The cytotoxic activity of cisplatin, carboplatin and teniposide alone and combined determined on four human small cell lung cancer cell lines by the clonogenic assay

Despite Blocking Doxorubicin-Induced Vascular Damage, Quercetin Ameliorates Its Antibreast Cancer Activity

Quercetin is a naturally occurring flavonol present in many foods. Doxorubicin is an effective anticancer agent despite its dose-limiting cardiovascular toxicity. Herein, we investigated the potential protective effects of quercetin against doxorubicin-induced vascular toxicity and its effect on the therapeutic cytotoxic profile of doxorubicin in breast cancer cell lines. The incubation of isolated aortic rings with doxorubicin produced concentration-dependent exaggeration of vasoconstriction responses to phenylephrine but impaired vasodilation responses to acetylcholine. Coincubation with quercetin completely blocked the exaggerated vasoconstriction responses and the impaired vasodilation. In addition, doxorubicin incubation increased reactive oxygen species generation from the isolated aorta, while coincubation with quercetin inhibited ROS generation back to normal values. On the other hand, quercetin in combination with doxorubicin, doubled the IC₅₀ of doxorubicin alone in MCF-7 cells from 0.4 ± 0.03 to 0.8 ± 0.06 μM. To a lesser extent, the IC₅₀ of doxorubicin did not change after combination with quercetin in MDA-MB-231 cells. These findings indicate a significant antagonistic interaction between quercetin and doxorubicin in the aforementioned cell lines. Only in T47D cells, quercetin combination with doxorubicin was an additive interaction (CI − value = 1.17). Yet, quercetin significantly impaired the immediate phase of intracellular ROS generation by doxorubicin within breast cancer cells from 125.2 ± 3.6% to 102.5 ± 3.9% of control cells. Using annexin-V/FITC staining technique, the quercetin/doxorubicin combination showed a significantly lower percent of apoptotic cells compared to doxorubicin alone treated cells. Cell cycle distribution in breast cancer cells was performed using DNA content flowcytometry after propidium iodide staining. Quercetin induced significant accumulation of cells in the S phase as well as in the G₂/M phase within both MCF-7 and MDA-MB-231 cell lines and interfered with doxorubicin-induced cell cycle effects. Interestingly, quercetin was found to inhibit the P-glycoprotein ATPase subunit with a consequent enhanced intracellular concentration of doxorubicin in MDA-MB-231 and T47D cells. In conclusion, quercetin, despite its potent vascular protective activity against doxorubicin, was found to influence doxorubicin-induced antibreast cancer effects via pharmacodynamic as well as cellular pharmacokinetic aspects.

5-fluorouracil-carboplatin Continuous Infusion as Salvage Chemotherapy in Advanced Cancer

Aims and Background

5-fluorouracil given by continuous infusion allows higher dose delivery, causes less myelosuppression and may interfere with repair of DNA damage caused by carboplatin. With this rationale, we conducted a phase II study to test the activity and toxicity of 5-fluorouracil and carboplatin given in continuous infusion to patients with advanced cancer and pretreated with at least two chemotherapy regimens.

Methods

Forty patients with advanced tumors (21 colon, 4 stomach, 3 breast, 3 bladder, 3 ovary, and 6 at other sites) were entered in the trial. 5-fluorouracil (200 mg/m2) and carboplatin (20 mg/m2) were administered by continuous infusion from days 1 to 20, every 4 weeks.

Results

All patients were assessable for response and toxicity. A total of 138 courses of chemotherapy were administered, with a mean of 3.5 per patient (range, 2-9). Toxicity, assessed using WHO criteria, was as follows: nausea and vomiting grade 2-3 in 34% of patients, alopecia grade 2-3 in 96%, and neutropenia grade 3-4 in 26%. One patient (2.5%) had a complete response to therapy and 7 (17.5%) had a partial response (response rate 20%; 95% Cl, 9.06-35.68%). Disease stability and progression occurred in 12 (30%) and 20 (50%) patients, respectively. Median time to progression was 5.6 months (range, 2.8-45.9+), with a median survival time of 7.7 months (range, 1.5-45.9+).

Conclusions

Outpatient treatment with a combination of 5-fluorouracil and carboplatin in continuous infusion was active as salvage treatment for advanced tumors and may give prolonged palliation of symptoms with manageable toxicity.

