Effects of cis-dichlorodiammine platinum (II) on DNA synthesis in kidney and other tissues of normal and tumour-bearing rats

Redefining Role of 5-Fluorouracil and Exploring the Impact of Taxanes and Cisplatin in Locally Advanced and Recurrent Carcinoma Cervix in Concurrent Setting With Radiotherapy: A Literature Review

Cervical carcinoma is the fourth most frequent cancer among women worldwide while it is common in rural India. The irony of the situation is that it continues to present in a locally advanced stage with bulky disease posing a significant challenge to the current treatment modalities despite various screening programs. Concurrent chemoradiotherapy is the mainstay of treatment for locally advanced carcinoma cervix. However, the appropriate dosing schedules, along with the salutation of the chemotherapeutic agent, remain a matter of debate to date. The use of chemotherapy in the neoadjuvant and adjuvant setting promises to improve progression-free survival and overall survival. The article aims to review various chemotherapy and their regimens in the treatment of carcinoma of the cervix.

In vitro modulation of cisplatin resistance by cytokines

We have previously demonstrated that treatment of wild-type (wt) T98G malignant glioma cells with Cisplatin (CDDP) led to a resistant phenotype. It has been demonstrated that interleukin 1 (IL-1) potentiates the cytotoxic effect of CDDP and that IL-6 decreases cytotoxicity by inhibition of apoptosis in cancer cells. Here we examined the influence of IL-1 and IL-6 on the sensitivity of resistant and wt T98G cells. Using semi-quantitative PCR reactions in three independent experiments, resistant glioma cells revealed a decreased IL-1alpha (50.3+/-7.2), IL-1beta (56.0+/-4.0) and IL-6 (44. 3+/-18.2) mRNA content compared to wt cells (100%;P<0.05). Resistant and wt cells were positive for the receptors IL-1RI and IL-6R (PCR). To investigate whether IL-1alpha, IL-1beta or IL-6 changes the sensitivity of the resistant and wt cells towards CDDP, cells were incubated up to 7 days with 10(-5) M CDDP and with different concentrations (0, 0.01, 0.1, 1 ng/ml) of cytokine. Sensitivity was tested in a colorimetric assay (MTT). IL-6 did not influence the sensitivity towards CDDP of either wt or resistant cells, while IL-1alpha and IL-1beta enhanced sensitivity of resistant cells to CDDP. These data suggest that autocrine IL-1 production is involved in the mechanisms of resistance in T98G cells.

Enhancement of sensitivity of urinary bladder tumor cells to cisplatin by c-myc antisense oligonucleotide

BACKGROUND

Tumor cells have different degrees of sensitivity and resistance to anticancer agents. The acquisition of drug resistance is a major concern in cancer treatment. Because the c-myc oncogene has been implicated in the poor prognosis of some human tumors including urinary bladder tumor (UBT), the resistance of such tumors to anticancer drugs may correlate with c-myc expression. The present study tested this hypothesis by examining the effect of c-myc antisense oligonucleotide treatment on the sensitivity of human UBT cells to anticancer chemotherapeutic agents.

METHODS

Cytotoxicity determined by a 1-day microculture tetrazolium dye assay. C-myc mRNA was examined by Northern blot analysis.

RESULTS

The T24 human UBT cell line constitutively expresses c-myc mRNA. Treatment of T24 cells with the c-myc antisense oligonucleotide resulted in a significant inhibition of the expression of c-myc mRNA. Treatment of T24 cells with the c-myc antisense oligonucleotide in combination with mitomycin C, Adriamycin, or 5-fluorouracil (5-FU) did not overcome their resistance to these anticancer chemotherapeutic agents. However, combination treatment with the c-myc antisense oligonucleotide and CDDP resulted in a synergistic cytotoxic effect on T24 cells and two freshly derived UBT cells. Further, treatment of CDDP-resistant T24 cells (T24/CDDP) with c-myc antisense oligonucleotide and CDDP reversed the resistance. Pretreatment of T24 cells with either agent and then treatment with the second agent resulted in the same cytotoxic activity as achieved in the presence of the two agents. The combination of c-myc antisense oligonucleotide and carboplatin also resulted in a synergistic cytotoxic effect on T24 cells, and the combination of c-myc antisense oligonucleotide and trans-diamminedichloro-platinum resulted in an additive cytotoxic effect. Incubation of T24 or T24/CDDP cells with the c-myc antisense oligonucleotide increased the intracellular accumulation of CDDP, but not the accumulation of 5-FU.

