Nicotinamide exerts different acute effects on microcirculatory function and tissue oxygenation in rat tumors

The Role of Imaging Biomarkers to Guide Pharmacological Interventions Targeting Tumor Hypoxia

Hypoxia is a common feature of solid tumors that contributes to angiogenesis, invasiveness, metastasis, altered metabolism and genomic instability. As hypoxia is a major actor in tumor progression and resistance to radiotherapy, chemotherapy and immunotherapy, multiple approaches have emerged to target tumor hypoxia. It includes among others pharmacological interventions designed to alleviate tumor hypoxia at the time of radiation therapy, prodrugs that are selectively activated in hypoxic cells or inhibitors of molecular targets involved in hypoxic cell survival (i.e., hypoxia inducible factors HIFs, PI3K/AKT/mTOR pathway, unfolded protein response). While numerous strategies were successful in pre-clinical models, their translation in the clinical practice has been disappointing so far. This therapeutic failure often results from the absence of appropriate stratification of patients that could benefit from targeted interventions. Companion diagnostics may help at different levels of the research and development, and in matching a patient to a specific intervention targeting hypoxia. In this review, we discuss the relative merits of the existing hypoxia biomarkers, their current status and the challenges for their future validation as companion diagnostics adapted to the nature of the intervention.

Validation of diffuse correlation spectroscopy sensitivity to nicotinamide-induced blood flow elevation in the murine hindlimb using the fluorescent microsphere technique

Nicotinamide has been shown to affect blood flow in both tumor and normal tissues, including skeletal muscle. Intraperitoneal injection of nicotinamide was used as a simple intervention to test the sensitivity of noninvasive diffuse correlation spectroscopy (DCS) to changes in blood flow in the murine left quadriceps femoris skeletal muscle. DCS was then compared with the gold-standard fluorescent microsphere (FM) technique for validation. The nicotinamide dose-response experiment showed that relative blood flow measured by DCS increased following treatment with 500- and 1000-mg  /  kg nicotinamide. The DCS and FM technique comparison showed that blood flow index measured by DCS was correlated with FM counts quantified by image analysis. The results of this study show that DCS is sensitive to nicotinamide-induced blood flow elevation in the murine left quadriceps femoris. Additionally, the results of the comparison were consistent with similar studies in higher-order animal models, suggesting that mouse models can be effectively employed to investigate the utility of DCS for various blood flow measurement applications.

Inhibition of PARP-1 by olaparib (AZD2281) increases the radiosensitivity of a lung tumor xenograft

PARP-1 is a critical enzyme in the repair of DNA strand breaks. Inhibition of PARP-1 increases the effectiveness of radiation in killing tumor cells. However, although the mechanism(s) are well understood for these radiosensitizing effects in vitro, the underlying mechanism(s) in vivo are less clear. Nicotinamide, a drug structurally related to the first generation PARP-1 inhibitor, 3-aminobenzamide, reduces tumor hypoxia by preventing transient cessations in tumor blood flow, thus improving tumor oxygenation and sensitivity to radiotherapy. Here, we investigate whether olaparib, a potent PARP-1 inhibitor, enhances radiotherapy, not only by inhibiting DNA repair but also by changing tumor vascular hemodynamics in non-small cell lung carcinoma (NSCLC). In irradiated Calu-6 and A549 cells, olaparib enhanced the cytotoxic effects of radiation (sensitizer enhancement ratio at 10% survival = 1.5 and 1.3) and DNA double-strand breaks persisted for at least 24 hours after treatment. Combination treatment of Calu-6 xenografts with olaparib and fractionated radiotherapy caused significant tumor regression (P = 0.007) relative to radiotherapy alone. To determine whether this radiosensitization was solely due to effects on DNA repair, we used a dorsal window chamber model to establish the drug/radiation effects on vessel dynamics. Olaparib alone, when given as single or multiple daily doses, or in combination with fractionated radiotherapy, increased the perfusion of tumor blood vessels. Furthermore, an ex vivo assay in phenylephrine preconstricted arteries confirmed olaparib to have higher vasodilatory properties than nicotinamide. This study suggests that olaparib warrants consideration for further development in combination with radiotherapy in clinical oncology settings such as NSCLC.

Nifedipine improves blood flow and oxygen supply, but not steady-state oxygenation of tumours in perfusion pressure-controlled isolated limb perfusion

Isolated limb perfusion allows the direct application of therapeutic agents to a tumour-bearing extremity. The present study investigated whether the dihydropyridine-type Ca(2+)-channel blocker nifedipine could improve blood flow and oxygenation status of experimental tumours during isolated limb perfusion. Perfusion was performed by cannulation of the femoral artery and vein in rats bearing DS-sarcoma on the hind foot dorsum. Perfusion rate was adjusted to maintain a perfusion pressure of 100-140 mmHg throughout the experiment. Following equilibration, nifedipine was continuously infused for 30 min (8.3 microg min(-1) kg(-1) BW). During constant-pressure isolated limb perfusion, nifedipine can significantly increase perfusion rate (+100%) and RBC flux (+60%) through experimental leg tumours. "Steal phenomena" in favour of the surrounding normal tissue and oedema formation were not observed. Despite the increased oxygen availability (+63%) seen upon application of this calcium channel blocker, nifedipine does not result in a substantial reduction of tumour hypoxia, most probably due to an increase in O(2) uptake with rising O(2) supply to the tumour-bearing hind limb. Nifedipine application during isolated limb perfusion can enhance tumour microcirculation and may therefore promote the delivery (pharmacokinetics) of anti-cancer drugs to the tumour and by this improve the efficacy of pressure-controlled isolated limb perfusion.