A pilot study of three-cycle consolidation chemotherapy with paclitaxel and platinum in epithelial ovarian cancer patients with clinical complete response after paclitaxel and platinum chemotherapy

The aim of this study is to evaluate the efficacy of three additional cycles of paclitaxel and platinum chemotherapy in epithelial ovarian cancer patients with clinical complete response (CR). Patients with histologically confirmed epithelial ovarian cancer stages II-IV with clinical CR after primary surgery and six cycles of chemotherapy with paclitaxel/platinum entered into the study. Three cycles of paclitaxel/platinum (cisplatin, carboplatin) were administered as a consolidation chemotherapy only in patients who agreed to the informed consent. Patients without further treatment served as controls. A total of 81 patients entered into the study. According to the informed consent, 42 patients were treated by the consolidation chemotherapy, and 39 patients were followed up without further treatment. The median actuarial disease-free survival for the patients with and without consolidation chemotherapy was 25.0 months and 26.0 months, respectively (P= 0.80). The median overall survival is not reached. World Health Organization grade 3-4 toxicities in the consolidation arm were increased but showed no significant differences statistically. Although the sample size is small and not randomized, these results suggest that three cycles of consolidation chemotherapy with paclitaxel/platinum might not provide a favorable outcome in patients with a clinical CR.

Direct isolation of poly(A)+ RNA from 4 M guanidine thiocyanate-lysed cell extracts using locked nucleic acid-oligo(T) capture

LNA oligonucleotides constitute a class of bicyclic RNA analogues having an exceptionally high affinity for their complementary DNA and RNA target molecules. We here report a novel method for highly efficient isolation of intact poly(A)+ RNA using an LNA-substituted oligo(dT) affinity ligand, based on the increased affinity of LNA-T for complementary poly(A) tracts. Poly(A)+ RNA was isolated directly from 4 M guanidine thiocyanate-lysed Caenorhabditis elegans worm extracts as well as from lysed human K562 and vincristine-resistant K562/VCR leukemia cells using LNA_2.T oligonucleotide as an affinity probe, in which every second thymidine was substituted by LNA thymidine. In accordance with the significantly increased stability of the LNA_2.T-A duplexes in 4 M GuSCN, we obtained a 30- to 50-fold mRNA yield increase using the LNA-substituted oligo(T) affinity probe compared with DNA-oligo(dT)-selected mRNA samples. The LNA_2.T affinity probe was, furthermore, highly efficient in isolation of poly(A)+ RNA in a low salt concentration range of 50-100 mM NaCl in poly(A) binding buffer, as validated by selecting the mRNA pools from total RNA samples extracted from different Saccharomyces cerevisiae strains, followed by northern blot analysis. Finally, we demonstrated the utility of the LNA-oligo(T)-selected mRNA in quantitative real-time PCR by analysing the relative expression levels of the human mdr1 multidrug resistance gene in the two K562 cell lines employing pre-validated Taqman assays. Successful use of the NH2-modified LNA_2.T probe in isolation of human mRNA implies that the LNA-oligo(T) method could be automated for streamlined, high throughput expression profiling by real-time PCR by covalently coupling the LNA affinity probe to solid, pre-activated surfaces, such as microtiter plate wells or magnetic particles.

Phase II trial of dose intensification with cisplatin and carboplatin plus vindesine in patients with inoperable non-small cell lung cancer

We conducted a phase II trial of a regimen that combined cisplatin (CDDP), carboplatin (CBDCA), and vindesine (VDS) in previously untreated patients with non-small cell lung cancer (NSCLC) to evaluate the efficacy and safety of the regimen. Thirty-five patients with inoperable NSCLC entered the study. Cisplatin (CDDP 80 mg/m2) was administered on day 1, and CBDCA 100 mg/m2 and VDS 2 mg/m2) were administered on days 2, 3, and 8. We observed one complete response (CR) and 16 partial responses (PR), with a total response rate of 49% [95% confidence interval (CI) 31-66%]. The overall median survival was 58 weeks; the 1-year survival rate was 60%, and the 2-year survival rate was 23%. Major toxicities were hematologic; leukopenia of grades 3 and 4 occurred in 66% patients, and thrombocytopenia occurred in 23%. Therefore, the dose-intensified regimen of CDDP, CBDCA, and VDS was active in treating patients with inoperable NSCLC, with demonstration of a favorable median survival time.

