Uptake and retention kinetics of para-fluorine-18-fluorobenzylguanidine in isolated rat heart

Para-[18F]fluorobenzylguanidine ([18F]PFBG) is a newly developed tracer for imaging myocardial sympathetic neuronal innervation. This study investigated the uptake and retention mechanisms of [18F]PFBG in perfused, isolated rat heart.

METHODS

Fluorine-18-PFBG was administered to working rat hearts within the perfusion medium at a constant activity concentration (1.5-2 MBq/liter) for 8 min, followed by a washout period (50 min). External scintillation probes with coincidence detection circuitry were used to measure myocardial radioactivity. Six groups of hearts (n = 6, except in Group 6) were studied: (Group 1) control; (Group 2) 100 nM desipramine (DMI); (Group 3) 0.8 microM SKF550; (Group 4) DMI + SKF550; (Group 5) SKF550 + 1.0 microM Ro 4-1284; and (Group 6) SKF550 with DMI chase at 30 min (n = 4).

RESULTS

Groups 2, 3 and 4 showed a mean reduction of 19% (uptake-1 blockade), 58% (uptake-2 blockade) and 95% (uptake-1 and uptake-2 blockade) in uptake rates, respectively, compared with control (p < 0.01). A further 33% reduction in the uptake rate was noted with vesicular transport inhibition (Group 5 compared with 3, p = 0.054). Biphasic clearance consisting of rapid (T1/2 = 5.32 +/- 1.1 min) and slow (T1/2 = 35.2 +/- 9.6 min) components were noted in control hearts. The rapid (T1/2 = 1.6 +/- 0.3 min) and slow (T1/2 = 10.9 +/- 1.4 min) clearance rates were accelerated (p < 0.0001) in Group 5 compared to control. DMI chase conditions (Group 6) caused an inhibition of [18F]PFBG washout (p = 0.004) suggesting a role for reverse transport through the uptake-1 carrier.

CONCLUSION

Fluorine-18-PFBG is specifically accumulated by sympathetic nerve terminals. However, further work is recommended in humans to evaluate the potential implications of specific extraneuronal uptake of [18F]PFBG through the uptake-2 mechanism.

Radioiodination of internalizing monoclonal antibodies using N-succinimidyl 5-iodo-3-pyridinecarboxylate

Monoclonal antibodies (mAbs) that internalize following binding to cell-surface receptors require radiolabeling approaches that minimize loss of radioactivity from the cell after intracellular processing. One class of internalizing mAbs of great interest for imaging and radioimmunotherapy are those specific for EGFRvIII, a truncated form of the epidermal growth factor receptor found on gliomas, non-small cell lung carcinomas, breast carcinomas, and ovarian carcinomas. Because lysosomes are known to retain positively charged compounds, N-succinimidyl 5-iodo-3-pyridinecarboxylate (SIPC) might be ideal for radioiodination of these mAbs because of the positive charge on its pyridine ring. To investigate this hypothesis, the anti-EGFRvIII mAb L8A4 was labeled using SIPC, and internalization assays were performed using the EGFRvIII-positive cell lines HC2 20 d2 and NR6M. Compared with L8A4 labeled using Iodogen or N-succinimidyl 3-iodobenzoate, SIPC increased intracellular retention of activity by up to 65%. Reverse-phase high-performance liquid chromatography analyses indicated that a significantly higher fraction of the low molecular weight catabolites from mAbs labeled via SIPC were retained within cells (SIPC, 28.1%; Iodogen, 7.6% at 1 h). With SIPC, the primary labeled species in cell lysates was the 5-iodonicotinic acid (INA)-lysine conjugate, whereas in the supernatant, both INA-lysine and INA were seen. A 3-4-fold higher percentage of these catabolites were charged at lysosomal pH in comparison with those from mAb labeled using N-succinimidyl 3-iodobenzoate, in concert with the differences in cellular retention observed between these two labeling methods. In mice bearing HC2 20 d2 xenografts, a significant improvement in tumor retention of radioiodine and tumor:normal tissue ratios was seen when L8A4 was labeled using SIPC instead of the Iodogen method. These results suggest that SIPC is a promising reagent for the radioiodination of anti-EGFRvIII L8A4 and, possibly, other internalizing mAbs.

Chimeric anti-tenascin antibody 81C6: increased tumor localization compared with its murine parent

When labeled using the Iodogen method, a chimeric antibody composed of the human IgG2 constant region and the variable regions of murine anti-tenascin 81C6 exhibited superior uptake in human glioma xenografts compared with its murine parent. In the current study, three paired-label experiments were performed in athymic mice with subcutaneous D-54 MG human glioma xenografts to evaluate further the properties of radioiodinated chimeric 81C6. These studies demonstrated that (a) the enhanced tumor uptake of chimeric 81C6 is specific; (b) when labeling was performed using N-succinimidyl 3-iodobenzoate, chimeric 81C6 again showed preferential accumulation in tumor compared with murine 81C6; and (c) the tumor uptake advantage observed previously with murine 81C6 for N-succinimidyl 3-iodobenzoate compared with Iodogen labeling did not occur with chimeric 81C6.

Radiotoxicity of systematically administered [211At]astatide in B6C3F1 and BALB/c (nu/nu) mice: a long-term survival study with histologic analysis

PURPOSE

The present study undertook to establish the dose (LD) of systematically administered (via tail vein) sodium [211At]astatide that would kill 10% (LD10) of exposed animals in two mouse models and to evaluate the resulting histologic lesions.

METHODS AND MATERIALS

Three dose escalation experiments were carried out using groups of 10 3- to 4-week-old, 20 +/- 2 g B6C3F1 mice, and one dose escalation experiment was carried out with groups of 10 4- to 6-week-old, 22 +/- 2 g BALB/c (nu/nu) mice. All animals were weighed daily and checked twice daily for general health; autopsies were performed within 12 h of death.

RESULTS

The LD10 (95% confidence interval) level of free [211At]astatide at 360 days was 15.1 microCi (5.2-19.1 microCi) in B6C3F1 mice and was associated with a 37.8% weight difference from saline controls (p < 0.001). In the BALB/c (nu/nu) mice, the LD10 at 360 days was 7.7 microCi (0-14.2 microCi), while a dose of 10 microCi (0.42 microCi g(-1)) was associated with a 9.44% weight difference vs. saline controls (p < 0.05). Exclusive of the well-known effects on thyroid, [211At]astatide activity levels were associated with severe bone marrow depression, testicular atrophy, focal alopecia, and nuclear atypia of the epidermoid mucosa of the fore-stomach in the B6C3F1 mice; at activity levels approximating LD10 at 360 days, mild changes in the heart, liver, stomach, and spleen were observed. For BALB/c (nu/nu) mice, administration of 10 microCi was associated at autopsy with mild histologic lesions in the heart, stomach, liver, and spleen.