ARCON: a novel biology-based approach in radiotherapy

Two mechanisms of radiotherapy resistance which are of major importance in various tumour types are tumour-cell repopulation and hypoxia. ARCON (accelerated radiotherapy with carbogen and nicotinamide) is a new therapeutic strategy that combines radiation treatment modifications, with the aim of counteracting these resistance mechanisms. To limit clonogenic repopulation during therapy, the overall duration of the radiotherapy is reduced, generally by delivering several fractions per day. This accelerated radiotherapy is combined with inhalation of hyperoxic gas to decrease diffusion-limited hypoxia, and nicotinamide, a vasoactive agent, to decrease perfusion-limited hypoxia. Preclinical studies have been done to test the enhancing effects of these three components of ARCON, individually and in combination, in several experimentally induced tumours and normal tissues. In a mouse mammary carcinoma, the tumour-control rate obtained with ARCON was the same as that with conventional treatment, but with a radiation dose almost 50% lower. Phase 1 and 2 clinical trials have shown the feasibility and tolerability of ARCON, and have produced promising results in terms of tumour control. In particular in cancers of the head and neck and bladder, the local tumour-control rates are higher than in other studies, and phase 3 trials for these tumour types are underway. In conjunction with these trials, hypoxia markers detectable by immunohistochemistry are being tested for their potential use in predictive assays to select patients for ARCON and other hypoxia-modifying therapies.

Effects of breathing a hyperoxic hypercapnic gas mixture on blood oxygenation and vascularity of head-and-neck tumors as measured by magnetic resonance imaging

PURPOSE

For head-and-neck tumors, breathing a hyperoxic hypercapnic gas mixture and administration of nicotinamide has been shown to result in a significantly improved tumor response to accelerated radiotherapy (ARCON, Accelerated Radiotherapy with CarbOgen and Nicotinamide). This may be caused by improved tumor oxygenation, possibly mediated by vascular effects. In this study, both blood oxygenation and vascular effects of breathing a hyperoxic hypercapnic gas mixture (98% O2 + 2% CO2) were assessed by magnetic resonance imaging (MRI) in patients with head-and-neck tumors.

METHODS AND MATERIALS

Tumor vascularity and oxygenation were investigated by dynamic gadolinium contrast-enhanced MRI and blood oxygen level dependent (BOLD) MRI, respectively. Eleven patients with primary head-and-neck tumors were each measured twice; with and without breathing the hyperoxic hypercapnic gas mixture.

RESULTS

BOLD MR imaging revealed a significant increase of the MRI time constant of transverse magnetization decay (T2*) in the tumor during hypercapnic hyperoxygenation, which correlates to a decrease of the deoxyhemoglobin concentration. No changes in overall tumor vascularity were observed, as measured by the gadolinium contrast uptake rate in the tumor.

CONCLUSION

Breathing a hyperoxic hypercapnic gas mixture improves tumor blood oxygenation in patients with head-and-neck tumors, which may contribute to the success of the ARCON therapy.

Effects of the interaction between carbogen and nicotinamide on R3230 Ac tumor blood flow in Fischer 344 rats

Braun, R. D., Lanzen, J. L., Turnage, J. A., Rosner, G. and Dewhirst, M. W. Effects of the Interaction between Carbogen and Nicotinamide on R3230 Ac Tumor Blood Flow in Fischer 344 Rats. Radiat. Res. 155, 724-733 (2001). The purpose of this study was to determine whether there are interactions between carbogen breathing and various doses of nicotinamide at the level of the tumor arteriole that might contribute to the improvement in tumor blood flow and pO(2) that is often seen with this combination treatment. R3230 adenocarcinomas were implanted and grown to 4-5 mm in dorsal skin flap window chambers in F344 rats. Saline or 65, 200 or 500 mg/kg nicotinamide was injected i.p. while the rat breathed air through a face mask. After 20 min, either the breathing gas was switched to carbogen for 60 min or the animal remained on air. Measured end points included diameter of tumor arterioles, tumor perfusion, mean arterial blood pressure, and heart rate. None of the measured parameters were affected by injection of saline or nicotinamide, except at the highest nicotinamide dose (500 mg/kg). Mean arterial blood pressure showed a median decrease of 25% when 500 mg/kg nicotinamide was given. Diameter of tumor arterioles decreased significantly from 5-15 min after 500 mg/kg nicotinamide was given but was back to baseline by 20 min. Blood flow decreased significantly 5-20 min after administration of 500 mg/kg nicotinamide compared to the baseline prior to injection. Carbogen breathing resulted in a small increase in mean arterial blood pressure in all groups. There was a transient decrease in the diameter of tumor arterioles and blood flow during the first 5 min of carbogen breathing that was statistically significant in several groups. In the group injected with 500 mg/kg nicotinamide, the diameter of tumor arterioles increased by about 10% during the first 25 min of carbogen breathing, and blood flow increased by a median of 75% over the level prior to carbogen breathing up to 40 min after carbogen breathing. The increase in flow in this group was most likely caused by the concomitant arteriolar vasodilation. Thus there was direct evidence for an interaction between carbogen breathing and nicotinamide, but only at the dose of 500 mg/kg nicotinamide. Since this dose yields plasma levels of nicotinamide that are higher than can be tolerated clinically, it is uncertain whether these changes in arteriolar diameter and blood flow would occur in human tumors.