Combined carboplatin and cisplatin therapy in patients with advanced non-small cell lung cancer

Combining cisplatin and carboplatin may eliminate some of the toxic effects of each agent and permit the use of higher doses, because these agents have different pharmacodynamics and dose-limiting toxicities. We investigated the safety and efficacy of these agents in combination. The toxicity profile was evaluated in a Phase I trial in 18 patients with advanced non-small cell lung cancer (NSCLC). Carboplatin was administered in doses ranging from 200 to 400 mg/m2 on day 1 and cisplatin at a dose of 80 mg/m2 on day 3. Only one cycle of chemotherapy was administered. Thrombocytopenia was the dose-limiting toxic effect. The maximal tolerated dose of carboplatin was 350 mg/m2. We then investigated the efficacy of the optimal dose of this combined chemotherapy in a Phase II trial in 13 patients. We used a carboplatin dose of 300 mg/m2 for safety in the Phase II trial. Three of 13 patients developed grade 3-4 hematologic toxicity, which was improved without major complications. A partial response was observed in 5 of 13 patients (38.5%). Combination chemotherapy with carboplatin (day 1, 300 mg/m2) and cisplatin (day 3, 80 mg/m2) showed promising effects in patients with advanced NSCLC.

Evaluation of cisplatin, carboplatin, and etoposide in metastatic nonsmall cell lung carcinoma. A phase II study of the Southwest Oncology Group

BACKGROUND

The combined use of cisplatin and carboplatin chemotherapy offers a unique means of platinum dose intensification. Response rates using either of these agents in combination with etoposide are comparable. In a Phase II trial, the authors investigated the combination of cisplatin and carboplatin with etoposide for the treatment of patients with advanced nonsmall cell lung carcinoma.

METHODS

Eligible patients were chemotherapy naive and had histologically confirmed, evaluable, or measurable selected Stage IIIB and Stage IV nonsmall cell lung carcinoma. Based upon the results of an earlier Phase I and II pilot study, patients received carboplatin, 225 mg/m2, on Day 1; cisplatin, 50 mg/m2, on Days 2 and 3; and etoposide, 75 mg/m2, on Days 1, 2, and 3 every-4-weeks.

RESULTS

Eighty-three patients (75 eligible patients) received chemotherapy with cisplatin, carboplatin, and etoposide. Two patients refused therapy after registration and were not analyzable. Thirty-six of the remaining 75 patients had Grade 4 toxicities, mostly hematologic, and 6 patients died of toxicity. The confirmed response rate was 24% (95% confidence interval, 15-35%). Median progression-free survival was 4 months and the median survival was 8 months.

CONCLUSIONS

Combination cisplatin, carboplatin, and etoposide chemotherapy appears to be no better than cisplatin/etoposide or carboplatin/etoposide for the treatment of patients with nonsmall cell lung carcinoma. The toxicity of this regimen may be higher, and therefore it cannot be recommended for general use.

Platinum analogue combination chemotherapy: cisplatin, carboplatin, and methotrexate in patients with metastatic urothelial tract tumors. A phase II trial with evaluation of prognostic factors

BACKGROUND

Cisplatin is one of the single drugs that has shown the best documented effect in treating patients with locally recurrent or metastatic urothelial cancer. To the authors' knowledge, the effect of the combination of different platinum analogues in treating transitional cell carcinoma has not been evaluated previously neither experimentally nor in clinical studies.

METHODS

A Phase II trial of carboplatin (200 mg/m2), cisplatin (100 mg/m2), and methotrexate (250 mg/m2) with folinic acid rescue every 3 weeks was performed on 55 previously untreated patients with metastatic or locally recurrent urothelial cell carcinoma.

RESULTS

A response (complete response and partial response) was achieved in 21 of 51 evaluable patients (41%; 95% confidence limits, 28-56%). Twelve patients had no change, whereas 18 had progressive disease. Eight patients (16%) achieved a complete response, and most of these survived more than 2 years. No patient with poor performance (performance status score > or = 2) or bone metastases achieved a complete response. The median survival for all patients was 8.4 months. Multivariate survival analyses showed that performance status and alkaline phosphatase levels were significant prognostic factors for survival.

CONCLUSION

Combination therapy with cisplatin, carboplatin, and methotrexate is feasible but offers no advantage over other combinations with cisplatin and methotrexate in treating metastatic urothelial cell cancer. It is important to select patients for treatment carefully, and further studies of prognostic factors in these patients are warranted.

