Preparation and investigation of tumor affinity, uptake kinetic and transport mechanism of iodine-123-labelled amino acid derivatives in human pancreatic carcinoma and glioblastoma cells

A Convenient Route to New (Radio)Fluorinated and (Radio)Iodinated Cyclic Tyrosine Analogs

The use of radiolabeled non-natural amino acids can provide high contrast SPECT/PET metabolic imaging of solid tumors. Among them, radiohalogenated tyrosine analogs (i.e., [¹²³I]IMT, [¹⁸F]FET, [¹⁸F]FDOPA, [¹²³I]8-iodo-L-TIC(OH), etc.) are of particular interest. While radioiodinated derivatives, such as [¹²³I]IMT, are easily available via electrophilic aromatic substitutions, the production of radiofluorinated aryl tyrosine analogs was a long-standing challenge for radiochemists before the development of innovative radiofluorination processes using arylboronate, arylstannane or iodoniums salts as precursors. Surprisingly, despite these methodological advances, no radiofluorinated analogs have been reported for [¹²³I]8-iodo-L-TIC(OH), a very promising radiotracer for SPECT imaging of prostatic tumors. This work describes a convenient synthetic pathway to obtain new radioiodinated and radiofluorinated derivatives of TIC(OH), as well as their non-radiolabeled counterparts. Using organotin compounds as key intermediates, [¹²⁵I]5-iodo-L-TIC(OH), [¹²⁵I]6-iodo-L-TIC(OH) and [¹²⁵I]8-iodo-L-TIC(OH) were efficiently prepared with good radiochemical yield (RCY, 51–78%), high radiochemical purity (RCP, >98%), molar activity (Am, >1.5–2.9 GBq/µmol) and enantiomeric excess (e.e. >99%). The corresponding [¹⁸F]fluoro-L-TIC(OH) derivatives were also successfully obtained by radiofluorination of the organotin precursors in the presence of tetrakis(pyridine)copper(II) triflate and nucleophilic [¹⁸F]F⁻ with 19–28% RCY d.c., high RCP (>98.9%), Am (20–107 GBq/µmol) and e.e. (>99%).

Amino acid transport system - A substrate predicts the therapeutic effects of particle radiotherapy

L-[methyl-¹¹C]Methionine (¹¹C-Met) is useful for estimating the therapeutic efficacy of particle radiotherapy at early stages of the treatment. Given the short half-life of ¹¹C, the development of longer-lived ¹⁸F- and ¹²³I-labeled probes that afford diagnostic information similar to ¹¹C-Met, are being sought. Tumor uptake of ¹¹C-Met is involved in many cellular functions such as amino acid transport System-L, protein synthesis, and transmethylation. Among these processes, since the energy-dependent intracellular functions involved with ¹¹C-Met are more reflective of the radiotherapeutic effects, we evaluated the activity of the amino acid transport System-A as an another energy-dependent cellular function in order to estimate radiotherapeutic effects. In this study, using a carbon-ion beam as the radiation source, the activity of System-A was evaluated by a specific System-A substrate, alpha-[1-¹⁴C]-methyl-aminoisobutyric acid (¹⁴C-MeAIB). Cellular growth and the accumulation of ¹⁴C-MeAIB or ¹⁴C-Met were evaluated over time in vitro in cultured human salivary gland (HSG) tumor cells (3-Gy) or in vivo in murine xenografts of HSG tumors (6- or 25-Gy) before and after irradiation with the carbon-ion beam. Post 3-Gy irradiation, in vitro accumulation of ¹⁴C-Met and ¹⁴C-MeAIB decreased over a 5-day period. In xenografts of HSG tumors in mice, tumor re-growth was observed in vivo on day-10 after a 6-Gy irradiation dose, but no re-growth was detected after the 25-Gy irradiation dose. Consistent with the growth results, the in vivo tumor accumulation of ¹⁴C-MeAIB did not decrease after the 6-Gy irradiation dose, whereas a significant decrease was observed after the 25-Gy irradiation dose. These results indicate that the activity of energy dependent System-A transporter may reflect the therapeutic efficacy of carbon-ion radiotherapy and suggests that longer half-life radionuclide-labeled probes for System-A may also provide widely available probes to evaluate the effects of particle radiotherapy on tumors at early stage of the treatment.