CONCLUSIONS

These studies provide a basis for the design of further investigations of [211At]-labeled compounds as therapeutic agents.

Hepatotoxicity due to antituberculosis therapy. Clinical profile and reintroduction of therapy

The clinical profile of antituberculosis treatment (ATT)-induced hepatotoxicity is variable, and the reintroduction of ATT in patients who have developed such injury is controversial. We conducted a prospective study to determine the clinical profile in patients with ATT-induced hepatotoxicity and to test a predefined strategy of reintroduction of ATT. Seventy-two consecutive patients with clinical evidence of ATT-induced hepatotoxicity were included. Jaundice was the presenting symptom in 44 (61%) patients; prodromal symptoms were present in 28 (39%). Serious complications developed in 12 (16.6%) patients (fulminant hepatic failure in seven, subacute hepatic failure in four, hepatic encephalopathy in one). Nine patients (three males, six females) died from these complications. The mean duration of treatment before the onset of hepatitis was significantly longer in the group that died (53.22 +/- 36.22 days) than in the rest of the patients (31.07 +/- 30.30 days; p < 0.01). Malnutrition was present in 37 of the 72 patients. After resolution of drug induced hepatitis, reintroduction of isoniazid and rifampicin was possible in 41 of 44 patients. Thus, our results showed that ATT-induced hepatitis carried significant morbidity and mortality, that malnutrition was common in patients with ATT-related hepatitis, and that potentially hepatotoxic antituberculosis agents could be safely reintroduced after recovery from hepatitis.

Enhanced binding and inertness to dehalogenation of alpha-melanotropic peptides labeled using N-succinimidyl 3-iodobenzoate

Two peptides of potential utility for targeting melanoma cells, alpha-melanocyte-stimulating hormone (alpha-MSH) and its more potent analogue [Nle4,D-Phe7]-alpha-MSH, were radioiodinated in 45-65% yield using N-succinimidyl 3-[125I]iodobenzoate (SIB). To determine whether this labeling method resulted in improved in vitro and in vivo characteristics, these peptides also were labeled with 131I by direct iodination with the iodogen method. For alpha-MSH, the rapid tissue clearance of both radionuclides in mice was consistent with rapid degradation of the peptide; however, significantly lower levels of 125I were observed in thyroid and stomach, reflecting a greater inertness to deiodination. More extensive comparisons were performed with [Nle4,D-Phe7]-alpha-MSH. The in vitro binding of [Nle4,D-Phe7,Lys11-(125I)IBA]-alpha-MSH (prepared using SIB) to the murine B-16 melanoma cell line, 34.1 +/- 4.7%, was more than twice as high as that for [Tyr2(131I),Nle4,D-Phe7]-alpha-MSH (15.0 +/- 0.1%), and its KD was more than 10-fold lower than that for conventionally labeled peptide (10 +/- 5 versus 140 +/- 14 pM). The normal tissue clearance of [Nle4,D-Phe7,Lys11-(125I)IBA]-alpha-MSH in mice was faster than that of [Tyr2(131I),-Nle4,D-Phe7]-alpha-MSH. The 19-40-fold lower activity concentrations of [Nle4,D-Phe7,Lys11-(125I)IBA]-alpha-MSH in tissues accumulating free iodide (thyroid and stomach) suggest a greater inertness of this peptide to deiodination. The primary urinary catabolite of [Nle4,D-Phe7, Lys11-(125I)IBA]-alpha-MSH was the lysine conjugate of iodobenzoic acid, whereas radioiodide was the chief catabolite generated from [Tyr2(131I),Nle4,D-Phe7]-alpha-MSH. We conclude that further evaluation of [Nle4,D-Phe7,Lys11-(125I)IBA]-alpha-MSH for targeting alpha-MSH receptors is warranted and that SIB may be a useful method for the radioiodination of peptides.

Para-[18F]fluorobenzylguanidine kinetics in a canine coronary artery occlusion model

BACKGROUND

The kinetics of para-[18F]fluorobenzylguanidine ([18F]PFBG) were investigated in a canine coronary artery occlusion model.

METHODS AND RESULTS

Five dogs were imaged by positron emission tomography (PET) before and after complete surgical ligation of the left anterior descending coronary artery. PET studies included a 10-minute dynamic [13N]NH3 perfusion scan, followed 1 hour later by 3-hour dynamic [18F]PFBG scanning. [18F]PFBG and [13N]NH3 images demonstrated homogeneous myocardial uptake/perfusion before infarction. One hundred eighty minutes after [18F]PFBG administration, myocardial accumulation was decreased by 60% (day 2, 0.0065% +/- 0.0015% injected dose/ml) and 58% (day 16, 0.0069% +/- 0.003% injected dose/ml) compared with a similar myocardial region of interest from the preinfarction (0.016% +/- 0.005% injected dose/ml) study. Myocardial accumulation of [13N]NH3 at 9 minutes showed a 52% (day 2) and 7% (day 16) decrease compared with the preinfarction study. The accumulation of [18F]PFBG in the infarction was decreased significantly at 120 and 180 minutes on all postinfarction studies (p = 0.01). In three dogs a significant decrease in the myocardial norepinephrine concentration was documented in the area of infarction (237 +/- 94 ng/gm) versus the noninfarcted (1018 +/- 48 ng/gm) myocardium (p = 0.001).

CONCLUSIONS

A decreased accumulation of [18F]PFBG was observed in the area of myocardial infarct in this canine model. The magnitude of the decrease in [18F]PFBG was larger than that seen with [13N]NH3 on day 16 after infarction, suggesting reperfusion and persistent sympathetic neuronal dysfunction.

Microscopic colitis is a cause of large bowel diarrhea in Northern India

Chronic diarrhea is a common clinical problem. To determine the possible causes in North India, we studied prospectively 71 patients with chronic diarrhea of the large bowel type. A definite diagnosis could be established in 70 patients. Ulcerative colitis was found in 18 patients, colorectal malignancies in three, colonic polyps in three, and irritable bowel syndrome in 32. In addition, seven patients with seronegative polyarthritis and chronic diarrhea were found to have chronic inflammation of the colon on histology. Two patients had pseudodiarrhea, and no diagnosis could be established in one patient. The remaining five patients with chronic diarrhea showed histologic evidence of chronic colonic inflammation with predominantly mononuclear cell infiltration of the lamina propria and increased intraepithelial lymphocytes, but results of their radiologic and endoscopic studies were normal. These five patients were classified as having microscopic (lymphocytic) colitis. We conclude that the causes of chronic diarrhea in North India patients are similar to a large extent to those seen in Western populations. Microscopic (lymphocytic) colitis is a definite clinicopathologic entity that should be considered in the differential diagnosis of chronic diarrhea.

