Search PubMed⌕ Search

Biomedical subjects

T S Lawrence

Publications and source records attributed to T S Lawrence.

At least 73 records · Page 4Linked to original sources
PubMed1996

Gemcitabine and radiosensitization in human tumor cells.

Gemcitabine is a nucleoside analogue with excellent clinical activity against solid tumors. Within the cell, gemcitabine is rapidly phosphorylated to its active di- and triphosphate metabolites. Cytotoxicity with gemcitabine appears to be related to multiple effects on DNA replication, where gemcitabine triphosphate can serve as both an inhibitor and substrate for DNA synthesis. Gemcitabine diphosphate inhibits ribonucleotide reductase, producing decreases in cellular dNTP pool levels in a cell-specific manner. These two major characteristics of gemcitabine, reduction in cellular dNTP pools and incorporation into DNA, are features of other antimetabolites antitumor agents which also exhibit radiosensitizing properties. Based on these favorable metabolic characteristics and the clinical activity of gemcitabine in tumor types which are commonly treated with radiation, the ability of gemcitabine to enhance X-radiation induced cytotoxicity was evaluated. Gemcitabine has been shown to be a potent radiosensitizer in a variety of tumor cell lines, including HT-29 colorectal carcinoma, pancreatic cancer, breast, non-small cell lung and head and neck cancer cell lines. Gemcitabine was most effective as a radiosensitizer when administered at least 2 hours prior to irradiation. For most cell lines, radiosensitization was evident at non-cytotoxic concentrations. The extent of radiosensitization increased with both increasing gemcitabine concentration and duration of exposure. Radiosensitization did not require redistribution of cells into a more radiosensitive phase of the cell cycle. The major metabolic effects observed under radiosensitizing conditions were the accumulation of high levels of gemcitabine triphosphate, and a selective decrease in the cellular dATP pool. The pattern of dATP decrease paralleled the increase in radiosensitization, whereas the level of gemcitabine triphosphate was not associated with the enhanced sensitivity to radiation. Compared to other radiosensitizers, the advantage of gemcitabine is that is can induce radiosensitization at concentrations that are 1000 times lower than typical plasma levels obtained with this drug. These studies will be used as guidelines for developing clinical trials of gemcitabine with radiation.

Antimetabolites, Antineoplastic↗
PubMed1996

Dependence of fluorodeoxyuridine-induced cytotoxicity and megabase DNA fragment formation on S phase progression in HT29 cells.

The relationship between cell cycle progression and induction of DNA double-strand breaks and cytotoxicity by exposure to fluorodeoxyuridine (FdUrd) was studied in HT29 human colon cancer cells. Fractionation of drug-treated populations by centrifugal elutriation yielded subpopulations having widely divergent abilities to progress through S phase in the presence of the drug. One of these subpopulations, which appeared to undergo coordinated growth arrest, was resistant to FdUrd cytotoxicity and DNA damage. In contrast, the subpopulation which was able to progress furthest through S phase in the presence of FdUrd underwent unbalanced growth arrest (i.e., increase in size and mass out of proportion to DNA synthesis), and displayed both DNA double-strand break formation (assayed by pulsed field gel electrophoresis) and loss of clonogenicity. When cells were elutriated prior to drug treatment, producing fractions enriched in cells at various cell cycle stages, no significant differences in sensitivity to FdUrd-induced cytotoxicity were detected among elutriation fractions. These findings support the model that, in HT29 cells, progression into and through S phase during drug treatment is an important determinant of FdUrd-induced DNA damage and cytotoxicity, but that the cell cycle position at the start of drug exposure is not a critical factor for these effects.

Antimetabolites, Antineoplastic↗
PubMed1996

Fluorodeoxyuridine-mediated cytotoxicity and radiosensitization require S phase progression.