Biological evaluation of new [(18) F]F-labeled synthetic amino acid derivatives as oncologic radiotracers

The present study evaluated the tumoral uptake of the novel synthetic amino acid positron emission tomography (PET) tracers (S)-2-amino-3-(4-([(18) F]fluoromethyl)-1H-1,2,3-triazol-1-yl)propanoic acid (AMC-101), (S)-2-amino-4-(4-([(18) F]fluoromethyl)-1H-1,2,3-triazol-1-yl)butanoic acid (AMC-102), and (S)-2-amino-5-(4-([(18) F]fluoromethyl)-1H-1,2,3-triazol-1-yl)pentanoic acid (AMC-103), all of which are (S)-2-amino-(4-([(18) F]fluoromethyl)-1H-1,2,3-triazol-1-yl)alkyl acids. In vitro cellular uptake was investigated using the rat glioma cell lines 9L and C6. In vitro competitive inhibition tests were performed to identify the involvement of specific amino acid transporters. In vivo dynamic PET images of 9L xenograft tumor-bearing model mice were acquired over 2 h after AMC administration. [(18) F]FDOPA PET studies were performed with and without S-carbidopa pretreatment for comparison. All three AMCs exhibited good in vitro cell uptake through the L and alanine-serine-cysteine transporters and enabled clear tumor visualization on PET, leaving the brain devoid of the tracer. Thirty minutes after injection, the mean tumor standardized uptake values were 1.59 ± 0.05, 1.89 ± 0.27, and 1.74 ± 0.13 for AMC-101, AMC-102, and AMC-103, respectively. Although the tumor uptake values of AMCs were lower than that of [(18) F]FDOPA with S-carbidopa pretreatment, AMCs enabled higher contrast images with lower background activity compared with [(18) F]FDOPA with S-carbidopa pretreatment. Our results indicate the potential uses of these new synthetic amino acids as oncologic radiotracers.

Systemic treatment with 4-211Atphenylalanine enhances survival of rats with intracranial glioblastoma

OBJECTIVE

Increased amino acid transport in brain tumours is used for diagnostic purposes. It has been shown that the α-emitting radionuclide astatine-211 labeled to L-phenylalanine is taken up by glioblastoma cells. We here tested, if systemic treatment with 4-[211At]astatine-phenylalanine (At-Phe) has a beneficial effect on survival of rats with intracranial glioblastoma.

ANIMALS, METHODS

The rat glioblastoma cell line BT4Ca was implanted into the prefrontal cortex of female BDIX rats by stereotaxic microinjection (10,000 cells/3 µl; n = 83). 3 days after implantation At-Phe or phosphate buffered saline were injected intravenously. A third group was treated twice, i.e., on day 3 and 10. Health condition was assessed each day by using a score system. Rats were sacrificed on days 6, 10, 13 and 17 after implantation, or when showing premortal health condition to measure tumour volume and necrosis. The proliferation index (PI) was assessed after immunohistochemical staining of Ki-67.

RESULTS

Survival time of rats treated twice with At-Phe was significantly prolonged. Additionally, both At-Phe-treated groups remained significantly longer in a better health condition. Rats with poor health status had larger tumours than rats with fair health condition. Overall, irrespective of treatment the PI was reduced in rats with poor health condition. Necrosis was larger in rats treated twice with At-Phe.

CONCLUSION

Intravenous treatment with At-Phe enhanced survival time of rats with intracranial glioblastomas and improved health condition. These results encourage studies using local treatment of intracranial glioblastoma with At-Phe, either by repeated local injection or by intracavital application after tumour resection.