CONCLUSIONS

This study demonstrates that combination treatment with c-myc antisense oligonucleotide and CDDP can overcome the CDDP-resistance of UBT cells and that the increased intracellular accumulation of CDDP by c-myc antisense oligonucleotide may play a role in the enhanced cytotoxicity obtained. The synergistic effect obtained with established UBT cells and freshly isolated UBT cells suggests that combination treatment with c-myc antisense oligonucleotide and CDDP may have clinical application in the therapy of CDDP-resistant UBT.

Overcoming TNF-alpha and drug resistance of human renal cell carcinoma cells by treatment with pentoxifylline in combination with TNF-alpha or drugs: the role of TNF-alpha mRNA downregulation in tumor cell sensitization

Previous studies have demonstrated that one of the possible mechanisms responsible for the resistance of tumor cells to tumor necrosis factor-alpha (TNF-alpha) is the expression of TNF-alpha mRNA and/or protein. Pentoxifylline (PTX) suppressed TNF-alpha gene transcription and downregulates the expression of TNF-alpha mRNA and the secretion of TNF-alpha protein in macrophages and monocytes. This study investigates whether PTX downregulates the expression of TNF-alpha mRNA and/or protein in renal cell carcinoma (RCC) cells and whether PTX enhances the sensitivity of TNF-alpha-resistant RCC cells to TNF-alpha. Further, we explored whether PTX enhances the sensitivity of RCC cells to agents other than TNF-alpha by downregulation of the expression of TNF-alpha mRNA and protein. The R4 human RCC cell line constitutively expressed TNF-alpha mRNA and protein and was resistant to TNF-alpha. When R4 cells were incubated with PTX, the level of TNF-alpha mRNA and protein was markedly reduced. Pentoxifylline and TNF-alpha together overcame the resistance of R4 cells to TNF-alpha. The R11 human RCC cell line did not constitutively express TNF-alpha mRNA or protein, and was resistant to TNF-alpha. The expression of TNF-alpha mRNA in R11 cells, but not the production of TNF-alpha protein, was induced by TNF-alpha. When PTX was used in combination with TNF-alpha, the level of TNF-alpha mRNA induced by TNF-alpha was markedly reduced. The combination of PTX and TNF-alpha overcame the resistance of R11 cells to TNF-alpha. Pentoxifylline also enhanced the sensitivity of R4 cells to interferon-alpha. Pentoxifylline and anti-TNF-alpha monoclonal antibody augmented the sensitivity of R4 cells to cis-diamminedichloroplatinum (II) (CDDP). This study demonstrated that PTX, in combination with TNF-alpha, IFN-alpha or CDDP, overcame the drug resistance to RCC cells and that downregulation of TNF-alpha mRNA by PTX may be related to the cytotoxicity enhanced by the combination. The implications of these findings for clinical therapy are discussed.

Overcoming cis-diamminedichloroplatinum (II) resistance of human ovarian tumor cells by combination treatment with cis-diamminedichloroplatinum (II) and tumor necrosis factor-alpha

BACKGROUND

Previous studies have demonstrated that tumor cells have different degrees of sensitivity and resistance to various cytotoxic agents. The acquisition of drug resistance is a major concern in cancer treatment. The current study investigates the cytotoxic effect of cis-diamminedichloroplatinum (II) (CDDP) and tumor necrosis factor-alpha (TNF-alpha) used in combination on CDDP-resistant human ovarian tumor cell lines.

METHODS

Cytotoxicity was determined by the microculture tetrazolium dye assay. Synergy was assessed by isobolographic analysis. TNF-mRNA was examined by Northern blot analysis.

RESULTS

Treatment of the CDDP-resistant C30 cells with CDDP and TNF-alpha overcame the resistance of C30 cells to CDDP or TNF-alpha. In addition, the combination of CDDP and TNF-alpha resulted in a synergistic effect on the C30-resistant line, the CDDP-sensitive parental cell line A2780, and two freshly derived ovarian carcinoma cell cultures. Treatment of C30 cells with CDDP followed by TNF-alpha showed a synergistic effect, whereas treatment with TNF-alpha followed by CDDP demonstrated a less cytotoxic effect. A possible mechanism of resistance to TNF-alpha in tumor cells is the induction of TNF-alpha mRNA and protein. C30 cells do not produce mRNA constitutively for TNF-alpha; however, treatment of C30 cells with TNF-alpha induces the expression of TNF-alpha mRNA. When CDDP was used in combination with TNF-alpha, the level of TNF-alpha mRNA induced by TNF-alpha was reduced significantly.