Radioimmunotherapy of breast cancer xenografts with monoclonal antibody ICR12 against c-erbB2 p185: comparison of iodogen and N-succinimidyl 4-methyl-3-(tri-n-butylstannyl)benzoate radioiodination methods

C-erbB2 p185 is a proto-oncogene product expressed in 25-30% of human invasive breast cancers that is associated with poor prognosis and resistance to endocrine therapy and chemotherapy. It is minimally expressed in normal adult tissues (M. F. Press et al., Oncogene, 5: 953-962, 1990). For this reason, it is an attractive target for radioimmunotherapy and other antibody-directed therapies. ICR12 is a rat IgG2a monoclonal antibody directed against a protein epitope of the external domain of the c-erbB2 p185. We performed experiments to optimize the direct iodination of ICR12 with 131I using the IodoGen method, and we found impairment of immunoreactive fraction with increasing specific activity. N-Succinimidyl 4-methyl-3-(tri-n-butylstannyl)benzoate (MATE) is a tin ester that can be radioiodinated easily and then coupled to the epsilon-amino group of lysine residues. This method has been shown to have improved uptake in tumors compared with antibody labeled by direct iodination (P. K. Garg et al., Nucl. Med. Biol., 20: 379-387, 1993). ICR12 could be labeled up to 16 mCi/mg by this technique without loss of immunoreactive fraction. Whole-body retention of MATE-labeled ICR12 was less than IodoGen (P < 0.0001). Radioimmunotherapy experiments in athymic mice bearing established MDA MB 361 human breast cancer xenografts showed growth inhibition for > 24 days at a dose of 600 microCi/mouse (P < 0.0001) when labeled by the IodoGen technique, and 12 days using the MATE method (P < 0.0001).

Effect of endoscopic variceal sclerotherapy on esophageal motor functions and gastroesophageal reflux

Sclerotherapy results in significant local complications, both immediate and delayed. This study was designed to examine the esophageal pathophysiology underlying these complications.

METHODS

We prospectively evaluated esophageal transit, motility abnormalities and gastroesophageal reflux (GER) with barium studies and esophageal functional scintigraphy in 24 patients (20 men, 4 women; mean age 33 +/- 12.4 yr) before sclerotherapy (Phase I), after two sessions (Phase II), following variceal eradication (Phase III) and 4 wk later (Phase IV).

RESULTS

Varices were obliterated after 5.6 +/- 1.9 sessions of intravariceal sclerotherapy performed weekly with 1% polidocanol (17.3 ml per session). There was no baseline Phase I dysmotility or reflux. Phase II studies recorded a marked delay of esophageal global and segmental (mid and distal) transit time in 98.2% of patients by scintigraphy and 90% by barium studies. Incoordinate contractions and aperistalsis were observed in 0, 66.7%, 58.3% and 33.8% of patients from Phases I-IV studies, respectively. Barium studies revealed tertiary waves and reverse peristalsis in 0, 50%, and 75% of patients from Phases I-III; strictures were observed in 0, 1, and 3 patients during Phases I-III. GER was detected scintigraphically in 0, 58.3%, 25% and 16.6% during Phases I-IV sequentially. In contrast, barium studies grossly underestimated GER (0, 5% and 15% at phases I-III).

CONCLUSION

There was strong concordance between esophageal symptoms, transit, motility abnormalities and GER (p < 0.05). Variceal eradication (Phases III and IV) was associated with a gradual recovery of esophageal symptoms, ulcers and all abnormal scintigraphic parameters. Sclerosant-induced chemical esophagitis in association with peptic esophagitis due to gross reflux following sclerotherapy possibly can explain the symptoms in most patients.

Catabolism of radioiodinated murine monoclonal antibody F(ab')2 fragment labeled using N-succinimidyl 3-iodobenzoate and Iodogen methods

The F(ab')2 fragment of monoclonal antibody (MAb) Me1-14 was labeled with 125I using the Iodogen method and by reaction with N-succinimidyl 3-[125I]iodobenzoate (SIB). The labeled catabolites generated after exposure to tissue homogenates in vitro and following administration of labeled F(ab')2 into normal mice were investigated by size-exclusion HPLC, gel electrophoresis, and reverse-phase HPLC. Rapid conversion of F(ab')2 to Fab was observed with both labeling methods. With F(ab')2 labeled using the Iodogen method, the primary low molecular weight catabolites appeared to be [125I]iodide and, to a lesser extent, mono[125I]iodotyrosine. With SIB, [125I]iodide and [125I]iodobenzoic acid (IBA) as well as the glycine and lysine conjugates of IBA were all observed. Differences in low molecular weight catabolic products could explain the more rapid normal tissue clearance with MAbs and MAb fragments labeled with SIB compared with those labeled using iodogen.

Role of omeprazole in prevention and treatment of postendoscopic variceal sclerotherapy esophageal complications. Double-blind randomized study

Endoscopic variceal sclerotherapy-related esophageal complications are quite common. The potential efficacy of omeprazole in the prevention and treatment of postsclerotherapy esophageal complications was evaluated in 47 patients with portal hypertension in randomized, placebo-controlled study. Twenty-one patients in the omeprazole group and 23 patients in the placebo group completed the study. The two treatment groups were similar in regards to the etiology of portal hypertension, Child's class, and clinical characteristics. Esophageal ulcers developed in 16 patients in the omeprazole group (2.43 ulcers/patient) and 18 patients in the placebo group (2.39 ulcers/patient). Most of the ulcers (> 90%) healed within 14 days in each group. Esophageal strictures requiring dilatation developed in two and one patient in the omeprazole and placebo groups, respectively. There was no statistically significant difference in regards to the complication rate between the two groups. We conclude that omeprazole is not effective for the prevention or treatment of postsclerotherapy esophageal complications.

Cholesterol bile duct stones with no stones in the gallbladder

Common bile duct stones are usually associated with stones in the gallbladder. During the period 1989-1991, however, we encountered 17 patients with common bile duct (CBD) stones without gallbladder stones who had presented with obstructive jaundice and cholangitis. Their ages ranged between 30 and 72 years; 10 were female and seven male. Five of them had a deceptive presentation and were initially misdiagnosed as having a malignant lesion. It was endoscopic retrograde cholangiopancreatography that correctly diagnosed the presence of CBD stones in all 17 patients. Therapeutic sphincterotomy led to subsidence of cholangitis in 16 patients and retrieval of stones in 13 of them. Three patients required nasobiliary decompression because stones could not be retrieved. One patient required emergency surgery due to flare-up of cholangitis. Cholesterol concentration of the retrieved stones was 70-92% of the dry weight. Thus, these 17 patients formed a distinct subgroup who had cholesterol CBD stones with stoneless gallbladder, and five of these 17 patients had presentations mimicking malignant lesions.