Inhibition of cADP-ribose formation produces vasodilation in bovine coronary arteries

cADP-ribose (cADPR) induces the release of Ca(2+) from the intracellular stores of coronary artery smooth muscle cells. However, little is known about the role of cADPR-mediated intracellular Ca(2+) release in the control of vascular tone. The present study examined the effects of nicotinamide, a specific inhibitor of ADP-ribosylcyclase, on the vascular tone of bovine coronary arteries. A bovine coronary artery homogenate stimulated the conversion of nicotinamide guanine dinucleotide into cGDP-ribose, which is a measure of ADP-ribosylcyclase activity. Nicotinamide significantly inhibited the formation of cGDP-ribose in a concentration-dependent manner: at a concentration of 10 mmol/L, it reduced the conversion rate from 3.34+/-0.11 nmol. min(-1). mg(-1) of protein in control cells to 1.42+/-0.11 nmol. min(-1). mg(-1) of protein in treated cells, a 58% reduction. In U46619-precontracted coronary artery rings, nicotinamide produced concentration-dependent relaxation. Complete relaxation with nicotinamide occurred at a dose of 8 mmol/L; the median inhibitory concentration (IC(50)) was 1.7 mmol/L. In the presence of a cell membrane-permeant cADPR antagonist, 8-bromo-cADPR, nicotinamide-induced vasorelaxation was markedly attenuated. Pretreatment of the arterial rings with ryanodine (50 micromol/L) significantly blunted the vasorelaxation response to nicotinamide. However, iloprost- and adenosine-induced vasorelaxation was not altered by 8-bromo-cADPR. Moreover, nicotinamide significantly attenuated KCl- or Bay K8644-induced vasoconstriction by 60% and 70%, respectively. These results suggest that the inhibition of cADPR formation by nicotinamide produces vasorelaxation and blunts KCl- and Bay K8644-induced vasoconstriction in coronary arteries and that the cADPR-mediated Ca(2+) signaling pathway plays a role in the control of vascular tone in coronary circulation.

Magnetic resonance imaging of perfusion in rat cerebral 9L tumor after nicotinamide administration

PURPOSE

To investigate the effect of nicotinamide on normal brain and 9L tumor blood flow in the rat using magnetic resonance imaging (MRI) and arterial spin tagging.

METHODS AND MATERIALS

Using MRI at 7 Tesla, measurements of blood perfusion were determined from two-dimensional maps of intracerebral 9L rat tumors and normal Fischer rat brains. The spatial and temporal influence of nicotinamide, 500 mg/kg i.p., on cerebral blood flow (CBF) was studied in normal brain and tumors between 5 and 21 days after tumor implantation. The MRI CBF measurements employed a variable tip-angle-gradient-recalled echo (VTA-GRE-CBF) readout of the magnetization of the tissue slice. The VTA-GRE-CBF required 8 minutes for a blood flow image with inplane resolution of 250 microm x 500 microm x 2 mm.

RESULTS

Normal brain blood flow decreased following the administration of nicotinamide. In contrast, tumor blood flow remained unaffected in the time following nicotinamide administration. Consequently, the blood flowing in the tumor relative to that in normal brain demonstrated a significant and selective increase in response to nicotinamide administration. Relative tumor blood flow increased at 10 minutes after nicotinamide injection compared with predrug levels and remained elevated for at least 1 hour.

CONCLUSION

The results suggest that nicotinamide will not enhance radiosensitivity of brain tumors. The results support the use of nicotinamide to improve delivery of anticancer therapeutics through its ability to selectively increase tumor blood flow relative to that in normal brain.

Blood flow and oxygenation status of human tumors. Clinical investigations

PURPOSE

There is a large body of evidence suggesting that blood flow and oxygenation of human tumors are important research topics which may explain, in particular, resistance to radiation and to many antineoplastic drugs, which can limit the curability of solid tumors by radiotherapy and chemotherapy.

MATERIALS AND METHODS

This manuscript reviews the clinical investigations which have been performed regarding blood flow and oxygenation status of human tumors in radiation oncology.

RESULTS

The possible uses and limitations of the prognostic significance and the changes under therapy measuring blood flow and oxygenation in human tumors were discussed. In addition, several approaches were summarized, which can improve the microvascular O2 availability and perfusion-limited O2 delivery.

CONCLUSION

The clinical data concerning the prognostic significance of blood flow, vascular function and oxygenation of human tumors are relevant for patient selection in clinical oncology. Strategies to improve traditional cancer therapy by modulation of the oxygenation status remain quite promising but more critical research and sophisticated clinical studies are necessary before its true potential is known.

Modulation of tumor oxygenation

There is a large body of evidence suggesting that deficiencies in the O2 supply of tumors exist due to restrictions (i) in the O2 delivery by perfusion and/or diffusion, and (ii) in the O2 transport capacity. Whereas the former are mostly based on inadequate and heterogeneous microcirculatory functions, the latter are predominantly due to tumor-associated anemia. Possible uses and limitations of measures are discussed which can increase the microvascular O2 content and thus may preferentially serve to enhance diffusion-limited O2 availability. In addition, means are described for improving and increasing the uniformity of microcirculation thus possibly enhancing perfusion-limited O2 delivery. Reducing cellular respiration rate should be of benefit in both pathophysiological conditions. Because both types of O2 limitation coexist in solid tumors, appropriate combinations should be aimed at eradicating tumor hypoxia which is present in at least one third of cancers in the clinical setting.

Lack of perfusion enhancement after administration of nicotinamide and carbogen in patients with glioblastoma: a 99mTc-HMPAO SPECT study

BACKGROUND

Nicotinamide (NAM) and carbogen both have been shown to enhance the radiation effect in rodent tumour models and are currently being tested in clinical trials. These agents have demonstrated to act against hypoxia and one of their underlying mechanisms could be an increase of tumour blood perfusion.

PURPOSE

To analyse the effect of both agents on normal brain perfusion and tumour perfusion in patients with glioblastoma.