CONCLUSIONS

This study shows that the combination of CDDP and TNF-alpha can overcome the CDDP resistance of tumor cells and that downregulation of TNF-alpha mRNA by CDDP may play a role in the enhanced cytotoxicity seen with the combination of CDDP and TNF-alpha. The synergistic effect obtained with established ovarian tumor cell lines and in short-term cultures of freshly isolated ovarian tumors suggests that combination treatment with TNF-alpha and CDDP may have clinical applications in the treatment of drug-resistant tumors.

Enhanced susceptibility of cis-diamminedichloroplatinum-treated K562 cells to lysis by peripheral blood lymphocytes and lymphokine activated killer cells

BACKGROUND

Previous studies have reported that cis-diamminedichloroplatinum (II) (CDDP) exhibits various immunomodulating activities. The current study investigates the effect of CDDP on the susceptibility of K562 cells to lysis by peripheral blood lymphocytes (PBL), natural killer (NK) cells, and lymphokine activated killer (LAK) cells.

METHODS

Cytotoxicity was determined by the 51Cr release assay.

RESULTS

Treatment of K562 cells with CDDP at 10 micrograms/ml or more for 3 hours or more enhanced their susceptibility to lysis by PBL. This CDDP-mediated enhancement of lysis was observed by PBL derived from healthy donors and from patients with urinary bladder tumor or with other malignant and nonmalignant urologic diseases. The CDDP-induced enhancement of K562 cell susceptibility to lysis by PBL also was observed when purified NK cells and LAK cells were used as effector cells. The CDDP analog, carboplatin, enhanced the susceptibility of K562 cells to lysis by PBL, but treatment with transdiamminedichloroplatinum (II) had no effect. Several experiments were done to investigate the mechanism of the enhanced susceptibility of CDDP-treated K562 cells to lysis by PBL. Treatment of K562 cells with CDDP had no effect on the expression of major histocompatibility complex (MHC) Class I, MHC Class II, neural cellular adhesion molecule, and leukocyte function antigen-1 on the tumor cells. The frequency of target cell conjugates to PBL was not changed by CDDP-treated K562 cells. Pretreatment of K562 cells with CDDP and lysosomotrophic agents (L-leucine-methyl-ester or chloroquine) abrogated their enhanced susceptibility to lysis by PBL. CDDP treatment of K562 cells did not augment their sensitivity to alpha-interferon, gamma-interferon, tumor necrosis factor alpha, or natural killer cytotoxic factor (NKCF). Treatment of effector cells with CDDP had no effect on their cytotoxic function.

CONCLUSIONS

These results demonstrate that CDDP has a direct effect on the K562 target cells, rendering them more susceptible to lysis by PBL, NK cells, and LAK cells. In addition, the result suggest that CDDP-mediated enhancement of target cell lysis is not attributable to changes of surface membrane antigen expression or recruitment of precursor cells but to processing of CDDP by the cells. The possible mechanisms of the effect of CDDP on K562 cells and clinical implications are discussed.

Modulation by cis-diamminedichloroplatinum (II) of the susceptibilities of human T24 lined and freshly separated autologous urinary bladder transitional carcinoma cells to peripheral blood lymphocytes and lymphokine activated killer cells

The effects of cis-diamminedichloroplatinum (II) on the susceptibilities of human T24 lined and freshly separated autologous urinary bladder transitional carcinoma cells to lysis by peripheral blood lymphocytes of patients with urinary bladder cancer was analysed in a 12-hour 51Cr release assay. Treatment of T24 cells with cis-diamminedichloroplatinum (II) at 10 micrograms./ml. for three hours enhanced their susceptibility to peripheral blood lymphocytes and lymphokine activated killer cells. Kinetics studies demonstrated that the enhancement of their susceptibility became noticeable by three hours and continued until 12 hours. The susceptibilities of autologous tumor cells to both large granular lymphocytes and T lymphocytes were also enhanced by treatment of them with cis-diamminedichloroplatinum (II). There was no significant difference in the number of peripheral blood lymphocytes binding to cis-diamminedichloroplatinum (II)-treated T24 cells as compared with untreated T24 cells. Treatment of T24 cells with mitomycin C did not change their natural killer sensitivity. Pretreatment of T24 cells with cis-diamminedichloroplatinum (II) and lysosomotrophic agents (L-leucin-methyl-ester or chloroquine) reduced the enhancement of their susceptibility to natural killer cells by cis-diamminedichloroplatinum (II) alone. On the other hand, pretreatment of peripheral blood lymphocytes with cis-diamminedichloroplatinum (II) had no influence on the cytotoxicity against T24 cells. These results indicate that cis-diamminedichloroplatinum (II) may have an augmenting effect on the susceptibility of tumor cells to the cell-mediated cytotoxicity partly through a modification of cell membrane independently of its antimetabolic activity and this modification may be one of the possible mechanisms responsible for tumor regression after chemotherapy with cis-diamminedichloroplatinum (II).