Continuous carboplatin infusion during 6 weeks' radiotherapy in locally inoperable non-small-cell lung cancer: a phase I and pharmacokinetic study

A phase I study was performed in 21 patients with previously untreated, locally inoperable, non-small-cell lung cancer (NSCLC) with ambulatory continuous carboplatin infusion together with continuous thoracic irradiation over 6 weeks. A dose range for carboplatin of 15 mg m-2 day-1 during the last 21 days (first level), during the last 31 days (second level), or during 6 weeks of the radiation period (third level) and thereafter 20 or 25 mg m-2 day-1 during 6 weeks of radiation (fourth and fifth level) was used. The total radiation dose was 60 Gy given as 2 Gy day-1 for 5 days week-1. The first three patients received radiotherapy without carboplatin. WHO grade III/IV leucopenia and thrombocytopenia occurred in the last two dose levels in two out of six and one out of six patients with 20 mg m-2 day-1 respectively, and in all three patients with 25 mg m-2 day-1 (dose-limiting toxicity). One local infection around the port and a subclavian vein thrombosis occurred. Radiation toxicity of the lung and oesophagus did not seem to be influenced by carboplatin treatment. Out of 21 patients one had a complete response (CR), ten partial response (PR), six stable disease (SD) and four progressive disease (PD). Total (TPt) and ultrafilterable plasma platinum (UPt) were measured in the last three dose levels with atomic absorption spectrophotometry with Zeeman correction. The mean (s.d.) level for TPt for 6 weeks at 15, 20 and 25 mg m-2 day-1 was 0.76 (0.15), 0.78 (0.19) and 0.90 (0.22) mg l-1 for UPt 0.10 (0.03), 0.12 (0.02) and 0.20 (0.03) mg l-1 respectively. TPt concentration levelled off after 3 weeks. The mean (s.d.) CLTB for UPt was 281 +/- 21 ml min-1 and correlated with glomerular filtration rate (r = 0.61, P = 0.03). As estimated with the sigmoid Emax model defined by the Hill equation the percentage reduction in platelets correlated with the area under the curve for UPt (r = 0.77). The maximum tolerable dose of carboplatin with concomitant continuous 60 Gy radiotherapy is 25 mg m-2 day-1; the recommended dose for phase II or III studies is 20 mg m-2 day-1 day for 6 weeks.

Mitomycin-ifosfamide-cisplatinum (MIP) vs MIP-interferon vs cisplatinum-carboplatin in metastatic non-small-cell lung cancer: a FONICAP randomised phase II study. Italian Lung Cancer Task Force

The FONICAP group is screening, with randomised phase II studies, the activity of new chemotherapy programmes for advanced non-small-cell lung cancer (NSCLC) looking for regimens with > 30% activity. In the present study, three regimens were tested: MIP (mitomycin 6 mg m-2, ifosfamide 3 g m-2, cisplatinum 80 mg m-2 on day 1 every 28 days); MIP-IFN (MIP and interferon alpha-2b 3 MU s.c. three times a week); and PC (cisplatinum 60 mg m-2 and carboplatin 400 mg m-2 on day 1 every 28 days). Overall 93 chemotherapy-naive patients were enrolled: 23 received MIP, 27 received MIP-IFN and 43 received PC. Eighty per cent of the patients had stage IV and 20% stage IIIb disease (positive pleural effusion or supraclavicular nodes). Response rates were as follows: MIP = 9% (95% CI 1-28%), MIP-IFN = 7% (95% CI 1-24%) and PC = 14% (95% CI 5-28%). The overall median survival was 183 days. Grade III-IV leucopenia was observed in 36% of patients treated with MIP-IFN vs 10% in the other two arms, and thrombocytopenia grade III-IV was reported in nearly 10% of patients overall. In conclusion, (1) all three regimens investigated have poor activity (< 30%); (2) when tested in multicentre randomised phase II trials, MIP displays lower activity than in phase II trials; (3) PC has similar activity to other platinum-containing regimens; (4) randomised phase II studies are a reliable and quick method of determining the anti-tumour activity of novel chemotherapeutic regimens in NSCLC.