Antituberculosis treatment-induced hepatotoxicity: role of predictive factors

Antituberculosis drug-induced hepatotoxicity is quite common. However, factors predicting its development are still controversial. The objective of the present study was to evaluate the role of certain factors (age and sex of the patient, alcoholism, chronic liver disease, hepatitis B virus carrier status, acetylator status, nutritional status and antituberculosis treatment (ATT) regimen) in predicting the development of ATT-induced hepatitis. In a case-control study, 60 consecutive patients with evidence of ATT-induced hepatitis were studied to assess the possible association of the above-mentioned factors with ATT-induced hepatitis. Body mass index was found to be significantly lower in ATT-induced hepatitis patients (17.2 +/- 2.7) than in controls (19.5 +/- 3.3) (p < 0.05). Pyrazinamide was used in addition to isoniazid and rifampicin in a significantly higher percentage of patients in the ATT-induced hepatitis group (70%) as compared with those in the control group (42%). No significant differences were observed between the two groups with regard to the rest of the parameters.

Preparation and preliminary evaluation of 4-[211At]astato-N-piperidinoethyl benzamide

The potential therapeutic agent, 4-[211At]astato-N-piperidinoethyl benzamide (4-APAB) was synthesized via a halodestannylation reaction. Radiochemical yields were 69% for a 5 min reaction and reached 74% by 25 min, whereas 82% radiochemical yields were obtained under similar reaction conditions for radioiodination. A simplified procedure was adopted for the purification of the target compound. In vitro binding of 4-APAB to SK-MEL 28 melanoma and D247 glioma cell lines was 20.7 +/- 1.3% and 12.2 +/- 1.3%, respectively. In comparison, binding of 4-[131I]iodo-N-piperidinoethyl benzamide (4-IPAB) to SK-Mel 28 cells was 13.9 +/- 1.9%. Paired label biodistribution studies were performed in normal Balb/c mice using 4-IPAB and 4-APAB. Thyroid uptake at 1, 2, and 6 h was significantly higher for 4-APAB. Differences in liver accumulation between the two compounds were small but statistically significant at most time points. A higher accumulation of 211At compared with 131I was observed in lungs and spleen at all time points studied. These results indicate that 4-APAB is not stable in vivo, suggesting the need for a better sigma receptor ligand for use in 211At.

Anti tubercular treatment induced hepatotoxicity: does acetylator status matter?

Anti tubercular drug related hepatotoxicity is common. The mechanism of injury and factors predisposing to its development are not fully understood. Forty patients with anti tubercular drugs related hepatotoxicity were studied to see the clinical and biochemical profile of these patients and to find out the significance of acetylator phenotype in the development of hepatotoxicity. Mean age of patients with liver damage (37.82 +/- 10.0 years) was similar to those without liver damage (36.48 +/- 12.5 years). Pyrazinamide appeared to increase the hepatotoxicity of isoniazid and rifampicin. The percentage of rapid acetylators and slow acetylators among patients with hepatotoxicity (70% and 30% respectively) was similar to controls (66.6% rapid and 33.3% slow acetylators). Acetylator phenotype probably has no role in anti tubercular drugs induced hepatotoxicity.

Preclinical evaluation and PET imaging of 18F-labeled Mel-14 F(ab')2 fragment in normal dogs

The F(ab')2 fragment of monoclonal antibody Mel-14, reactive with human melanomas and gliomas, was labeled with 18F using two acylation agents, N-succinimidyl 8-[(4'-[18F]fluorobenzyl)amino]suberate (SFBS) and N-succinimidyl 4-[18F]fluorobenzoate (SFB). The immunoreactivity and affinity for Mel-14 F(ab')2 labeled using the two methods were similar. As a prelude to human clinical evaluation, PET imaging, tissue distribution and pharmacokinetic measurements were performed in two groups of normal foxhounds. Similar in vivo behavior was seen for Mel-14 F(ab')2 labeled using SFBS and SFB. Radiation dosimetry calculations suggest that a 10 mCi dose could be used for this F(ab')2 fragment labeled using either acylation agent.

Nonsurgical drainage for biliary obstruction

Biliary obstruction is a common and potential fatal condition. Its pathological effects include depressed immunity, impaired phagocytic activity and reduced Kupffer cell function with consequent endotoxemia, septicemia and renal failure. Over the last decade however, non-surgical biliary drainage procedures performed with radiologic or endoscopic guidance emerged as alternative to surgical of therapy in many situations, particularly palliation of malignant strictures because of their lower morbidity and mortality rates. Endoscopic stent placement is preferred over percutaneous transhepatic catheter drainage in general. If endoscopic procedure is not possible or fails percutaneous transhepatic biliary drainage or combined radiological-endoscopic procedure should be employed. Surgery is currently reserved only for curative resection/palliative drainage in young and fit patients. Preoperative biliary drainage aimed at reducing post-operative morbidity and mortality is not universally accepted and needs further study. Benign strictures are increasingly being dilated non-surgically with temporary stenting, especially in patients with failed surgery, recurrent strictures, contraindication to surgery and pre-liver transplant strictures eg primary sclerosing cholangitis. A cohesive team consisting of a surgeon, a radiologist and an endoscopist is required to achieve the best possible results.

Radioimmunotherapy of neoplastic meningitis in rats using an alpha-particle-emitting immunoconjugate

Because of their short range and high linear energy transfer, alpha-particles may be particularly effective in the treatment of neoplastic meningitis. Monoclonal antibody 81C6 was labeled with alpha-particle-emitting 211At using N-succinimidyl3-[211At]astatobenzoate, and the efficacy and toxicity of this immunoconjugate were evaluated in an athymic rat model. Animals were given injections via a chronic indwelling catheter with 5 x 10(5) TE-671 human rhabdomyosarcoma cells and treated 8 days later with single intrathecal doses of either saline or 4-18 microCi of 211At-labeled specific 81C6 antibody or isotype-matched control 211At-labeled 45.6 antibody. In the first experiment, 4, 7, and 13 microCi 211At-labeled 81C6 produced statistically significant (P = 0.004-0.02) increases in median survival of 33, 29, and 51%, respectively, as compared with saline. Two of 10 animals receiving the 13-microCi dose lived for 6 months before being killed for histological analysis. In the second experiment, 12 microCi of 211At-labeled 45.6 did not increase median survival significantly relative to saline control, while 12 microCi of 211At-labeled 81C6 increased median survival by 113% (P < 0.005) and resulted in 33% apparent cures. Five of 10 animals receiving 18 microCi of 211At-labeled 81C6 survived until they were killed at 295 days. An additional study was performed in animals given intrathecal injections of 5 x 10(6) TE-671 cells and given a single dose of 18 microCi of 211At-labeled 81C6 or 211At-labeled 45.6. At this higher cell number, significantly prolonged survival was still seen for specific antibody as compared with saline (P < 0.001) and control antibody (P < 0.05). These results suggest that treatment with 211At-labeled monoclonal antibodies may be a valuable approach for neoplastic meningitis.