The effects of the platinum anti-tumour agents on renal cell kinetics and the response to a second cytotoxic agent

Following 15 mg kg-1 cisplatin to mice, the labelling index (LI) in the kidney decreased from 0.4% to less than 0.01% at 1-3 d, increased to 1.9% at day 16 and returned to control levels by day 30. Cytotoxicity was assessed by counts of viable tubule cross-sections and recovery was incomplete up to 14 months after treatment. Cisplatin treatment impaired the regeneration response to a dose of 16 mg kg-1 uranyl nitrate (UN) given 14 d after cisplatin when assessed by an increase in LI and sub-capsular tubule count. However, there was recovery with greater time intervals. At nine months after cisplatin there was no difference in response to UN of controls and mice previously treated by cisplatin. Prior treatment with paraplatin or iproplatin at LD50 doses produced not only no histopathological changes but also no impairment of the subsequent responses to UN.

The influence of cisplatin and unilateral nephrectomy on the response of the rat kidney to irradiation

Cisplatin was administered as a single i.p. dose of 5 mg/kg to WAG/Rij rats at intervals of 7 days or 0.5 h before, or 7 days after graded X-ray doses to the left kidney. The right kidney was surgically removed 4 weeks after irradiation. Renal function was determined by measuring total urine volume excreted in 24 h, urine osmolality and serum urea. The severity of alterations in the various anatomic compartments of the kidney induced by the various treatments were graded histologically. The administration of 2 or 5 mg/kg cisplatin alone did not alter any of the kidney function parameters. Isoeffective radiation doses calculated for each of the functional parameters continuously decreased with increasing time after treatment. Differences between the isoeffective doses for the three combined treatments and for treatment with irradiation alone were only observed for urine osmolality and urine volume which primarily are tubular-related functional parameters. The histopathological grading studies also indicated that enhancement by cisplatin of radiation-induced damage was almost entirely confined to the tubules. The results of this study indicate that the sequence and length of time between treatments is an important variable in the development of cisplatin plus radiation-induced renal injury, but that none of the tested combinations showed a more than additive toxicity.

Acute systemic toxicity of combined cis-diamminedichloroplatinum and hyperthermia in the rat

To investigate the previously observed increased morbidity and mortality of combined cis-diamminedichloroplatinum (cis-DDP) and hyperthermia, BD IX rats were given 4 mg/kg cis-DDP i.p., waterbath hind leg heating (44 degrees C, 60 min) with resultant whole body hyperthermia, or combined treatment with or without systemic cooling. Cardiac blood and histopathologic sections of kidney, small intestine and liver were examined in rats sacrificed 2, 3 and 5 days after and femur bone marrow 5 days after treatment. In a separate experiment, the effect of systemic hyperthermia on renal function was tested. The most significant finding was a marked increase in cis-DDP induced renal damage by systemic hyperthermia, expressed as elevated creatinine levels and quantitatively enhanced proximal tubular necrosis. As both systemic hyperthermia and cis-DDP can result in primarily altered renal haemodynamics, it is postulated that relative tubular epithelial hypoxia and increased tubular exposure time to cis-DDP due to reduced tubular filtrate flow rate are likely mechanisms for the increased toxicity.

Isolation perfusion with cisplatin for malignant melanoma of the limbs

The role of hyperthermic isolation perfusion in the treatment of advanced melanoma of the limbs is clearly important. However, attention has in the past been focussed mainly on surgical aspects of the procedure. Whereas, equivalent interest in chemotherapy considerations has been lacking. A group of 15 patients have been treated by hyperthermic isolated perfusion for advanced melanoma of the limbs. The chemotherapeutic agent used was cisplatin. The follow-up period was 6 to 60 months. None of these patients had any sign or symptoms of toxicity from the drug. Twelve of these 15 patients are free of disease currently. There was one case of mortality due to unrelated cause and two cases of mortality due to disseminated disease which developed 6 months and 30 months, respectively, posttreatment. Since not uncommon toxic side effects from other drugs used for this procedure is significant, these results suggest that cisplatin may be used as an alternative chemotherapeutic agent in isolation perfusion for advanced melanoma of the extremities.