MATERIALS AND METHODS

Nineteen patients with glioblastoma were studied with 99mtechnetium-hexamethylpropyleneamine oxime single photon emission computed tomography (99mTc-HMPAO SPECT) before and after administration of carbogen and/or NAM. Another six patients were studied with the same procedure but without any flow modulator and were used as controls.

RESULTS

Although the variations between patients were large, no significant enhancement in mean tumour and normal brain perfusion could be demonstrated with NAM or carbogen compared to the control patients. Also no consistent changes in the mean perfusion ratio between tumour and surrounding normal brain were found, suggesting an absence of a selective perfusion effect.

CONCLUSIONS

No significant influence of carbogen and/or NAM on tumour perfusion and normal brain perfusion could be detected with SPECT in patients with glioblastoma.

Impact of nicotinamide on human tumour hypoxic fraction measured using the comet assay

BACKGROUND AND PURPOSE

Nicotinamide has been shown to reduce hypoxia in experimental tumours, but there are no data that measure the hypoxic fraction at the time of irradiation in humans. This study investigates whether nicotinamide with radiation can reduce human tumour hypoxia.

MATERIALS AND METHODS

Twenty-two patients undergoing palliative radiotherapy for treatment of accessible metastatic tumours were exposed to two doses of radiation (3.5-8 Gy, median 6 Gy) separated by 1-6 days. Directly on completion of the first dose, two fine needle aspirate biopsies (FNAB) were taken and analyzed for hypoxic fraction using the alkaline comet assay. On the second day of radiation, 13 patients were given 80 mg/kg nicotinamide post-operatively on an empty stomach 2 h before treatment; the remaining nine patients acted as controls. A second comparative pair of aspirates were obtained immediately on completion of the second fraction.

RESULTS

Sixteen tumours were suitable for analysis (nine nicotinamide and seven controls). Marked inter-tumour variations in hypoxic fraction were noted (0-67%). Both nicotinamide treated tumours and controls demonstrated a significant increase in the percentage of cells containing heavily damaged DNA following the second dose of radiation (P = 0.01). A significant reduction in mean hypoxic fraction after the second radiation treatment was noted (22 to 13%, P = 0.04). This reduction was predominantly due to the nicotinamide treated group, where mean hypoxic fraction fell from 25 to 11% (P = 0.08) compared to the much smaller change in the radiation only control group, 18 to 15% (P = 0.3).

CONCLUSIONS

The decrease in hypoxic fraction suggests that nicotinamide can improve tumour hypoxia measured at the time of irradiation. Exposure to the first dose of radiation, or an effect of the first FNAB on microregional tumour blood flow may also contribute.

Nicotinamide reduces tumour interstitial fluid pressure in a dose- and time-dependent manner

Nicotinamide radiosensitizes a number of experimental tumours, and increases blood flow and mean pO2 in some tumours. It has been suggested that nicotinamide reduces tumour interstitial fluid pressure (IFP), thereby reducing transient vessel non-perfusion and acute hypoxia, and radiosensitizing tumours. To test this hypothesis, tumour IFP, transient vessel non-perfusion, and radiosensitivity after nicotinamide administration were examined in the murine carcinoma NT. Nicotinamide at doses of 500 and 1000 mg kg-1 significantly reduced tumour IFP within 20 min of administration, with recovery to control values by 60-80 min; 100 mg kg-1 had no effect. The percentage of previously non-perfused vessels that became perfused 20 min after administering 1000 mg kg-1 of nicotinamide significantly exceeded the percentage that became perfused within 20 min in the absence of nicotinamide. By 90 min after nicotinamide administration, this differential effect was abolished. The correlation in the time courses of reduced IFP and increased vessel perfusion after nicotinamide administration suggest that decreased IFP may accompany vessel reperfusion. However, 1000 mg kg-1 of nicotinamide radiosensitized the NT carcinoma 80 min after administration, whilst no radiosensitization was seen within 10 min. Thus it is unlikely that increased vessel perfusion is the sole mechanism of nicotinamide-induced radiosensitization in this tumour.

Carbogen and nicotinamide in the treatment of bladder cancer with radical radiotherapy

Carbogen and nicotinamide have been evaluated in a phase II study as hypoxia-modifying agents during radical radiotherapy for bladder cancer using a standard daily 20-fraction schedule. Three groups of patients have received (a) nicotinamide alone, given orally in a dose of 80 mg kg(-1) daily with 52.5 Gy in 20 fractions over 4 weeks, (b) carbogen alone, with 50 Gy in 20 fractions over 4 weeks, and (c) carbogen and nicotinamide, with 50-52.5 Gy in 20 fractions over 4 weeks. Ten patients were treated in each group. All patients completed carbogen and radiotherapy as prescribed, but only 45% completed daily nicotinamide over the 4-week treatment period. The end points of this study were acute bowel and bladder morbidity and local control at cystoscopy 6 months after treatment. An expected level of acute bowel and bladder morbidity was seen that reverted to normal in most patients by 12 weeks with no difference between the three treatment groups. Complete response rates at 6 months were seven out of ten (100%) in the nicotinamide alone group, nine out of ten (90%) in the carbogen alone group and seven out of ten (70%) in the carbogen and nicotinamide group. It is concluded that carbogen and nicotinamide may improve the results of daily fractionated radiotherapy in bladder cancer and that further evaluation is required.