The role of cell cycle redistribution in fluoropyrimidine-mediated radiosensitization remains unresolved. To determine if radiosensitization resulted from the redistribution of cells into a sensitive phase of the cell cycle, we assessed fluorodeoxyuridine (FdUrd)-mediated radiosensitization in flow-sorted mid-S phase HT29 human colon cancer cells. We hypothesized that if FdUrd-mediated radiosensitization were strictly the result of cell cycle redistribution, FdUrd-treated mid-S phase cells would remain as radioresistant as mid-S phase cells cultured in the absence of drug. However, we found that the mid-S phase cells from FdUrd-treated populations were markedly radiosensitized. To assess the role of S phase progression in radiosensitization, we exposed FdUrd-treated cells to aphidicolin, an inhibitor of DNA polymerase alpha, prior to irradiation. We found that aphidicolin blocked the radiosensitizing (and cytotoxic) effects of FdUrd. These results, combined with our previous observations that FdUrd-treated cells at the G1/S boundary are minimally sensitized, appear to disprove the hypothesis that sensitization results strictly from cell cycle distribution. Furthermore, they suggest that a key aspect of both FdUrd-mediated radiosensitization and cytotoxicity is S phase progression on a damaged DNA template.

Antimetabolites, Antineoplastic↗
PubMed1996

Radiosensitization of human solid tumor cell lines with gemcitabine.

Gemcitabine has high clinical activity in several solid tumors that are treated with radiotherapy and/or chemotherapy. The mode of action of gemcitabine involves a number of intracellular changes that are shared by other radiation sensitizing anticancer drugs. There is therefore a clear rationale for investigating the interaction between gemcitabine and radiation in human cancer cell lines in vitro. Gemcitabine has been shown to be a potent radiosensitizer in human colorectal, pancreatic, and other solid tumor cell lines. Gemcitabine produces radiation enhancement ratios that are higher than those of other established radio-sensitizing agents, and this radiation enhancement occurs at low noncytotoxic concentrations. Radiosensitization increases with dose and with duration of exposure to gemcitabine, and is greatest when exposure to gemcitabine precedes radiation. The primary radiosensitizing effect of gemcitabine seems to be associated with depletion of endogenous nucleotide pools; deoxyadenosine triphosphate reduction is particularly striking, even at the lowest concentrations of gemcitabine. These studies suggest a number of approaches that are relevant to the clinical use of gemcitabine in patients with solid tumors.

Antimetabolites, Antineoplastic↗
PubMed1995

Incorporation of 5-bromo-2'-deoxyuridine into colorectal liver metastases and liver in patients receiving a 7-day hepatic arterial infusion.

Preclinical and clinical data suggest that the combination of hepatic arterial bromodeoxyuridine (BrdUrd), a thymidine analogue radiation sensitizer, and high-dose three-dimensional conformal radiation therapy offer a high potential for improving the local control of intrahepatic cancers. A key step in the design of a successful protocol is to determine in patients the conditions for BrdUrd administration that would be expected to produce selective radiosensitization of the tumor. Therefore, we designed a clinical trial to assess BrdUrd incorporation into the DNA of hepatic colorectal metastases and normal liver after a 7-day continuous BrdUrd infusion at a dose rate of 25 mg/kg/day (the maximal tolerated dose for a 14-day infusion) for patients undergoing laparotomy for either resection of liver metastases or hepatic arterial catheter and pump placement. Thirteen patients were entered into this study. We found that the average replacement of thymidine by BrdUrd in the tumor and normal liver were 11.6 +/- 1.2% and 1.1 +/- 0.2%, respectively. This extent of incorporation would be expected to produce a single fraction radiation enhancement of 1.5 in the tumor without detectable sensitization of the normal liver. Immunohistochemical staining for BrdUrd revealed heterogeneity of incorporation with a range of approximately 60-80% of the cells labeled in different regions of the specimens. These findings suggest that hepatic arterial BrdUrd given at this dose and schedule has a high likelihood of producing clinically significant radiosensitization for patients with hepatic metastases from colorectal cancer. Furthermore, the demonstrated selectivity of tumor perfusion that can be obtained with hepatic arterial infusion combined with the high proliferative rate of colorectal metastases (versus normal liver) suggests that these patients may be good candidates for tumor-directed gene transfer therapy by using regionally delivered retroviral vectors.