An alternative and expedient synthesis of radioiodinated 4-iodophenylalanine

Radiolabeled amino acids have been used extensively in oncology both as diagnostic and therapeutic agents. In our pursuit to develop radiopharmaceuticals to target breast cancer, we were interested in determining the uptake of radioiodinated 4-iodophenylalanine, among other labeled amino acids, in breast cancer cells. In this work, we have developed an alternative method for the synthesis of this agent. The novel tin precursor, (S)-tert-butyl 2-(tert-butoxycarbonylamino)-3-(4-(tributylstannyl)phenyl)propanoate (3) was synthesized from the known, corresponding iodo derivative. Initially, the labeled 4-iodophenylalanine was synthesized from the above tin precursor in two steps with radiochemical yields of 91.6 ± 2.7% and 83.7 ± 1.7% (n=5), for the radioiodination (first) and deprotection (second) step, respectively. Subsequently, it was synthesized in a single step with an average radiochemical yield of 94.8 ± 3.4% (n=5). After incubation with MCF-7 breast cancer cells for 60 min, an uptake of up to 49.0 ± 0.7% of the input dose was seen; in comparison, the uptake of [¹⁴C]phenylalanine under the same conditions was 55.9 ± 0.5%. Furthermore, the uptake of both tracers was inhibited to a similar degree in a concentration-dependent manner by both unlabeled phenylalanine and 4-iodophenylalanine. With [¹⁴C]phenylalanine as the tracer, IC₅₀ values of 1.45 and 2.50 mM were obtained for Phe and I-Phe, respectively, and these values for [¹²⁵I]I-Phe inhibition were 1.3 and 1.0 mM. In conclusion, an improved and convenient method for the synthesis of no-carrier-added 4-[(⁎)I]phenylalanine was developed and the radiotracer prepared by this route demonstrated an amino acid transporter-mediated uptake in MCF-7 breast cancer cells in vitro that was comparable to that of [¹⁴C]phenylalanine.

Prospective study of p-[123I]iodo-L-phenylalanine and SPECT for the evaluation of newly diagnosed cerebral lesions: specific confirmation of glioma

PURPOSE

The differentiation between gliomas, metastases and gliotic or inflammatory lesions by imaging techniques remains a challenge. Gliomas frequently exhibit increased uptake of radiolabelled amino acids and are thus amenable to PET or SPECT imaging. Recently, p-[123I]iodo-L-phenylalanine (IPA) was validated for the visualization of glioma by SPECT and received orphan drug status. Here we investigated its diagnostic performance for differentiating indeterminate brain lesions.

METHODS

This prospective open study included 67 patients with newly diagnosed brain lesions suspicious for glioma (34 without and 33 with contrast enhancement in the MRI scan). Patients received 250 MBq IPA intravenously after overnight fasting. SPECT images at 30 min and 3 h post-injection were iteratively reconstructed and visually interpreted after image fusion with an MRI brain scan (fluid-attenuated inversion recovery sequence or T1-weighted contrast-enhanced image). Findings were correlated with results of stereotactic or open biopsies or serial imaging.

RESULTS

Twenty-seven low-grade (2 WHO I, 25 WHO II) and 24 high-grade gliomas (1 WHO III, 23 WHO IV), 3 metastases originating from lung cancer as well as 13 non-neoplastic lesions were proven. All non-neoplastic lesions and all metastases were negative with IPA SPECT. Forty gliomas were true-positive (TP) and 11 false-negative (FN) findings (8 WHO II, 1 WHO III, 2 WHO IV) occurred. There were no false-positive (FP) findings. For the differentiation of primary brain tumours and non-neoplastic lesions, sensitivity and specificity were 78 and 100%. In 34 lesions without contrast enhancement in MRI, IPA SPECT resulted in 17 TP, 8 true-negative, 9 FN and no FP findings (sensitivity 65%, specificity 100%).

CONCLUSION

In patients with suspected glioma, IPA SPECT shows a high specificity, but especially in low-grade gliomas FN findings may occur. Due to the high positive predictive value a positive finding allows a suspected glioma to be confirmed.

Synthesis, radiolabeling, and biological evaluation of (R)- and (S)-2-amino-3-[(18)F]fluoro-2-methylpropanoic acid (FAMP) and (R)- and (S)-3-[(18)F]fluoro-2-methyl-2-N-(methylamino)propanoic acid (NMeFAMP) as potential PET radioligands for imaging brain tumors

The non-natural amino acids (R)- and (S)-2-amino-3-fluoro-2-methylpropanoic acid 5 and (R)- and (S)-3-fluoro-2-methyl-2-N-(methylamino)propanoic acid 8 were synthesized in shorter reaction sequences than in the original report starting from enantiomerically pure (S)- and (R)-alpha-methyl-serine, respectively. The reaction sequence provided the cyclic sulfamidate precursors for radiosynthesis of (R)- and (S)-[(18)F]5 and (R)- and (S)-[(18)F]8 in fewer steps than in the original report. (R)- and (S)-[(18)F]5 and(R)- and (S)-[(18)F]8 were synthesized by no-carrier-added nucleophilic [(18)F]fluorination in 52-66% decay-corrected yields with radiochemical purity over 99%. The cell assays showed that all four compounds were substrates for amino acid transport and enter 9L rat gliosarcoma cells in vitro at least in part by system A amino acid transport. The biodistribution studies demonstrated that in vivo tumor to normal brain ratios for all compounds were high with ratios of 20:1 to115:1 in rats with intracranial 9L tumors. The (R)-enantiomers of [(18)F]5 and [(18)F]8 demonstrated higher tumor uptake in vivo compared to the (S)-enantiomers.