Cisplatin nephrotoxicity: insights into mechanism

Cis-dichlorodiammine platinum (II), or cisplatin, is currently among the most widely used agents in the chemotherapy of cancer. The chief limit to its greater efficacy is its nephrotoxicity. Acute and chronic nephrotoxicity of cisplatin occurs in man and animals especially after repeated administration. Morphological damage is restricted to the P3 segment of the proximal tubule. Abnormalities of water and solute reclamation and transglomerular passage of fluid are commonly associated with cisplatin nephrotoxicity. The vulnerability of the kidney to cisplatin may be related to its function as the primary excretory organ for platinum. Platinum binds to multiple cellular organelles and macromolecules, yet the precise mechanism of its cytotoxicity has not been delineated. Because abnormalities in renal function are preceded by a period where gross renal function appears normal, it is an ideal model to study the early physiological and biochemical determinants of metal induced acute renal failure.

The effects of cis-diamminedichloroplatinum (II) and radiation on the proliferation kinetics of mouse duodenal crypt cells and on a partially synchronized crypt cell population

The effects of c-DDP and radiation, given alone or in combination, on the proliferation kinetics of mouse duodenal crypt cells were investigated by repeated labelling with 3H-thymidine. Crypt cells surviving a single dose of X-rays or c-DDP alone, appeared to proliferate faster than untreated controls. When both agents were given in combination, the proliferation rate of surviving crypt cells was markedly reduced as a result of a significant increase in the DNA synthesis time. The influence of the proliferation rate of crypt cells on the effects of c-DDP and X rays was analyzed using the crypt microcolony assay. Mice were given single doses or 2 fractions of X rays with 2 different time intervals (4 and 48 hours) either alone or in combination with c-DDP given 30 minutes before the second fraction. The lethal effect of the drug was approximately the same in cells that were in accelerated proliferation after the first irradiation as in unstimulated cells. A tendency for less cell killing by the drug was observed 3.5 hours after the first irradiation. To rule out cell age dependent effects of c-DDP, crypt cell survival was assessed after irradiation was given alone or in combination with the drug on a partially synchronized crypt cell population, obtained by repeated injections of hydroxyurea. A predominant lethal effect of c-DDP was found in cells in the late G1-phase. These findings are important to consider both from a mechanistic point of view as for clinical application of the agents.

Renal and haemopoietic proliferative defects as a delayed consequence of cis-platin, adriamycin and daunomycin treatments

The long-term effects of Adriamycin (ADR), daunomycin (DMN) and cis-dichlorodiammine platinum (II) (DDP) on the ability of murine renal tubular epithelium and erythropoiesis to respond to an acute proliferative stress was investigated. Folic acid (FA) and acute anaemia induced by bleeding were used as acute proliferative stimuli for renal-tubule epithelium and erythropoiesis respectively. The ability of these normal cell-renewal systems to mount a regenerative proliferative response was evaluated by radioisotopic, morphological and gravimetric techniques 4 months after drug treatment. The results indicate that pretreatment with these agents produce a long-lasting reduction in the ability of these cell-renewal systems to mount regenerative proliferation. In the kidney, the ability to respond to FA was most severely compromised by ADR and DDP, whereas in the erythropoietic system all 3 agents induced a long-lasting proliferative defect.

Proliferative defects in renal and intestinal epithelium after cis-dichlorodiammine platinum (II)

The effects of cis-dichlorodiammine platinum II (DDP) on the intestinal mucosa and the kidney were studied after single and multiple treatments with intervals of 7-45 days. After a single treatment, the jejunal epithelium underwent a transient interruption of cell proliferation followed by a hyperplastic recovery and return to control proliferative rate on Day 7. Subsequent treatments led to suboptimal recovery for all treatment intervals. In contrast, DDP induced a 6-fold increase in [3H]dT incorporation in the kidney by Day 7 which remained high until Day 21, and returned to near-control values by Day 45. After a single DDP treatment, the "recovery potential" of kidneys, measured by the proliferative response to folic-acid stress, demonstrated suboptimal proliferative reserve compartments for up to 45 days. The distinction between acute and delayed sensitivity to subsequent drug treatment was more apparent in the DDP-treated kidney than in the intestinal epithelium.