Adult↗
PubMed1995

The treatment of colorectal liver metastases with conformal radiation therapy and regional chemotherapy.

PURPOSE: Whole-liver radiation, with or without chemotherapy, has been of modest benefit in the treatment of unresectable hepatic metastases from colorectal cancer. A Phase I/II study combining escalating doses of conformally planned radiation therapy (RT) with intraarterial hepatic (IAH) fluorodeoxyuridine (FdUrd) was performed. METHODS AND MATERIALS: Twenty-two patients with unresectable hepatic metastases from colorectal cancer, 14 of whom had progressed after previous chemotherapy (2 with prior IAH FdUrd), were treated with concurrent IAH FdUrd (0.2 mg/kg/day) and conformal hepatic radiation therapy (1.5-1.65 Gy/fraction twice a day). The total dose of radiation given to the tumor (48-72.6 Gy) depended on the fraction of normal liver excluded from the high-dose volume. All patients were assessed for response, toxicity, hepatobiliary relapse, and survival. Median potential follow-up was 42 months. RESULTS: Eleven of 22 patients demonstrated an objective response, with the remainder showing stable disease. Actuarial freedom from hepatic progression was 25% at 1 years. The most common acute toxicity was mild to moderate nausea and transient liver function test abnormalities. There were three patients with gastrointestinal bleeding (none requiring surgical intervention) after the completion of treatment. Overall median survival was 20 months. The presence of extrahepatic disease was associated with decreased survival (p < 0.01). CONCLUSIONS: Combined conformal radiation therapy and IAH FdUrd can produce an objective response in 50% of patients with hepatic metastases from colorectal cancer. However, response was not durable, and hepatic progression was frequent. Improvements in hepatic tumor control for patients with metastatic colorectal cancer may require higher doses of conformal radiation and/or improved radiosensitization. In an effort to increase radiosensitization, we have recently initiated a clinical trial combining IAH bromode-oxyuridine, a thymidine analog radiosensitizer, with conformal high dose radiation therapy.

Adult↗
PubMed1995

Hepatic toxicity resulting from cancer treatment.

Radiation-induced liver disease (RILD), often called radiation hepatitis, is a syndrome characterized by the development of anicteric ascites approximately 2 weeks to 4 months after hepatic irradiation. There has been a renewed interest in hepatic irradiation because of two significant advances in cancer treatment: three dimensional radiation therapy treatment planning and bone marrow transplantation using total body irradiation. RILD resulting from liver radiation can usually be distinguished clinically from that resulting from the preparative regime associated with bone marrow transplantation. However, both syndromes demonstrate the same pathological lesion: veno-occlusive disease. Recent evidence suggests that elevated transforming growth factor beta levels may play a role in the development of veno-occlusive disease. Three dimensional treatment planning offers the potential to determine the radiation dose and volume dependence of RILD, permitting the safe delivery of high doses of radiation to parts of the liver. The chief therapy for RILD is diuretics, although some advocate steroids for severe cases. The characteristics of RILD permit the development of a grading system modeled after the NCI Acute Common Toxicity Criteria, which incorporates standard criteria of hepatic dysfunction.

Dose-Response Relationship, Radiation↗
PubMed1995

Analysis of clinical complication data for radiation hepatitis using a parallel architecture model.