Synthesis and analysis of 2-[211At]-L-phenylalanine and 4-[211At]-L-phenylalanine and their uptake in human glioma cell cultures in-vitro

2-[211At]-L-phenylalanine and 4-[211At]-L-phenylalanine were prepared from the corresponding iodo and bromo derivatives using the Cu(+)-assisted nucleophilic exchange. 4-[211At]-L-phenylalanine was additionally prepared by destannylation of the BOC-derivatized 4-tributylstannyl-L-phenylalanine. Radiochemical yields of 2-[211At]-L-phenylalanine and 4-[211At]-L-phenylalanine by nucleophilic exchange were 52-74% and 65-85%. Radiochemical yield of 4-[211At]-L-phenylalanine by electrophilic destannylation was 35-50%. HPLC sequence analysis showed that 2-[211At]-L-phenylalanine followed the halogen sequence (F<Cl<Br<I<At) whereas 4-[211At]-L-phenylalanine eluted between 4-Br-L-phenylalanine and 4-I-L-phenylalanine (F<Cl<Br<At<I), independent on the production pathway. Uptake of 4-[211At]-L-phenylalanine and 4-[131I]-L-phenylalanine in DBTRG-05MG glioma cells was inhibited by l-phenylalanine 7-fold and 6-fold, respectively.

Uptake of 3-[125I]iodo-alpha-methyl-L-tyrosine into colon cancer DLD-1 cells: characterization and inhibitory effect of natural amino acids and amino acid-like drugs

INTRODUCTION

We examined 3-[(123)I]iodo-alpha-methyl-L-tyrosine ([(123)I]IMT) uptake and inhibition by amino acids and amino acid-like drugs in the human DLD-1 colon cancer cell line, to discuss correlation between the inhibition effect and structure.

METHODS

Expression of relevant neutral amino acid transporters was examined by real-time PCR with DLD-1 cells. The time course of [(125)I]IMT uptake, contributions of transport systems, concentration dependence and inhibition effects by amino acids and amino acid-like drugs (1 mM) on [(125)I]IMT uptake were examined.

RESULTS

Expression of system L (4F2hc, LAT1 and LAT2), system A (ATA1, ATA2) and system ASC (ASCT1) was strongly detected; system L (LAT3, LAT4) and MCT8 were weakly detected; and B(0)AT was not detected. [(125)I]IMT uptake in DLD-1 cells involved Na(+)-independent system L primarily and Na(+)-dependent system(s). Uptake of [(125)I]IMT in Na(+)-free buffer followed Michaelis-Menten kinetics, with a K(m) of 78 microM and V(max) of 333 pmol/10(6) cells per minute. Neutral D- and L-amino acids with branched or aromatic large side chains inhibited [(125)I]IMT uptake. Tyrosine analogues, tryptophan analogues, L-phenylalanine and p-halogeno-L-phenylalanines, and gamma amino acids [including 3,4-dihydroxy-L-phenylalanine (L-DOPA), DL-threo-beta-(3,4-dihydroxyphenyl)serine (DOPS), 4-[bis(2-chloroethyl)amino]-L-phenylalanine and 1-(aminomethyl)-cyclohexaneacetic acid] strongly inhibited [(125)I]IMT uptake, but L-tyrosine methyl ester and R(+)/S(-)-baclofen weakly inhibited uptake. The substrates of system ASC and A did not inhibit [(125)I]IMT uptake except L-serine and D/L-cysteine.

CONCLUSIONS

[(125)I]IMT uptake in DLD-1 cells involves mostly LAT1 and its substrates' (including amino acid-like drugs derived from tyrosine, tryptophan and phenylalanine) affinity to transport via LAT1. Whether transport of gamma amino acid analogues is involved in LAT1 depends on the structure of the group corresponding to the amino acid residue. Beta-hydroxylation may confer reduction of transport affinity of tyrosine analogues via LAT1.