PET imaging of osteosarcoma in dogs using a fluorine-18-labeled monoclonal antibody Fab fragment

Four dogs with histologically confirmed osteogenic sarcoma were studied with PET following intravenous injection of the 18F-labeled Fab fragment of TP-3, a monoclonal antibody specific for human and canine osteosarcomas.

METHODS

The antibody fragment was labeled using the N-succinimidyl 8-[(4'-[18F]fluorobenzyl)amino]suberate acylation agent. Blood clearance of activity was biphasic in all dogs but half-times were variable (T1/2 beta = 2-13 hr). Catabolism of labeled Fab was reflected by the decrease in protein-associated activity in serum from more than 90% at 1 min to 60%-80% at 4 hr.

RESULTS

PET images demonstrated increased accumulation of 18F at the primary tumor site relative to normal contralateral bone in one dog as early as 15 min after injection. Biopsies obtained after euthanasia indicated higher uptake at the edges of the tumor as observed on the PET scans. Tumor uptake was 1-3 x 10(-3)% injected dose/g, a level similar to that reported for other Fab fragments in human tumors. In the three dogs with metastatic disease, early PET images reflected activity in the blood pool but later uptake was observed in suspected metastatic sites.

CONCLUSIONS

These results, although preliminary, suggest that PET imaging of 18F-labeled antibody fragments is feasible and that dogs with spontaneous tumors could be a valuable model for preclinical research with radioimmunoconjugates.

Synthesis and preliminary evaluation of para- and meta-[18F]fluorobenzylguanidine

meta-[18F]Fluorobenzylguanidine ([18F]MFBG) and para-[18F]fluorobenzylguanidine ([18F]PFBG) were synthesized in three steps beginning with a fluoro for nitro exchange reaction on 3- and 4-nitrobenzonitrile, respectively. Overall radiochemical yields were 10-15% for [18F]MFBG and 50-55% for [18F]PFBG in a total synthesis time of 60 min. However, impurities interfered with the binding of the product to target cells. A new route was adopted for the synthesis of [18F]PFBG using 4-nitrilophenyl trimethylammonium trifluoromethanesulfonate (Q.S.) as the starting material. In addition to shortening the overall synthesis time by 10 min, this precursor also eliminated problems associated with the presence of small amounts of starting material in the preparation. In vitro binding of [18F]PFBG prepared by the Q.S. method to SK-N-SH, human neuroblastoma cells was 26.5 +/- 1.1%, compared to 16.9 +/- 1.6% when the nitro precursor was used. Selective uptake of both 18F-labeled isomers in the heart and adrenal was seen in mice. At 4 h, adrenal and heart uptake of [18F]PFBG prepared using Q.S. was 20.3 +/- 4.8 and 5.9 +/- 0.8% ID/g respectively, compared to 23.8 +/- 5.0 and 10.5 +/- 1.7% ID/g for [18F]MFBG. Based on the 5-fold higher radiochemical yields obtained with [18F]PFBG, this isomer would appear to be the more practical choice; however, in vitro and in vivo results suggest that [18F]MFBG exhibits greater similarities to MIBG.

Radioiodination of a monoclonal antibody using N-succinimidyl 5-iodo-3-pyridinecarboxylate

The potential utility of N-succinimidyl 5-iodo-3-pyridinecarboxylate (SIPC) for the radioiodination of monoclonal antibodies was investigated. Paired-label studies were performed using the anti-tenascin antibody 81C6 in athymic mice bearing subcutaneous D-54 MG human glioma xenografts. Radiolabeling was also done using N-succinimidyl 3-iodobenzoate (SIB). Radioiodination of SIPC and SIB both proceeded in 60-80% yield, but protein coupling efficiencies with SIB were higher (76 +/- 16 vs 60 +/- 7%). Immunoreactivity and affinity of both preparations were similar. Using SIPC, thyroid uptake was quite low, decreasing from 0.3% at day 1 to 0.05% at day 8. Tumor uptake reached 46 +/- 11% injected dose/g at day 1 but declined gradually thereafter. This apparent decline reflected the rapid growth of these xenografts since tumor accumulation expressed as percentage of injected dose remained nearly constant up to day 9. These results suggest that SIPC, like SIB, offers significant advantages for labeling antibodies when compared with conventional protein iodination methods.

Measuring astatine-211 distributions with SPECT

We have investigated standard SPECT techniques (rotating gamma cameras, multi-hole collimators, and filtered backprojection reconstruction) for imaging astatine-211 distributions. Since 211At emits alpha particles, this nuclide has potential for use in radiotherapy. The capability of imaging this nuclide would allow in vivo evaluation of the distribution and stability of potential 211At-labelled radiotherapeutic agents. 211At decay yields x-rays in the 77-92 keV range in addition to 500-900 keV gamma rays. This study evaluates the feasibility of SPECT imaging using the x-ray emissions of 211At. We have evaluated several collimators, with the determination that the medium-energy collimators we used are suitable, with 7% penetration (uncollimated counts versus collimated counts). Several phantoms were imaged and attenuation coefficients were measured (narrow-beam mu = 0.182 cm-1 for 77-80 keV x-rays in water). Reconstructed images demonstrate qualitative capabilities and a simple quantitative study demonstrates good correction for attenuation and scatter (approximately 10% error), at low count densities, at least for the phantom geometries used in this study.

N-succinimidyl 4-methyl-3-(tri-n-butylstannyl)benzoate: synthesis and potential utility for the radioiodination of monoclonal antibodies

N-succinimidyl 4-methyl-3-(tri-n-butylstannyl)benzoate (MATE) was synthesized in two steps from 4-methyl-3-iodobenzoic acid. Radioiododestannylation of MATE proceeded more slowly than N-succinimidyl 3-(tri-n-butylstannyl)benzoate (ATE), but for reaction periods of 10 min or more, identical yields were obtained. Paired-label biodistribution studies were performed in mice with an intact monoclonal antibody and an F(ab')2 fragment labeled using MATE, ATE and Iodogen. Thyroid uptake with MATE was low, comparable to that seen with ATE, and considerably lower than that observed when the Iodogen method was used. With the F(ab')2 fragment, kidney uptake using MATE was 8-fold higher than that observed when either the ATE or Iodogen methods were used.