Combined carboplatin and cisplatin. Limited prospects for dose intensification

BACKGROUND

The relative lack of overlapping toxicities and less-than-complete cross-resistance of tumors treated with both carboplatin and cisplatin may allow these two analogues to be given in combination to exploit platinum dose intensity therapeutics. Early experience with combined platinum regimens, however, found myelosuppression, particularly severe thrombocytopenia, to be dose-limiting. It was postulated that a 2-day interval between carboplatin and cisplatin would allow for near complete clearance of the former before cisplatin administration and a potential gain in dose intensity.

METHODS

Other carboplatin-cisplatin regimens produced Grade 3-4 toxicity in 20% of patients. By defining 95% confidence limits around this observed rate of Grade 3-4 toxicity, the accrual needs of this study were determined in a two-stage process. Sixteen patients with advanced malignancies were entered onto a trial of 300 mg/m2 of carboplatin on day 1, followed by 125 mg/m2 of cisplatin on day 3 every 28 days. Hematologic and nonhematologic toxicity was closely monitored, including the use of serial audiograms, to allow appropriate dose modification.

RESULTS

A total of 40 courses of combination platinum therapy was administered to 15 patients who were evaluable for toxicity. Higher-than-anticipated ototoxicity and neurosensory toxicity was observed. WHO Grade 3 ototoxicity (hearing loss) was documented in 12 of 15 patients (80.0%, 95% confidence interval [CI]: 52.0-97.0%) and emerged as the dose-limiting side effect of this regimen. High-frequency hearing loss, as demonstrated by conventional audiograms, was universal among all 12 patients who received at least 2 courses of combination platinum therapy (100%, 95% CI: 73.5-100%). Grade 2 or 3 neurosensory toxicity also was observed in 4 of 15 patients. Hematologic toxicity was manageable. WHO Grade 3-4 neutropenia or thrombocytopenia occurred in only 14% and 11%, respectively, of 40 courses. There was no evidence of cumulative marrow toxicity. Calculated dose intensities (mg/m2/week) were 94 +/- 26.0 for carboplatin, 39.3 +/- 12.4 for cisplatin, and 64.0 +/- 19.2 for the combination (expressed as cisplatin equivalents). Objective responses (complete response+partial response) occurred in 8 of 16 subjects (50.0%, 95% CI: 24.7-75.4%), with 1 patient achieving a complete response of 14+months.

CONCLUSIONS

The schedule of day 1 carboplatin plus day 3 cisplatin every 4 weeks appeared to allow a higher platinum dose intensity with less myelotoxicity than previously reported schedules combining these two analogues. Ototoxicity, however, was unexpectedly severe and limits future prospects for the use of combined platinum analogues to achieve dose intensification.

Doxorubicin sensitivity pattern in a panel of small-cell lung-cancer cell lines: correlation to etoposide and vincristine sensitivity and inverse correlation to carmustine sensitivity

The aim of our investigations is to evaluate whether the sensitivity patterns of small-cell lung-cancer (SCLC) cell lines in vitro can be used in evaluating new drugs and in selecting drugs for the optimization of combination therapy. In our attempts to obtain a panel of cell lines demonstrating differential patterns in sensitivity, we have developed three SCLC lines exhibiting different types of multidrug resistance (MDR). In the present investigations we compared the sensitivity patterns shown by five wild-type SCLC lines and three MDR lines in response to six different types of drugs: doxorubicin, cytarabine, carmustine, cisplatin, vincristine, and etoposide. In the wild-type SCLC cell lines, the range of variation in sensitivity to all drugs was within a factor of 10. Cell lines showing low sensitivity to doxorubicin also exhibited low sensitivity to etoposide and vincristine, and vice versa. In contrast, the pattern of sensitivity to carmustine was almost the opposite of that to doxorubicin. A tendency to an inverse relationship between doxorubicin and carmustine sensitivity was also observed when doxorubicin sensitivity was reduced in near stationary cells and in cells exposed to the metabolic inhibitor 2-deoxy-D-glucose. In agreement with the pattern observed for the wild-type lines, all of the MDR sublines demonstrated collateral sensitivity to carmustine. As to cytarabine, the wild-type lines expressed a sensitivity pattern similar to that shown in response to doxorubicin. Interestingly, the opposite pattern was found in the MDR lines, as all three demonstrated cytarabine hypersensitivity. The combination of alkylating agents and "MDR" drugs are of proven clinical benefit in the treatment of solid tumors, as is the combination of anthracycline and cytarabine in acute myeloid leukemia. The experimentally derived sensitivity data on cytarabine, alkylating agents, and MDR drugs (i.e., etoposide, doxorubicin, vincristine) thus resemble the clinical experience with these drugs, and we conclude that the use of a clonogenic assay on the described panel of SCLC cell lines can give valuable information for the selection of agents for combination therapy.