Intracellular reactions affecting 2-amino-4-([(11)C]methylthio)butyric acid ([(11)C]methionine) response to carbon ion radiotherapy in C10 glioma cells

PURPOSE

The response of 2-amino-4-([(14)C]methylthio)butyric acid ([(14)C]Met) uptake and [(125)I]3-iodo-alpha-methyl-l-tyrosine ([(125)I]IMT) uptake to radiotherapy of C10 glioma cells was compared to elucidate the intracellular reactions that affect the response of 2-amino-4-([(11)C]methylthio)butyric acid ([(11)C]Met) uptake to radiotherapy.

METHODS

After irradiation of cultured (3 Gy) or xenografted C10 glioma cells (25 Gy) using a carbon ion beam, the accumulation of [(14)C]Met and [(125)I]IMT in the tumors was investigated. The radiometabolites in xenografted tumors after radiotherapy were analyzed by size-exclusion HPLC.

RESULTS

[(14)C]Met provided earlier responses to the carbon ion beam irradiation than [(125)I]IMT in both cultured and xenografted tumors. While [(125)I]IMT remained intact in xenografted tumor before and after irradiation, the radioactivity derived from [(14)C]Met was observed both in high molecular fractions and intact fractions, and the former decreased after irradiation.

CONCLUSION

The earlier response of [(11)C]Met uptake to tumor radiotherapy could be attributable to the decline in the intracellular energy-dependent reactions of tumors due to radiotherapy.

Improved synthesis of no-carrier-added p-[124I]iodo-L-phenylalanine and p-[131I]iodo-L-phenylalanine for nuclear medicine applications in malignant gliomas

This work describes the synthesis and the tumor affinity testing of no-carrier-added (n.c.a.) p-[(124)I]iodo-L-phenyalanine ([(124)I]IPA) and n.c.a. p-[(131)I]iodo-l-phenyalanine ([(131)I]IPA) as radiopharmaceuticals for imaging brain tumors with PET and for radionuclid-based therapy, respectively. Parameters for labeling were optimized with regard to the amount of precursor, temperature and time. Thereafter, n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA were investigated in rat F98 glioma and in primary human A1207 and HOM-T3868 glioblastoma cells in vitro, followed by an in vivo evaluation in CD1 nu/nu mice engrafted with human glioblastoma. No-carrier-added [(124)I]IPA and n.c.a. [(131)I]IPA were obtained in 90+/-6% radiochemical yield and >99% radiochemical purity by iododestannylation of N-Boc-4-(tri-n-butylstannyl)-L-phenylalanine methylester in the presence of chloramine-T, followed by hydrolysis of the protecting groups. The total synthesis time, including the HPLC separation and pharmacological formulation, was less than 60 min and compatible with a clinical routine production. Both amino acid tracers accumulated intensively in rat and in human glioma cells. The radioactivity incorporation in tumor cells following a 15-min incubation at 37 degrees C/pH 7.4 varied from 25% to 42% of the total loaded activity per 10(6) tumor cells (296-540 cpm/1000 cells). Inhibition experiments confirmed that n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA were taken up into tumor by the sodium-independent L- and ASC-type transporters. Biodistribution and whole-body imaging by a gamma-camera and a PET scanner demonstrated a high targeting level and a prolonged retention of n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA within the xenotransplanted human glioblastoma and a primarily renal excretion. However, an accurate delineation of the tumors in mice was not possible by our imaging systems. Radioactivity accumulation in the thyroid and in the stomach as a secondary indication of deiodination was less than 1% of the injected dose at 24h p.i., confirming the high in vivo stability of the radiopharmaceuticals. In conclusion, n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA are new promising radiopharmaceuticals, which can now be prepared in high radiochemical yields and high purity for widespread clinical applications. The specific and high-level targeting of n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA to glioma cells in vitro and to glioblastoma engrafts in vivo encourages further in vivo validations to ascertain their clinical potential as agent for imaging and quantitation of gliomas with PET, and for radionuclid-based therapy, respectively.

Intra-individual comparison of p-[123I]-iodo-L-phenylalanine and L-3-[123I]-iodo-α-methyl-tyrosine for SPECT imaging of gliomas

OBJECTIVES

Radioactive amino-acids accumulate in gliomas even with an intact blood-brain-barrier. L-3-[(123)I]-iodo-alpha-methyl-tyrosine (IMT) is well established for SPECT imaging of gliomas. Recently, we introduced p-[(123)I]-iodo-L-phenylalanine (IPA) for the characterisation of brain lesions. This study compares both tracers in glioma patients.