Effects of blood flow modifiers on tumor metabolism observed in vivo by proton magnetic resonance spectroscopic imaging

Perfusion plays a key role in tumor proliferation and therapeutic response. Tumor heterogeneity necessitates use of the highest spatial resolution to monitor metabolic correlates of blood flow changes. This is best achieved with 1H NMR spectroscopy, which permits noninvasive acquisition of high resolution spectroscopic images (SI) of subcutaneous tumors in a relatively short scan time (e.g., 12-25 microliters voxels with signal-to-noise ratio 7:1 in 30 min at 4.7 T). This study seeks to identify 1H spectroscopic indices of tumor blood flow. Proton SI of subcutaneous murine RIF-1 tumors were recorded (a) before and after administration of nicotinamide (1 g/kg) to increase blood flow, and (b) before and after hydralazine (10 mg/kg) to decrease flow. Nicotinamide produced a significant decrease in the total choline peak amplitudes, which subsequent high resolution NMR spectroscopy of tumor extracts revealed to be due to decreases in phosphocholine and glycerophosphocholine. The deamidation of nicotinamide to nicotinic acid, which is known to have hypolipidemic effects and to stimulate the formation of prostaglandins, may have sufficiently altered lipid metabolism to affect the in vivo concentration of the NMR-visible choline-containing compounds. The main effect of hydralazine was a significant increase of lactate, which is consistent with a reduction of tumor blood flow.

Radiotherapy with carbogen breathing and nicotinamide in head and neck cancer: feasibility and toxicity

The feasibility and early toxicity of radiotherapy with carbogen breathing and nicotinamide was tested in 74 head and neck cancer patients. Forty patients with laryngeal and hypopharyngeal tumors were treated with an accelerated schedule combined with carbogen alone (16) or with carbogen and nicotinamide (24). Thirty-four patients with far advanced unresectable tumors of the oral cavity and oropharynx received conventional radiotherapy with carbogen [16] or with carbogen and nicotinamide (18). Some enhancement of skin reaction was observed with nicotinamide but this remained well within limits of tolerance. With the accelerated regimen there was increased severity of mucosal damage expressed as confluent mucositis in 95% of patients which required healing times of 3-4 months in four patients. Eventually restoration of the mucosal lining was complete in all cases. Nausea and vomiting are the most frequent side effects of nicotinamide and were reported by 60% and 36% of the subjects, respectively. In 26% this was reason to discontinue drug intake. Severe renal dysfunction was associated with nicotinamide intake in two patients of this study and in one other patient who presented later. It is our conclusion that radiotherapy combined with carbogen and nicotinamide is a safe treatment with manageable side effects. We recommend not to give nicotinamide concomitantly with nephrotoxic medication or to patients who have impaired renal function. Preliminary tumor control rates are encouraging and clinical testing will be continued.

Should direct measurements of tumor oxygenation relate to the radiobiological hypoxic fraction of a tumor?

PURPOSE

Numerous previous studies have attempted to relate the radiobiological hypoxic fraction (HF) to direct measures of tumor oxygenation such as HbO2 saturations, tumor pO2 levels, or hypoxic cell labeling. Although correlations have been found within tumor lines, no overall relationships were seen across tumor lines. The current objective was to examine the effect on HF of changes in the fractions of the oxygenated and anoxic tumor cells that remain clonogenic.

METHODS AND MATERIALS

A mathematical model was developed that relates the HF to direct measures of tumor oxygenation. The primary assumptions were that: (a) the tumor is divided into distinct compartments of either fully oxygenated or fully anoxic cells, and (b) the survival of the oxygenated cells is negligible compared to that of the anoxic cells. Based on these assumptions, the HF is plotted as a function of the fractions of clonogenic or nonclonogenic, and oxygenated or anoxic cells.

RESULTS

If all cells are clonogenic, then the HF equals the fraction of anoxic cells. If a higher fraction of anoxic than oxygenated cells are nonclonogenic, then the HF will be overestimated by the fraction of the tumor measured to be anoxic using direct measuring techniques. If a higher fraction of the oxygenated than anoxic cells are nonclonogenic, the HF will be underestimated by the fraction of anoxic cells.

CONCLUSION

Correlations between the HF and direct measures of tumor oxygenation have been described within tumor lines evaluated under different physiological condition. However, such relationships can be totally unpredictable between different tumors if the fraction of the anoxic cells that is clonogenic varies substantially. Clearly, if tumor anoxia cannot be detected using direct measures, this is an accurate indication that the tumor is well oxygenated. When tumor anoxia is present, however, the conclusions are ambiguous. Even when a small fraction of the tumor is measured as anoxic, direct measures of tumor oxygenation may not be representative of the HF if a substantial proportion of nonclonogenic cells is present.

Nicotinamide pharmacokinetics in patients

Pharmacokinetic analyses were performed on blood samples of 12 patients undergoing treatment with nicotinamide, hyperthermia and radiation therapy for a variety of recurrent/metastatic cancers. Escalating oral doses of 3, 4, 5, 6 and 10 g of nicotinamide showed a linear relationship between maximum recorded plasma concentrations and the dose in grams (correlation coefficient, r = 0.91). Maximum plasma levels were observed by 30 min in most patients ingesting up to 6 g of nicotinamide. In marked contrast, five out of six patients ingesting 10 g of nicotinamide demonstrated increasing plasma levels at least up to 3 h post-ingestion. Doses up to 6 g were well tolerated and resulted in average maximum recorded plasma levels (mean +/- 1 SEM) of 156.4 +/- 33.6 micrograms/ml. Doses of 10 g were generally not well tolerated, but a high plasma level was maintained on average for at least 4 h. Plasma concentrations of the above order have been previously associated with maximal enhancement of radiation damage in mouse tumor models. This suggests that radiosensitization can be expected to occur in human tumors following oral administration of a safe and well tolerated dose of 6 g. However, at higher doses (i.e., 10 g), the pharmacokinetics, and perhaps radiosensitization, may differ markedly.