Localization of fluorine-18-labeled Mel-14 monoclonal antibody F(ab')2 fragment in a subcutaneous xenograft model

Positron emission tomography is an imaging method that might improve the effectiveness of radioimmunoscintigraphy and might provide more accurate estimates of monoclonal antibody dosimetry prior to therapy. Because of its widespread availability, 2-h half-life 18F could be a useful nuclide for labeling monoclonal antibody fragments, provided that adequate tumor uptake and satisfactory tumor:normal tissue ratios could be achieved rapidly. In this study, the tissue distribution of 18F-labeled Mel-14 F(ab')2, a monoclonal antibody reactive with gliomas, was evaluated in a s.c. athymic mouse human glioma xenograft model. 18F labeling was performed using N-succinimidyl-8-(4-[18F]fluorobenzylamino) suberate. For paired-label comparisons both in vitro and in vivo, Mel-14 F(ab')2 was also labeled using N-succinimidyl 3- [125I]- iodobenzoate. When 100-120 micrograms of disuccinimidyl suberate was used in the 18F-labeled acylation agent synthesis, the binding of 18F-labeled Mel-14 F(ab')2 to glioma homogenates was comparable to that of the radioiodinated fragment. Scatchard analyses indicated nearly identical affinity constants for fragments with both labels (18F, 6.4 x 10(8) M-1; 125I, 6.7 x 10(8) M-1). Tumor levels of 18F increased between 1 and 2 h and then were relatively constant between 2 and 6 h. When lower levels of disuccinimidyl suberate were used, there was an excellent correlation between 18F and 125I tumor uptake (r = 0.984, slope 1.03-1.04). At 4 h, tumor:normal tissue ratios for 18F-labeled Mel-14 F(ab')2 in liver, spleen, muscle, and brain were 2.3, 4.2, 14, and 40, respectively. Localization indices, determined by comparison with 18F-labeled nonspecific F(ab')2, were 3.7 at 4 h and 6.9 at 6 h for tumor and about 1 for normal tissues, indicating the specificity of 18F-labeled Mel-14 F(ab')2 tumor uptake.

Rare poisoning with cerebra thevetia (yellow oleander). Review of 13 cases of suicidal attempt

Clinical presentation, course and management of thirteen patients of suicidal poisoning with seeds of yellow-oleander were studied. All patients who took more than two seeds had gastro-intestinal as well as cardiovascular toxic effects. Patients responded well to symptomatic and supportive treatment when they had four or less than four seeds and reported to hospital within four hours intake. Progress was bad when patients took more than four seeds and reported after four hours of intake, cause of death in all the patients was cardiogenic shock.

Fluorine-18-antimyosin monoclonal antibody fragments: preliminary investigations in a canine myocardial infarct model

The purpose of this study was to determine in a canine model whether selective myocardial infarct uptake of 18F-labeled antimyosin monoclonal antibody fragments could be achieved in a time frame compatible with the short half-life of this nuclide. Antimyosin monoclonal antibody fragments were labeled with 18F using a succinimidyl [18F]fluorobenzylamine ester acylation agent. Six dogs had myocardial infarction induced by coronary artery occlusion and were reperfused prior to the intravenous administration of 0.6-4.7 mCi of 18F-labeled F(ab')2 (two dogs) or Fab (four dogs). Analysis of tissues obtained 2-4 hr after antibody administration revealed infarct:normal myocardium uptake ratios as high as 14-21:1 for F(ab')2 and 9-12:1 for Fab. Even with Fab, however, prolonged 18F activity in the blood pool interfered with delineation of infarcts by PET imaging. In one dog, perfusion imaging with [13N]ammonia before antimyosin administration was performed, and regions of normal and ischemic myocardium were determined. With these regions of interest, infarct:normal myocardium uptake ratios calculated from the 18F-labeled Fab images increased from 1.5:1 at 1 hr to 4.0:1 at 5 hr. We conclude that 18F-labeled antimyosin fragments may be of value for hot-spot imaging of damaged myocardium with PET; however, blood-pool subtraction techniques will probably be required.

4-Fluorobenzylamine and phenylalanine methyl ester conjugates of 2-nitroimidazole: evaluation as hypoxic cell radiosensitizers

We have synthesized two 2-nitroimidazole derivatives and evaluated their hypoxic radiosensitization properties. The first, a 4-fluorobenzylamine conjugate of 2-nitroimidazole (PK-110), was designed so that it could also be labeled with the F-18 and used for positron emission tomographic imaging of hypoxia. The second, the L-phenylalanine methyl ester conjugate of 2-nitroimidazole (PK-130), was designed in an attempt to exploit amino acid transport channels to enhance drug transport into the tumor. The effects of these drugs (and SR-2508, for comparison) in vitro on the aerobic and hypoxic radiosensitivity of Chinese hamster V79 cells were evaluated using clonogenic assays. PK-130 and PK-110 at 0.1 and 1.0 mM were more efficient hypoxic cell radiosensitizers than obtained with 1.0 mM SR-2508. Marginal aerobic radiosensitization was observed for 1.0 mM treatment with PK-130 and PK-110, however, no aerobic radiosensitization was observed at 0.1 mM. Glutathione (GSH) depletion (less than 5% of control levels) by L-buthionine sulfoximine (BSO) further enhanced the SER for both PK-130 and PK-110 at 0.1 mM to 3.2 +/- 0.63 and 2.4 +/- 0.16, respectively. The results of this study encourage the in vivo tumor radiosensitization evaluation of PK-130 and PK-110.