PURPOSE: The detailed knowledge of dose volume distributions available from the three-dimensional (3D) conformal radiation treatment of tumors in the liver (reported elsewhere) offers new opportunities to quantify the effect of volume on the probability of producing radiation hepatitis. We aim to test a new parallel architecture model of normal tissue complication probability (NTCP) with these data. METHODS AND MATERIALS: Complication data and dose volume histograms from a total of 93 patients with normal liver function, treated on a prospective protocol with 3D conformal radiation therapy and intraarterial hepatic fluorodeoxyuridine, were analyzed with a new parallel architecture model. Patient treatment fell into six categories differing in doses delivered and volumes irradiated. By modeling the radiosensitivity of liver subunits, we are able to use dose volume histograms to calculate the fraction of the liver damaged in each patient. A complication results if this fraction exceeds the patient's functional reserve. To determine the patient distribution of functional reserves and the subunit radiosensitivity, the maximum likelihood method was used to fit the observed complication data. RESULTS: The parallel model fit the complication data well, although uncertainties on the functional reserve distribution and subunit radiosensitivity are highly correlated. CONCLUSION: The observed radiation hepatitis complications show a threshold effect that can be described well with a parallel architecture model. However, additional independent studies are required to better determine the parameters defining the functional reserve distribution and subunit radiosensitivity.

Hepatitis↗
PubMed1995

Recent advances in radiation oncology.

Radiotherapy remains an important component of the management of malignant disease. Especially when combined with cytotoxic chemotherapy, limited surgical excision, or both, irradiation has been shown to control disease in the primary site and regional nodes without the need for surgical extirpation as frequently as in past years. New developments in three-dimensional treatment planning and the precise delivery of high-dose radiation promise to increase the benefit of radiation treatment. Finally, molecular studies of the cell's response to radiation and the phenomena of DNA damage and repair are providing explanations for heretofore unexplained radiobiologic observations. Such research is laying the groundwork for targeted manipulation of the cell's response to radiation, which will be tested in the near future.

Animals↗
PubMed1995

Preoperative radiation therapy and iododeoxyuridine for large retroperitoneal sarcomas.

PURPOSE: Local failure is frequent after conventional therapy for patients with retroperitoneal sarcomas. A Phase I/II multimodality approach was used, combining iododeoxyuridine (IdUrd) and radiation therapy, followed by attempted surgical resection, with the goal of improving local control. METHODS AND MATERIALS: Patients with retroperitoneal sarcomas were treated with three to five consecutive cycles of treatment. Each 14-day cycle consisted of a continuous intravenous infusion of IdUrd on days 1-5, twice a day radiation therapy (1.25 Gy/fraction) on days 8-12, and a break on day 13 and 14. Surgical resection was attempted after three or five cycles. Patients resected after three cycles received an additional two cycles of treatment with radiation directed to the tumor bed. IdUrd dose was escalated in Phase I fashion (1000 mg/m2/day, 1333 mg/m2/day, and 1600 mg/m2/day). The median potential follow-up was 31 months. RESULTS: Sixteen patients (13 with high grade tumors) were treated. The median maximum tumor size was 17 cm. Resection margins were negative in four patients, microscopically positive in four patients, and grossly positive in three patients. Five patients were not resected. The only grade 4 acute toxicity observed was vomiting which occurred in three patients receiving upper abdominal radiation. Postsurgical and long-term complications were rare. Median survival overall and for resected patients were 18 and 32 months, respectively. Local control was observed in three out of four patients with negative margins (9, 40+, and 51+ months), two out of four patients with microscopically positive margins (4 and 22 months), and one out of three patients with grossly positive margins (46+ months). The overall freedom from local progression was 45% at 24 months. CONCLUSION: Retroperitoneal sarcomas can be resected after preoperative radiation therapy and IdUrd, with encouraging local control in patients resected with negative or microscopically positive margins. The recommended dose using this drug and radiation schedule appears to be 1600 mg/m2/day, which will form the basis for a Phase II trial.

Actuarial Analysis↗
PubMed1995

Dependence of fluorodeoxyuridine-mediated radiosensitization on S phase progression.