Carbogen breathing with nicotinamide improves the oxygen status of tumours in patients

Nicotinamide and carbogen breathing are both effective radiosensitisers in experimental tumour models and are even more effective in combination. This study was to investigate the feasibility of using the agents in combination in patients and to measure their effect on tumour oxygenation. Twelve patients with advanced malignant disease were treated with 4-6 g of oral nicotinamide (NCT) in tablet formulation. Ten of these 12 patients breathed carbogen (95% oxygen, 5% carbon dioxide) for up to 20 min at presumed peak plasma NCT concentration (Cpeak) and had tumour oxygen partial pressure (pO2) measured using the Eppendorf pO2) histograph. The mean Cpeak values were 82, 115 and 150 micrograms ml-1 for NCT doses of 4, 5 and 6 g respectively and were dose dependent. The time of Cpeak was independent of dose with an overall mean of 2.4 h (range 0.7-4 h). NCT toxicity occurred in 9 out of 12 patients and was mild in all but one; carbogen was well tolerated in all patients. Following NCT only two patients had significant rises (P < 0.05) in tumour median pO2. During carbogen breathing, eight out of ten patients had early highly significant rises in pO2 (P < 0.0001), of which six continued to rise or remained in plateau until completion of gas breathing. Six patients had hypoxic pretreatment values less than 5 mmHg, which were completely abolished in three and reduced in two during carbogen breathing. In conclusion, the combination of NCT and carbogen breathing was generally well tolerated and gave rise to substantial rises in tumour pO2 which were maintained throughout gas breathing. These results should encourage further study of this potentially useful combination of agents as radiosensitisers in the clinic.

The effects of carbogen and nicotinamide on intravascular oxyhaemoglobin saturations in SCCVII and KHT murine tumours

Considerable effort has been focused on devising methods for manipulating tumour oxygenation and thereby improving tumour radiosensitivity. The combination of nicotinamide and carbogen has been proposed to oxygenate both chronically and acutely hypoxic cells in tumours. However, results have varied markedly with both tumour model and measurement technique. The current objectives were (1) to determine whether changes in radiosensitivity following oxygen manipulation correlated with changes in tumour oxygenation and (2) to assess whether oxygenation was preferentially improved in specific tumour micro-regions. Using two murine tumour lines, the SCCVII carcinoma and the KHT sarcoma, tumour intravascular HbO2 saturations were measured cryospectrophotometrically following nicotinamide, carbogen or the combination. Generally, nicotinamide had minor effects on oxygenation, arguing against a substantial effect on acute hypoxia, while carbogen and the combination produced marked and equivalent improvements in oxygen availability. These results demonstrate that changes in tumour radiosensitivity may not agree with corresponding changes in oxygenation, even within a given tumour model, and that the efficacy of a given manipulative agent may vary substantially with tumour line. One possible explanation for these findings is that different subpopulations of clonogenic vs non-clonogenic cells may be oxygenated by alternative treatments.

The role of tumor volume in 'reoxygenation' upon cyclophosphamide treatment

The effect of cyclophosphamide (CP) injection (60 mg/kg i.p., single dose) on volume growth and tissue oxygenation (pO2 distribution) was investigated in rat DS-sarcomas. CP was administered 4 days after subcutaneous (s.c.) tumor implantation (volume approximately 0.35 ml). Polarographic pO2 measurements were performed in the subcutis at the hind foot dorsum and in tumors 72 h after CP administration. The oxygenation status of these tissues was compared with that of saline-treated controls. CP-injection caused a mean growth delay of 11 days in DS-sarcomas and had no impact on the oxygenation status of the subcutis. In contrast, in s.c. growing DS-sarcomas the pO2 distribution improved significantly when treated tumors (0.59 ml volume) were compared with their untreated counterparts (1.15 ml volume). Comparison of the oxygenation data of CP-treated tumors with size-matched controls revealed an identical oxygenation status in the experimental tumors used. Thus, when 'reoxygenation' is discussed, one should consider whether it is solely the result of tumor shrinkage or a volume-independent phenomenon.

Nicotinamide and other benzamide analogs as agents for overcoming hypoxic cell radiation resistance in tumours. A review

Oxygen deficient hypoxic cells, which are resistant to sparsely ionising radiation, have now been identified in most animal and some human solid tumours and will influence the response of those tumours to radiation treatment. This hypoxia can be either chronic, arising from an oxygen diffusion limitation, or acute, resulting from transient stoppages in microregional blood flow. Although clinical attempts to overcome hypoxia have met with some success, the results have been far from satisfactory, and efforts are still being made to find better methods. Extensive experimental studies, especially in the last decade, have shown that nicotinamide and structurally related analogs can effectively sensitise murine tumours to both single and fractionated radiation treatments and that they do so in preference to the effects seen in mouse normal tissues. The earliest studies suggested that this enhancement of radiation damage was the result of an inhibition of the repair mechanisms, as was well documented in vitro. However, recent studies in mouse tumours have shown that the primary mode of action actually involves a reduction in tumour hypoxia. More specifically, these drugs prevent transient cessations in blood flow, thus inhibiting the development of acute hypoxia. This novel discovery led to the suggestion that the potential role of these agents as radiosensitizers would be when combined with treatments that overcame chronic hypoxia. The first attempt to demonstrate this combined nicotinamide with hyperthermia and found that the enhancement of radiation damage by both agents together was greater than that seen with each agent alone. Similar results were later seen for nicotinamide combined with a perfluorochemical emulsion, carbogen breathing, and pentoxifylline, and in all these studies the effects in tumours were always greater than those seen in appropriate normal tissues. Of all the analogs, it is nicotinamide itself which has been the most extensively studied as a radiosensitizer in vivo and the one that shows the greatest effect in animal tumours. It is also an agent that has been well established clinically for the treatment of a variety of disorders, with daily doses of up to 6 g being considered reasonably safe and associated with a low incidence of side effects. This human dose is equivalent to 100-200 mg/kg in mice and such doses will maximally sensitize murine tumours to radiation. These findings have now resulted in phase I/II clinical trials of nicotinamide, in combination with carbogen breathing, as a potential radiosensitizing treatment.