Improved therapeutic efficacy of a monoclonal antibody radioiodinated using N-succinimidyl-3-(tri-n-butylstannyl)benzoate

Improvements in efficacy of radioimmunotherapy will require increased tumor uptake relative to normal tissue. We previously demonstrated that labeling the IgG2b glioma-reactive antitenascin monoclonal antibody 81C6 with 131I using N-succinimidyl-3-(tri-n-butylstannyl)benzoate (ATE) increased tumor uptake and tumor-to-normal tissue ratios and decreased deiodination compared with labeling using Iodo-Gen. The present study was conducted to determine whether 131I 81C6 labeled using ATE (81C6 ATE) would demonstrate a therapeutic advantage over 131I 81C6 labeled using Iodo-Gen (81C6 IOD) in treating s.c. D-54 MG human glioma xenografts in athymic mice. The subclass IgG2b MAb 45.6 labeled with 131I using ATE (45.6 ATE) was used as a control. Animals were injected with saline or 500 microCi of 45.6 ATE (23 microCi/microgram), 81C6 ATE (26 microCi/microgram), or 81C6 IOD (24 microCi/microgram). With approximately 150 mm3 initial tumor volumes, growth delay for 81C6 ATE was significantly better by Wilcoxon rank sum analysis than saline (P = 0.0006 to 0.003), 45.6 ATE (P = 0.0006 to 0.002), and 81C6 IOD (P = 0.0008 to 0.007). Biodistribution data from similarly injected animals gave estimated radiation doses to tumor of 7723, 5200, and 1667 rad for 81C6 ATE, 81C6 IOD, and 45.6 ATE, respectively. In addition, 81C6 ATE administered at this dosage to animals with 50% larger initial tumors also improved tumor growth delay in comparison with 81C6 IOD given to animals with standard-size tumors. A similar experiment was conducted at 1000 microCi and, although radiation toxicity was noted in all labeled groups, two animals in the 81C6 ATE group had tumor regression for more than 240 days, and the other groups had no regressors. We conclude that the use of the ATE method may significantly improve the therapeutic efficacy of radioiodinated monoclonal antibodies.

Fluorine-18 labeling of monoclonal antibodies and fragments with preservation of immunoreactivity

A new method is reported for labeling proteins with the positron-emitting nuclide 18F. Initially, 4-[18F]-fluorobenzylamine was prepared in two steps from aqueous [18F]fluoride in high yield. The 18F acylation agent was formed by reaction of this product with disuccinimidyl suberate. Overall yields for the 4-[18F]fluorobenzylamine succinimidyl ester ([18F]SFBS), decay corrected to the end of cyclotron bombardment, were about 30% in a synthesis time of 60 min. After a 15-min reaction, 30-45% (decay corrected) of the [18F]SFBS could be coupled to intact antibodies and their F(ab')2 and Fab fragments. Coupling yields were dependent on protein concentration but not reaction time. HPLC purification of [18F]SFBS was necessary to obtain optimal coupling efficiency and immunoreactivity. The immunoreactivities of 18F-labeled F(ab')2 and Fab fragments of an antimyosin antibody were 89 +/- 5% and 75 +/- 9%, respectively. Biodistribution studies in normal mice demonstrated similar in vivo behavior of 18F-labeled antibody fragments and those labeled with 125I by using N-succinimidyl 3-[125I]iodobenzoate. These results indicate that this method may be useful for labeling monoclonal antibodies and other proteins and peptides with 18F.

N-succinimidyl 5-(trialkylstannyl)-3-pyridinecarboxylates: a new class of reagents for protein radioiodination

N-Succinimidyl 5-(trialkylstannyl)-3-pyridinecarboxylates (alkyl = Me, Bu) have been prepared and used as a precursor to label N-succinimidyl 5-[131I]iodo-3-pyridinecarboxylate (SIPC). SIPC was obtained in greater than 80% yield from either the methyl or butyl precursor with N-chlorosuccinimide and heating at 60-65 degrees C. Significantly lower yields were observed with tert-butyl hydroperoxide. After a 30-min incubation with [131I]SIPC at pH 8.5, goat IgG, an intact monoclonal antibody (MAb), and a MAb F(ab')2 fragment were labeled in 60-65% yield. Specific binding of the MAb and MAb fragment after SIPC labeling was identical with that observed with N-succinimidyl 3-iodobenzoate and higher than that reported previously for these MAbs after labeling by using the Iodogen method. When 5-[131I]iodonicotinic acid was injected into normal mice, thyroid uptake was less than 0.2% of the injected dose, reflecting the inertness of this compound to deiodination. Paired-label biodistribution studies indicate that for both the MAb and the F(ab')2 labeled by using SIPC, accumulation of activity in the thyroid and other tissues is comparable to that observed when these proteins were labeled by using N-succinimidyl 3-iodobenzoate. The results of this study suggest that SIPC may be a reagent for labeling MAbs with halogen nuclides.

Comparative tissue distribution in mice of the alpha-emitter 211At and 131I as labels of a monoclonal antibody and F(ab')2 fragment

Because it decays by the emission of short-range, high-energy alpha-particles, the radiohalogen 211At might be a particularly useful nuclide for some types of radioimmunotherapy. However, no suitable gamma-emitting nuclide of astatine exists which would permit either imaging prior to therapy to obtain radiation dosimetry estimates or performing experiments in paired-label format. Since iodine is the halogen above astatine in the periodic table, we investigated whether the in vivo distribution of 131I could be used to mimic the biodistribution of 211At. In this study, the N-succinimidyl 3-(trialkylstannyl)benzoate method was used to label C110 IgG, an antibody directed against carcinoembryonic antigen, and its (Fab')2 fragment with 211At and 131I. Paired-label experiments were performed in normal mice comparing the tissue distribution of 211At- versus 131I-labeled C110 IgG and F(ab')2 as well as [211At]astatide versus [131I]iodide and m-[211At]astatobenzoic acid versus m-[131I]iodobenzoic acid, potential catabolites of proteins radiohalogenated via the N-succinimidyl 3-(trialkylstannyl)benzoate method. With the exception of thyroid, retention of astatide in tissues was higher than that of iodide; and, with the halobenzoic acids, uptake of 211At was higher than 135I in thyroid, stomach, and spleen. Use of the N-succinimidyl 3-(trialkylstannyl)benzoate method to label C110 IgG with 211At and 131I resulted in similar distributions of the two nuclides. In contrast, loss of 211At from the F(ab')2 fragment was considerably more rapid than 131I, suggesting that different astatination methods may be required for use with F(ab')2 fragments.

Vasculitis--a complication of hypothyroidism

A case of hypothyroidism presenting with cutaneous vasculitis is described. Apart from typical findings of hypothyroidism, the patient had hypertension (HT). With thyroxine replacement therapy patient showed an improvement in hypothyroidism, HT and cutaneous vasculitis.