Recent evidence casts doubt on the hypotheses that fluoropyrimidine-mediated radiosensitization is related to cytotoxicity or to cell cycle redistribution into the G1/S boundary. We hypothesized that cells that are capable of progressing into S phase in the presence of fluorodeoxyuridine may also be more susceptible to radiation-induced damage. To test this hypothesis, fluorodeoxyuridine (FdUrd)-treated HT29 human colon cancer cells were separated by centrifugal elutriation into four fractions (1-4) containing a range of cells from those at the G1/S boundary (fraction 1) to those which had progressed approximately 11% into S phase (fraction 4). We found that fraction 4 cells showed significantly greater radiosensitization than fraction 1 cells. We also compared the effects of fluorodeoxyuridine on HT29 and SW620 human colon cancer cells. We found that, in contrast with HT29 cells, SW620 cells arrested at the G1/S boundary and were minimally radiosensitized. Finally, we found that an increase in sensitivity was correlated with a decrease in the rate of repair of DNA double-strand and single-strand breaks (assessed by asymmetric field inversion gel electrophoresis and alkaline elution respectively). These findings are consistent with the hypothesis that fluorodeoxyuridine-mediated radiosensitization depends on S phase progression and a decreased ability to repair radiation-induced DNA damage.

Cell Separation↗
PubMed1995

High-dose localized radiation therapy for treatment of hepatic malignant tumors: CT findings and their relation to radiation hepatitis.

OBJECTIVE: High-dose radiation therapy of the liver performed using overlapping portals defined by a three-dimensional treatment-planning system (conformal radiation therapy) is a new method of treating hepatic tumors. This study was performed to delineate the differences in the CT appearances of the liver after therapy compared with other methods of radiotherapy and to correlate imaging findings to clinical findings of radiation hepatitis. MATERIALS AND METHODS: Contrast-enhanced CT scans were obtained at 8- to 12-week intervals on 31 consecutive patients with primary or metastatic hepatic malignant tumors. All had undergone high-dose conformal radiation therapy and injection of fluorodeoxyuridine into the hepatic artery as part of the treatment for unresectable hepatic neoplasms. Tumor size, location, presence of changes within the target volume after therapy, presence of atrophy of the treated segments or hypertrophy of the untreated segments, ascites, and any changes in adjacent organs seen on serial CT scans obtained before and after treatment were recorded. Clinical records were reviewed for evidence of radiation hepatitis (nonmalignant ascites evident on physical examination and a twofold elevation of alkaline phosphatase in the anicteric patient). RESULTS: In 23 (74%) of the 31 patients, follow-up CT studies after treatment showed a low-attenuation area adjacent to the hepatic tumor in the target volume. In two patients with fatty infiltration of the liver, CT showed relative increased density in the treatment portal. A sharp, straight interface was rarely seen at the treatment margin. Maximal effect was seen 2-3 months after completion of therapy and persisted for up to 3 months. Atrophy in the treated segment or lobe was seen in four patients, hypertrophy of the untreated liver was seen in four patients, and both effects were seen in seven patients. Extrahepatic effects included segmental right renal atrophy in three patients and duodenal wall thickening in two patients. Only two patients (6%) in this series had clinical evidence of radiation hepatitis. CONCLUSION: High-dose localized radiotherapy of the liver results in reversible hypodense regions in the liver parenchyma within the target volume that do not have a sharp interface delineating the radiation portal. This appearance should not be confused with tumor progression or irreversible liver injury. The changes evident on CT scans after therapy are not predictive of radiation hepatitis.

Adult↗
PubMed1995

Radiosensitization of human tumor cells by gemcitabine in vitro.

Gemcitabine is a novel nucleoside analog that has shown clinical activity in a variety of solid tumors. The use of gemcitabine in combination with radiation was investigated in vitro using HT-29 colon carcinoma cells. Gemcitabine was demonstrated to be a potent radiosensitizer even at noncytotoxic concentrations. Increasing the concentration of gemcitabine and/or the duration of exposure increased the radiosensitization of cells. Biochemical studies indicated that a reduction in the dATP pool was the most likely mechanism of radiosensitization by gemcitabine. This effect is of great clinical importance and should be investigated in vivo.