Increased tumor oxygenation and radiation sensitivity in two rat tumors by a hemoglobin-based, oxygen-carrying preparation

The rat 13762 mammary carcinoma and the rat 9L gliosarcoma were grown subcutaneously in a hind limb of female, Fisher 344 rats. The oxygen content of the tumors was determined using an Eppendorf pO2 histograph. Fifty-to-sixty oxygen measurements were made per tumor and there were 8-to-10 animals per group. The percent of pO2 readings < or = 5 mmHg in the mammary carcinoma was 49%, this was decreased to 34% by administration of the hemoglobin preparation (8 ml/kg) and further decreased to 29% when carbogen (95% O2/5% CO2) breathing was added to administration of the hemoglobin preparation. The percent of pO2 readings < or = 5 mmHg in the gliosarcoma was 49%, this was decreased to 24% by administration of the hemoglobin preparation and further decreased to 0% when carbogen breathing was added to administration of the hemoglobin preparation. Therapeutic response was assessed over a single-dose range of radiation therapy (10, 20 and 30 Gray). The dose modifying factor produced by the hemoglobin preparation/air was 1.6 and by the hemoglobin preparation/carbogen was 2.7 in the rat 13762 mammary carcinoma. The dose modifying factor produced by the hemoglobin preparation/air was 1.9 and by the hemoglobin preparation/carbogen was 2.9 in the rat 9L gliosarcoma. Administration of a hemoglobin-based oxygen carrier reduced tumor hypoxia and increased tumor response to radiation therapy.

The effect of nicotinamide on microregional blood flow within tumours assessed using laser Doppler probes

Laser Doppler probes have been used to provide real-time spatial flow mapping of microregional erythrocyte flux within the murine Sarcoma F (SaF). The results demonstrate that fluctuations in microregional red blood cell flux are a common feature of SaF tumours, with approximately 50% of regions demonstrating a change of at least a factor of 2 over a one-hour sampling period and 16% of regions showing a greater than 5-fold change. Administration of 250 mg/kg nicotinamide induced a small reduction in the number of microregions showing fluctuations, 9% of regions demonstrated large fluctuations (i.e., > or = 5) relative to 16% in control tumours. At a dose of 500 mg/kg, the number of changes in microregional erythrocyte flux was increased. However, unlike either untreated or nicotinamide 250 mg/kg pretreated animals, these changes were mostly increases in microregional flow. Indeed, 12 of the 13 changes by a factor of 5 or greater, observed following nicotinamide at 500 mg/kg, were increases. This effect on microregional flow translated into only a 25% increase in macroregional flow determined by averaging all the regions sampled.

Effect of nicotinamide and pentoxifylline on normal tissue and FSA tumor oxygenation

Nicotinamide (NA) and pentoxifylline (PTX) sensitize experimental murine tumors to radiation without sensitizing normal tissues. They are presumed to exert this effect by reducing hypoxia in tumors. The present study evaluated the individual and combined effects of NA and PTX on oxygen levels in subcutaneous normal tissue and subcutaneous FSa fibrosarcoma tumors in the hind foot dorsum of C3H mice. Oxygen measurements were made using a polarographic needle electrode inserted into the tissue immediately before and/or 15-60 min after intraperitoneal administration of 500 mg/kg of NA, 50 mg/kg of PTX, or saline. The median tumor pO2 increased from a mean +/- S.E.M. of 4.1 +/- 1.1 mm Hg in saline-treated control mice to 6.8 +/- 1.9 mm Hg 15 min after NA, 7.6 +/- 1.4 mm Hg 60 min after PTX, and 6.7 +/- 1.1 mm Hg after NA and PTX in combination. PTX raised the median tumor pO2 level from 21% to 39% of the median subcutaneous normal tissue pO2 (p < 0.01). PTX also significantly reduced the proportion of tumor pO2 values < or = 2 mm Hg from 41 +/- 10% to 8 +/- 7% (p = 0.02). Although NA did increase the proportion of tumor that was well oxygenated, it did not significantly reduce the proportion of tumor pO2 values < or = 2 mm Hg (p = 0.34). The combination of NA and PTX did not add to the tumor oxygenation enhancement achieved by PTX alone. NA increased the median subcutaneous normal tissue pO2 by an average of 5.1 +/- 2.2 mm Hg from a baseline of 17.1 +/- 2.2 mm Hg (p = 0.04). PTX had no effect on the median normal tissue pO2 (p = 0.93). PTX showed greater therapeutic potential in this model system than did NA.

The radiosensitizer nicotinamide inhibits arterial vasoconstriction

Nicotinamide (NA) is currently entering clinical trials as a radiosensitizer. A major component of its activity is the improvement of tumour oxygenation resulting from a reduction in microregional ischaemia. NA is known to reduce arterial blood pressure in rodents, suggesting a vascular component in its mechanism of action. We have used an ex vivo system to study the direct action of NA on the contractile properties of vascular smooth muscle. Isolated pieces of rat tail artery were internally perfused with Krebs' solution at a constant flow rate so that constriction of the arterial smooth muscle could be measured as an increase in perfusion pressure. Transient vasoconstrictor responses lasting up to 10 min were induced with bolus injections (10 microliters) of phenylephrine, at concentrations ranging from 10(-5) to 10(-2) M, into the internal perfusate whereas a constant increase in vasoconstrictor tone, giving perfusion pressures of 43-84 mmHg, was induced by constantly perfusing with PE (5 x 10(-6) M) or raising the K+ concentration of the Krebs' solution to 122 mM. The addition of NA to the perfusate significantly reduced the size of the transient vasoconstrictor responses in a dose-dependent manner and induced the precontracted vessels to relax. This action of NA could not be blocked either by N omega-nitro-L-arginine methyl ester (L-NAME), indomethacin or propranolol. We conclude that direct effects on supplying blood vessels probably contribute to the oxygenating action of NA in tumours, though the precise mechanism remains obscure.

