10(-8) M), purines reduced MTX toxicity and at higher concentrations they potentiated MTX toxicity. The reduction of low-concentration MTX cytotoxicity by purines was associated with the reversal of MTX-induced changes in deoxyribonucleotide pools. On the other hand, potentiation of MTX toxicity by purines was associated with substantial increases in deoxyadenosine 5'-triphosphate levels in conjunction with low deoxythymidine 5'-triphosphate levels. The magnitude of increase in deoxyadenosine 5'-triphosphate levels tended to correlate with the degree of potentiation which varied between 5-fold and 200-fold, depending on cell type and the exogenous purine.