METHODS

Eleven patients with gliomas (1 WHO grade 1, 5 grade 2, 1 grade 3, 2 grade 4 gliomas, 1 unconfirmed upgrading and 1 post-therapeutic non-neoplastic lesion) underwent SPECT imaging with IPA (early and delayed acquisitions at 30 min and 3 h) and IMT (early only). Maximum tumour-to-brain ratios (TBR) were calculated using region-of-interest analysis to assess uptake of IMT and IPA. Imaging results were compared to histopathological findings.

RESULTS

Early TBRs of IMT and IPA were strongly correlated (r = 0.828, p = 0.002). TBRs were higher for IMT than IPA (1.95+/-0.50 versus 1.79+/-0.42; p < 0.05), but independent from tumour cell density (p > 0.1). Visual interpretation by different observers was more concordant for IMT-SPECT than IPA-SPECT (kappa 1.0 versus 0.774). No differences in early TBRs were observed between low-grade and high-grade gliomas for IMT (1.97+/-0.53 versus 2.21+/-0.44, p > 0.5) or IPA (1.70+/-0.23 versus 2.21+/-0.56, p = 0.167) with a trend to higher TBRs in low-grade tumours for IMT (p = 0.093). In contrast to the known wash-out of IMT, we observed persistent accumulation of IPA in gliomas.

CONCLUSIONS

IPA shows lower TBRs than IMT, especially in low-grade tumours, so IMT should be preferred for the delineation of low-grade gliomas by SPECT imaging. Due to its prolonged retention, however, IPA remains promising for therapeutic use in gliomas after labelling with I-131.

Use of [123I]-2-iodo-L-phenylalanine as a tumor imaging agent in two dogs with synovial cell sarcoma

[123I]-iodo-L-phenylalanine was successfully evaluated for gamma camera imaging in vivo in tumor-bearing athymic mice and in humans with brain tumors. Here, we report the use of this tracer in two dogs with synovial cell sarcoma of the tarsus. [123I]-iodo-L-phenylalanine was quantitatively prepared as a kit formulation using the Cu(1+) +-assisted nucleophilic exchange. Rapid [123I]-2-iodo-L-phenylalanine tumor accumulation was observed with good tumor to background contrast and rapid clearance in these two dogs. This radiopharmaceutical is a promising alternative tumor tracer to overcome the known limitations of 18F-fluorodeoxyglucose and, when labelled with radioiodine-131, has the potential to be used for therapeutic purposes.

In vitro and in vivo magnetic resonance detection of tumor cells by targeting glutamine transporters with Gd-based probes

The glutamine transporting system is up-regulated in tumor cells because cell proliferation requires the uptake of large quantities of glutamine. It has been found that the paramagnetic magnetic resonance imaging (MRI) reporter Gd-DOTAMA-C6-Gln, where the glutamine residue is covalently bound to the Gd chelate through a C6 spacer, accumulates in tumor cells both "in vitro" and "in vivo" experiments. The observation that the relaxivity of cellular pellets does not increase with the increase in the amounts of entrapped Gd chelate is taken as an indication that the internalization has occurred through receptor mediated endocytosis. The iv administration of Gd-DOTAMA-C6-Gln allowed the MRI visualization of tumor masses in A/J mice grafted with the murine neuroblastoma cell line Neuro-2a and in Her-2/neu transgenic mice developing multiple mammary carcinoma, respectively.

Preparation and tumor affinity testing of the radioiodinated tetrahydroisoquinoline derivative [123I]TIC(OH) for targeting prostate cancer