Possible mechanisms involved in tumor radiosensitization following nicotinamide administration

Despite continued interest in the administration of nicotinamide (NA) as a tumor-specific radiosensitizer (an effect thought to be related to increases in tumor blood flow and oxygenation), little is known about the underlying mechanism(s) of this effect. The aim of this study was to investigate metabolic changes following NA application in both tumor and normal tissues. Increased concentrations of NAD+ were measured in DS-sarcomas, liver, and kidney tissue, with no changes in concentrations in resting skeletal muscle. Further investigations also examined the concentrations of glucose, lactate, ATP, ADP and AMP in tumor and resting skeletal muscle tissue following NA application. Here, the only change detected was an increase in lactate levels in tumor tissue. The changes in NAD+ concentrations described correlate well with reported changes in tissue blood flow measured following NA. On the basis of changes in tumor blood flow, oxygenation and metabolite concentrations found in this and other recent studies, possible mechanisms for tumor radiosensitization following nicotinamide administration are considered.

Micro-regional mapping of HbO2 saturations and blood flow following nicotinamide administration

PURPOSE

Although nicotinamide administration has increased the radiosensitivity of experimental tumors, there is a scarcity of data detailing the underlying physiological mechanisms. The current study presents a method for quantifying both microregional distributions of intravascular HbO2 saturations and the presence or absence of blood flow in adjacent frozen tumor sections.

METHODS AND MATERIALS

Two murine tumor cell lines, KHT and SCCVII, were implanted and quick-frozen without the use of anesthetics. Nicotinamide was administered IP 1 h prior to freezing, and a fluorescent dye that preferentially stains cells adjacent to blood vessels was injected i.v. 1 min prior to freezing. To visualize the presence or absence of blood flow, six micron sections were first cut using a cryostat. The remaining frozen tumor block was then analyzed cryospectrophotometrically to determine intravascular HbO2 levels.

RESULTS

While KHT HbO2 levels increased somewhat predictably following nicotinamide, the response in SCCVII tumors varied with distance from the tumor surface. Near the periphery, SCCVII HbO2 levels increased, but nearer the tumor center, HbO2 levels actually decreased. Perfused blood vessels were uniformly distributed throughout the tumor volume except in regions of necrosis. Even vessels containing no measurable oxygen remained perfused, as evidenced by the presence of the fluorescent marker.

CONCLUSION

These results demonstrate that nicotinamide raises intravascular HbO2 saturations in both KHT and SCCVII tumors. This increase in oxygen delivery is not evenly distributed throughout the tumor volume in spite of a uniform distribution of perfused blood vessels. Blood flow in a substantial proportion of these vessels is most likely not sufficiently rapid to serve a functional purpose in terms of oxygen supply to the surrounding tumor tissue.

Nicotinamide and carbogen: relationship between pO2 and radiosensitivity in three tumour lines

The effects of carbogen breathing, nicotinamide injection and their combination on tumour radiosensitivity were correlated with changes in tumour O2 tension to determine the relationship between radiosensitivity and measured pO2. The radiosensitivity (in vivo-in vitro colony assay) and O2 tension (computerized pO2 histograph KIMOC 6650) of two human xenografted tumours (HRT18 and Na11+) and one murine tumour (EMT6) were measured under similar experimental conditions. A single dose of radiation was delivered (8 Gy for HRT18 and Na11+, 12 Gy for EMT6). Carbogen breathing, nicotinamide injection, and their combination all significantly increased radiosensitivity in the three cell lines (p < 0.05); the most efficient treatment was carbogen plus nicotinamide. The radiosensitization was optimum for EMT6 and Na11+. Mean and median pO2 increased with all three treatments, except for carbogen breathing in EMT6. Carbogen breathing had little effect on the proportion of low pO2 values, but induced pO2 values > 30 mmHg in all three tumour lines. Nicotinamide decreased the proportion of low pO2 values. This effect is larger with the combination carbogen plus nicotinamide. Almost all pO2 values < 2.5 mmHg were eliminated for HRT18 and EMT6. The relationship between radiosensitization and pO2 was significant when pO2 was expressed as the percentage of values below 7, 8, 9 and 10 mmHg for the three lines.

Reducing acute and chronic hypoxia in tumours by combining nicotinamide with carbogen breathing

The ability of nicotinamide and carbogen breathing to improve the radiation response of a C3H mammary carcinoma by reducing both acute and chronic hypoxia was investigated. Using a tumour growth delay assay the response of 200 mm3 foot tumours to local irradiation was found to be increased by either injecting nicotinamide (100-1,000 mg/kg) 20 min prior to irradiation, or by allowing mice to breathe carbogen for 10 min before and during the radiation treatment. The greatest radiosensitization occurred when nicotinamide and carbogen were combined. With a histological fluorescent staining technique nicotinamide was shown to prevent transient stoppages in microregional blood flow, and also appeared to improve tumour oxygenation as measured with an Eppendorf oxygen electrode, both effects being consistent with its ability to decrease perfusion limited acute hypoxia. Carbogen had no effect on vessel closure, but it significantly improved tumour oxygenation, which was indicative of it reducing diffusion limited chronic hypoxia.