Cell Cycle↗
PubMed1995

Effect of bromodeoxyuridine on radiation-induced DNA damage and repair based on DNA fragment size using pulsed-field gel electrophoresis.

We have used biphasic linear ramping pulsed-field gel electrophoresis (PFGE) to understand the effect of incorporation of bromodeoxyuridine (BrdUrd) on radiation-induced DNA damage and repair. This technique permits a determination of the fragment size distribution produced immediately after irradiation as well as during the repair period. We found that incorporation of BrdUrd increased the induction and decreased the repair of radiation damage. The fragment size distribution was consistent with a random breakage model. When we found that significantly more damage was detected after irradiation of deproteinized DNA compared to intact cells, we studied the effects of BrdUrd incorporation on the radiation response of cells or DNA at various phases of preparation for electrophoresis: cells adherent to the culture dish (A), trypsinized cells (B), agarose-embedded cells (C) and deproteinized DNA (D). Although there was a general tendency to detect more damage when irradiation was performed later in the preparation process, steps B and C were the only successive steps which were significantly different. These findings demonstrate that incorporation of BrdUrd randomly increases the induction of radiation damage and decreases its repair at the level of 200 kbp to 5 Mbp fragments. Furthermore, they confirm that the amount of damage detected depends upon the conditions of the cells or DNA at the time of irradiation.

Antimetabolites, Antineoplastic↗
PubMed1995

Current status of radiation sensitization by fluoropyrimidines.

Numerous clinical studies suggest that the combination of the fluoropyridimine fluorouracil and radiation is a more effective treatment for many cancers, especially gastrointestinal tumors, than is either modality alone. Also, the combination of fluorouracil and radiation has greatly expanded the possibilities for organ-preserving treatments, for example, in patients with bladder cancer. Another fluoropyridimine, floxuridine, administered via the hepatic artery, has been used in combination with irradiation in the treatment of patients with intrahepatic cancers and may prolong survival in patients with unresectable hepatobiliary malignancies. The beneficial effects of combining fluoropyrimidines and radiation are thought to be due to radiosensitization. More studies are needed to clarify the mechanism(s) of radiosensitization so as to optimize regimens in which the fluoropyrimidines are used.

Animals↗
PubMed1995

Lack of dependence of 5-fluorodeoxyuridine-mediated radiosensitization on cytotoxicity.

It has been proposed that fluoropyrimidine-mediated cytotoxicity and radiosensitization are closely correlated. We have shown that HT29 human colon cancer cells transfected with the E. coli dUTPase gene are resistant to 5-fluorodeoxyuridine (FdUrd)-mediated cytotoxicity, presumably through more effective elimination of dUTP. We used these cells to assess the association between radiosensitization and cytotoxicity produced by FdUrd. The radiation sensitivities of the clones expressing elevated dUTPase activity (dutE clones) were similar to those of untransfected HT29 cells or HT29 cells which had been transfected with only the expression vector for the E. coli gene (con clones). We found that FdUrd produced similar increases in radiation sensitivity regardless of dUTPase activity. Levels of dUTPase in the dutE clones remained elevated during the entire period of FdUrd exposure, demonstrating that the lack of difference between dutE and Con clones was not a reflection of down-regulation of dUTPase activity by FdUrd. Flow cytometry showed that all clones progressed past the G1/S-phase boundary and into early S phase during FdUrd treatment. These data suggest that the mechanisms of FdUrd-mediated cytotoxicity and radiosensitization are not closely linked. These findings, combined with our previous investigations, are consistent with the hypothesis that radiosensitization occurs in cells which progress past the G1/S-phase boundary in the presence of FdUrd.

Cell Survival↗