Enhanced tumor localization and in vivo stability of a monoclonal antibody radioiodinated using N-succinimidyl 3-(tri-n-butylstannyl)benzoate

Loss of radiolabel after in vivo administration of labeled monoclonal antibodies (MAbs) to cancer patients is a likely cause of the low levels of tumor uptake of MAb which have been observed. In this study, we have evaluated the utility of N-succinimidyl 3-(tri-n-butylstannyl)benzoate (ATE) for the radioiodination of 81C6, a MAb reactive with the extracellular matrix antigen tenascin associated with gliomas and other tumors. In vitro binding properties of MAb labeled via ATE were slightly better than those of the Iodogen preparations. Paired-label studies were performed in athymic mice bearing s.c. D-54 MG xenografts and injected with both 81C6 labeled with 125I using the ATE method and 131I using the Iodogen method. These studies demonstrated that use of the ATE method (a) decreased thyroid uptake by 40- to 100-fold, suggesting a lower rate of dehalogenation compared to MAb labeled using Iodogen; (b) increased tumor uptake by as much as a factor of 4 at Day 1 to more than 12-fold at Day 8; and (c) resulted in superior tumor-to-normal-tissue dose ratios. The specificity of MAb uptake was investigated in a paired-labeled study comparing the distribution of 81C6 and isotype-matched control 45.6, both labeled using the ATE procedure. Localization indices for tumor ranged between 6 at Day 1 to 34 at Day 7, values considerably higher than those reported previously for 81C6 and 45.6 radioiodinated using a conventional method (chloramine T). These results demonstrate that the ATE method may be a valuable approach for labeling MAbs with iodine nuclides.

Labeling monoclonal antibodies and F(ab')2 fragments with the alpha-particle-emitting nuclide astatine-211: preservation of immunoreactivity and in vivo localizing capacity

alpha-Particles such as those emitted by 211At may be advantageous for radioimmunotherapy since they are radiation of high linear energy transfer, depositing high energy over a short distance. Here we describe a strategy for labeling monoclonal antibodies and F(ab')2 fragments with 211At by means of the bifunctional reagent N-succinimidyl 3-(trimethylstannyl)benzoate. An intact antibody, 81C6, and the F(ab')2 fragment of Me1-14 (both reactive with human gliomas) were labeled with 211At in high yield and with a specific activity of up to 4 mCi/mg in a time frame compatible with the 7.2-hr half-life of 211At. Quantitative in vivo binding assays demonstrated that radioastatination was accomplished with maintenance of high specific binding and affinity. Comparison of the biodistribution of 211At-labeled Me1-14 F(ab')2 to that of a nonspecific antibody fragment labeled with 211At and 131I in athymic mice bearing D-54 MG human glioma xenografts demonstrated selective and specific targeting of 211At-labeled antibody in this human tumor model.

Labeling proteins using aryl iodide acylation agents: influence of meta vs para substitution on in vivo stability

The effect of para vs meta substitution on the biological behavior of an intact antibody and an F(ab')2 fragment was investigated. Paired-label studies were performed using 81C6 IgG and OC 125 F(ab')2 labeled using the N-succinimidyl esters of both p-[125I]- and m-[131I]iodobenzoate as well as with the potential catabolites, p-[125I]- and m-[131I]iodobenzoic acid. In all 3 studies, up to 55% lower uptake of 131I in thyroid and stomach was observed, suggesting that the m-substituted species were more inert to dehalogenation in vivo.

Synthesis of radioiodinated N-succinimidyl iodobenzoate: optimization for use in antibody labelling

N-succinimidyl-3-(tri-n-butylstannyl)benzoate (m-BuATE), N-succinimidyl-3-(tri-methylstannyl)benzoate (m-MeATE) and N-succinimidyl-4-(tri-n-butylstannyl)benzoate (p-BuATE) were synthesized and radioiodinated using either N-chlorosuccinimide (NCS) or t-butylhydroperoxide (TBHP) as the oxidant. Radiohalogenation of m-MeATE proceeded more rapidly than m-BuATE. NCS was the more efficient oxidant at reaction times less than 15 min; use of both TBHP and NCS resulted in nearly quantitative yields after 15 min when m-MeATE was used. Using NCS, achieving optimal antibody coupling and specific binding required purification of the active ester by HPLC; in contrast, with TBHP, only Sep-Pak purification was needed.

Evaluation of the cytotoxic potential of catechols and quinones structurally related to butylated hydroxyanisole

The cytotoxicity of 2- and 3-butylated hydroxyanisole (BHA) and 18 related aromatic compounds has been determined employing cultured P388 and KB cells. The phenolic compounds, 3-BHA and 2-BHA, had moderately low cytotoxic activity. Their corresponding catechols had ED50 values that were much lower than those of the parent compounds. This substantial increase in the cytotoxic activity is attributed to the presence of the catechol group, which is known to undergo one-electron oxidation readily to give the corresponding semiquinone radical. Other related catechols had similar cytotoxic activity. In general, derivatization of the catechol functionality resulted in a decrease of the cytotoxic potential of the compounds. Monoacetylation or monomethylation of the catechols gave products that were less potent cytotoxic agents than the parent compounds. Further loss of activity was observed when both hydroxy groups of the catechol function were blocked. Substitution of a methoxy group in place of a hydrogen atom in these compounds resulted in a significant increase of cytotoxicity, whereas the replacement of a methoxy group with a methyl group reduced the cytotoxicity. The catechols and quinones derived from 2-BHA were more active when compared with those derived from 3-BHA. The t-butyl group adjacent to the catechol or quinone moiety in the 3-BHA derivatives appeared to exert a significant steric effect toward the cytotoxic potential of these compounds. These results suggest the potential use of o-quinones and catechols as cytotoxic and antitumor agents.

Pharmacokinetic studies of amino acid analogues of 2-nitroimidazole, new hypoxic cell radiosensitizers

A series of new analogues of 2-nitroimidazole has been synthesized by inserting various amino acids at 1-position through an amide bond. The ethyl esters of N-alpha-[(2-nitro-1-imidazolyl)acetyl]-L-phenylalanine and N-alpha-[2-nitro-1-imidazolyl)acetyl]-L-tyrosine were found to be the most effective radiosensitizers in vitro against hypoxic Chinese hamster (V-79) cells. The sensitizer enhancement ratios (SER) of these derivatives were 2.2 and 2.3 respectively at 1 mM concentration after 2 hr exposure under hypoxia. However, the free acid of phenylalanine analogue was less active as a radiosensitizer and required 5 mM concentration to produce SER of 1.9. In contrast, the free acid of tyrosine analogue was inactive in this test system. The pharmacokinetic studies with the esters revealed their rapid hydrolysis in serum to the corresponding acids within 5 minutes as detected by HPLC. The pharmacokinetic parameters were therefore determined by employing the free acid analogues and solubilizing them as their sodium salts. The drugs were administered intraperitoneally at 0.5 mg/g dose level to C-57 mice bearing B16 melanoma. These agents were cleared from the plasma rapidly with an apparent t 1/2 of 18.8 and 15.6 min respectively. Peak tumor concentration of approximately 217 micrograms/g was achieved within 15 min with phenylalanine analogue. The tumor to brain ratio was 10:1 suggesting that this agent is excluded from CNS and that the phenylalanine analogue should be considered a potentially less neurotoxic radiosensitizer than misonidazole.