Cellular pharmacokinetics of carboplatin and cisplatin in relation to their cytotoxic action

We have studied the cellular pharmacokinetics of carboplatin (CBDCA), as part of the evaluation of the antitumor activity of CBDCA in cancers limited to the peritoneal cavity in comparison with cisplatin (cDDP). The uptake of CBDCA into L1210 (lymphosarcoma), CC531 (colonic carcinoma), COV413.B (human ovarian carcinoma) and NB1 (human neuroblastoma) cells was 1.5 to 13 times lower than the uptake of cDDP. The uptake of CBDCA into human ovarian carcinoma cells, taken directly from patients, was also 8-20 times lower than cDDP. Platinum concentrations, expressed as a percentage of the total intracellular Pt concentration, were similar for CBDCA and cDDP in cytosol and nucleus/membrane fractions. A second major difference between the drugs was their binding to DNA. Less CBDCA-DNA than cDDP-DNA adducts were formed after incubation at equimolar amounts of drug with isolated salmon sperm DNA (5-25 times less). A 16-69 times higher concentration of CBDCA than cDDP was needed to induce similar changes in cell growth activity (50% [3H]thymidine inhibition) in CC531 and COV413.B cells, indicating that equitoxicity can only be achieved when tumor cells are exposed to higher concentrations of CBDCA than cDDP. Similar toxicity was achieved in CC531 cells after incubation with a 16-fold higher CBDCA dose than cDDP. Comparable intracellular platinum concentrations, however, were obtained with a 10-fold higher CBDCA dose, suggesting that cellular pharmacokinetics of the drugs are different. Regarding drug uptake and pharmacokinetics the mechanism of action of CBDCA differed from cDDP at a cellular level.

Cisplatin-carboplatin therapy in extensive non-small cell lung cancer: a Cancer and Leukemia Group B study

The response to cisplatin in non-small cell lung cancer (NSCLC) is limited by the renal and neurological toxicities of this agent. Carboplatin has modest activity in NSCLC and when given in conventional doses has a different spectrum of toxicity. Both drugs were administered to 76 eligible patients with advanced NSCLC. No patient had been previously treated with chemotherapy. Cisplatin 50 mg/m2 and carboplatin 350 mg/m2 were administered every 28 days until disease progression occurred. There was 1 complete response and the overall response rate among the 68 evaluable for response patients was 13%. Neither histological subtype nor initial performance status was a significant factor influencing response. Median survival was 5.1 months with significant differences based on initial performance status but not on histological subtype. Severe or life-threatening leukopenia and thrombocytopenia occurred in 23% and 36% of the 76 patients, respectively. There were 2 toxic deaths, 1 each due to infection and haemorrhage. The efficacy of this combination is not different from that of carboplatin alone, and the combination may be of greater benefit in patients with more responsive tumours than NSCLC.

Reduced variation in the clonogenic assay obtained by standardization of the cell culture conditions prior to drug testing on human small cell lung cancer cell lines

An advantage of established tumor cell lines compared to fresh human tumor specimens used in sensitivity assessments is the possibility of repeated experiments. Ultimately a database of sensitivity profiles on a panel of cell lines can be made and the sensitivity to new drugs compared with historical data. A prerequisite of this strategy is a minimal interexperimental variation. The sensitivity of eight human small cell lung cancer cell lines to adriamycin, daunomycin, aclacinomycin A, and mitoxantrone was tested in the clonogenic assay. A covariation in the sensitivity to the drugs emphasized the importance of simultaneous drug testing on the same batch of cells. On one cell line (NCI-N592) the interexperimental variation was further evaluated and a significant correlation was found between preexposure culture conditions, size of S-phase, and sensitivity to adriamycin, daunomycin, and mitoxantrone. Rigorous standardization of the growth conditions prior to clonogenic assay reduced the variation in the sensitivity to adriamycin from a factor of five to only 10-15%. It is concluded that simultaneous experiments on the same batch of cells in drug comparisons should be used if possible. Specification and standardization of culture conditions are necessary in the comparison of drugs tested in different experiments. Inclusion of the same reference drug in all experiments may further increase the validity of comparisons in different experiments.