Effect of ultrasound-induced hyperthermia and cis-diamminedichloride platinum II on murine renal function

Murine renal function was evaluated after ultrasound-induced kidney hyperthermia (42.5 degrees C and 46.5 degrees C for 35 minutes) and the administration of cis-diamminedichloride platinum II (8 mg/kg). A quantitative immunonephelometric technique was employed to determine urinary total protein (TP) and albumin (Alb) 1-180 days post-treatment. Hyperthermia of 46.5 degrees C elevated urinary TP excretion significantly more than that of 42.5 degrees C cis-diamminedichloride platinum (Cis DDP) administration greatly increase urine TP with a peak mean TP concentration of 488 microgram/ml, four days after Cis DDP (normal range was 26-48 microgram/ml). This returned to normal by day 14. The TP excretion after 42.5 degrees C hyperthermia with concurrent Cis DDP was similar to Cis DDP alone, implying there was no potentiation of early or late Cis-DDP-induced renal damage by hyperthermia. Late mouse mortality was greater (P = 0.08) after Cis DDP alone, than Cis DDP with 42.5 degrees C hyperthermia. There was a statistically significant increase (P less than 0.05) in the Alb/TP excretion ratio after the addition of 42.5 degrees C hyperthermia to Cis DDP, implying an alteration in the site or nature of the Cis DDP renal lesion by hyperthermia.

Cis-diamminedichloroplatinum (II) ototoxicity in the guinea pig

Cochleas from 12 guinea pigs were evaluated using light, scanning, and transmission electron microscopy after systemic administration of cis-diamminedichloroplatinum (cis-DDP). Administration of cis-DDP resulted in loss of the Preyer reflex and degeneration of outer hair cells (OHC) with increased dose. The OHC degeneration was most pronounced in the basal turns of the cochlea with greatest severity in the inner row. Ultrastructural evidence of OHC degeneration included dilatation of the parietal membranes, softening of the cuticular plate, increased vacuolization and increased numbers of lysosome-like bodies in the apical portion of the cell. Supporting cells appeared more sensitive than OHC. Alteration of supporting cell ultrastructure preceded detectable change in OHC. Injury to the supporting cells was noted with intracellular vesiculation and increased autophagocytosis.

Effects of certain antitumor platinum compounds on kidney esterases

In a study of the biochemical mechanism of renal toxicity of certain antitumor platinum compounds, particularly cis-dichlorodiammineplatinum(II) (NSC-119875), qualitative and quantitative studies of the soluble nonspecific kidney esterases were carried out using (C57BL/L X DBA/2) mice. There was a major suppression of the testosterone-dependent esterases of treated male mice; these levels dropped to levels below those found in untreated females within 72 h after certain of the drugs were administered. This effect appeared to be in inverse relationship to the numerical value of the LD50 values of the compounds investigated.

Effects of cytotoxic agents on TdR incorporation and growth delay in human colonic tumour xenografts

The relationship between the utilization of 3H-thymidine in situ ([3H]-TdR fractional incorporation or TFI) and tumour growth delay after treatment with various cytotoxic agents has been examined. It is shown that (a) it is not possible to predict tumour growth delay, or to select the most effective agent, from changes in TFI 1 day after treatment; (b) there is a good correlation between tumour growth delay and the time for recovery of TFI to the pretreatment level; (c) there is a relationship within a tumour line between the depression of TFI 4 days after treatment and growth dealy induced by the same treatment. This relationship appears to be independent of the mechanism by which the agent exerts its cytotoxic effect.

Fractional incorporation of [3H]thymidine and DNA specific activity as assays of inhibition of tumour growth

The Fractional Incorporation (FI) of [3H] thymidine ([3H]TdR) has been examined in small lung tumours after cyclophosphamide (CY) treatment in vivo and compared to the DNA specific activity (SA) at different times after treatment. FI was found to correlate with the incidence of labelled cells after treatment, whereas SA did not, due to the loss of DNA from drug-killed cells 72 h after treatment. The FI is independent of the precursor concentration in the tissue, and therefore may give a better index of DNA synthesis in irregularly perfused tissues than SA. Following either CY or 60Co radiation treatment, the time necessary for FI to reach the pretreatment level is quite similar to the growth delay measured for the FI depression 45 h after treatment and growth delay has been established in the Lewis lung tumour, which would allow the prediction of growth delay induced by another agent to be made within 2 days of treatment.