This work describes the synthesis and tumor affinity testing of 8-[123I]iodo-l-1,2,3,4-tetrahydro-7-hydroxyisoquinoline-3-carboxylic acid [ITIC(OH)], a cyclic non-naturally occurring amino acid as an imaging probe for prostate cancer. Parameters for labelling were optimized with regard to the amount of precursor, the temperature and time. Thereafter, ITIC(OH) was evaluated in terms of its uptake in primary human PC-3 and DU-145 prostate cancer cells, followed by analysis of the underlying mechanisms of the radioactivity accumulation in tumor cells. No-carrier-added ITIC(OH) was obtained in 80+/-15% radiochemical yield and >98% radiochemical purity by a one-step radioiodination, using IODO-GEN as oxidant. The total synthesis time was less than 30 min, and compatible with a clinical routine production. ITIC(OH) accumulated intensively in primary human prostate cancer cells. The radioactivity incorporation in tumor following a 10-min incubation at 37 degrees C/pH 7.4 varied from 35% to 58% of the total loaded activity per 10(6) tumor cells (355-540 cpm/1000 cells). Inhibition experiments revealed that ITIC(OH) was taken up into tumor by an active transport different from the common amino acid carrier systems, including the sodium-dependent system A and B+,0, and the sodium-independent L- and ASC-type transporter. In contrast, the cellular incorporation was dependent on the membrane potential and correlated with the activity of the mitochondria. In conclusion, the specific and high-level accumulation of ITIC(OH) in human prostate carcinoma cells, indicates that the new radiopharmaceutical is a good candidate for further in vivo investigations to ascertain its potential as an imaging probe for prostate cancer by SPET.

Validation of brain tumour imaging with p-[123I]iodo-l-phenylalanine and SPECT

PURPOSE

The aims of this prospective study were to validate single-photon emission computed tomography (SPECT) with p-[(123)I]iodo-L-phenylalanine (IPA) in brain tumours and to evaluate its potential for the characterisation of indeterminate brain lesions.

METHODS

In 45 patients with indeterminate brain lesions or suspected progression of glioma, amino acid uptake was studied using IPA-SPECT and compared with the final diagnosis established by biopsy or serial imaging. After image fusion of IPA-SPECT and magnetic resonance imaging, the presence of tumour was visually determined by two independent observers. IPA uptake was quantified as the ratio between maximum uptake in the suspicious lesion and mean uptake in unaffected brain.

RESULTS

Primary brain tumours were present in 35 cases (12 low-grade and 23 high-grade gliomas). Non-neoplastic brain lesions were confirmed in seven cases (three dysplasias, three inflammatory lesions, one lesion after effective therapy). Visual analysis showed a high concordance between the two observers (kappa=0.90, p<0.001), with sensitivity and specificity of 86% and 100% for the discrimination of primary brain tumours and non-neoplastic lesions. At 30 min p.i., IPA uptake in primary brain tumours was higher than that in non-neoplastic lesions (1.70+/-0.36 vs 1.14+/-0.18, p<0.05). Brain metastases showed no increased uptake (1.13+/-0.22, n=3). The persistent retention of IPA in low-grade gliomas without disruption of the blood-brain barrier was visualised up to 24 h p.i. Low-grade and high-grade gliomas showed equivalent IPA uptake (1.72+/-0.37 vs 1.67+/-0.36 at 30 min, p=0.745).

CONCLUSION

IPA shows long and specific retention in gliomas. IPA is a promising and safe radiopharmaceutical for the visualisation of gliomas and the characterisation of indeterminate brain lesions.

Radioiodinated phenylalanine derivatives to image pancreatic cancer: a comparative study with [18F]fluoro-2-deoxy-d-glucose in human pancreatic carcinoma xenografts and in inflammation models

This work validated an in vivo model of human pancreatic cancer for preclinical studies and evaluated p-amino-3-[123I]iodo-L-phenylalanine (AIPA) and p-[123I]iodo-L-phenylalanine (IPA) as potential imaging agents for pancreatic cancer. The primary human pancreatic adenocarcinoma PaCa44 and PanC1 cells (1.5-2.5x10(6)) were inoculated either subcutaneously into the flank or orthotopically into the pancreas of severe combined immunodeficient (SCID) mice. Tumor formation was recorded by palpation and monitored by magnetic resonance imaging (MRI). After intravenous injection, tumor affinity and organ distribution of AIPA and IPA were compared with those of [18F]fluoro-2-deoxy-D-glucose (FDG) in tumor-bearing SCID mice and in concanavalin A (ConA)-induced inflammation models. All SCID mice developed a pancreatic tumor 2-4 weeks after cell implantation. All subcutaneously transplanted tumors were detected by MRI and confirmed histologically, whereas 90% and 68% of the histopathologically confirmed orthotopic PaCa44 and PanC1 tumors were accurately demonstrated by MRI. Tumor formation and spread after orthotopic implantation showed invasion into adjacent organs and metastases in different sites of the abdomen. In contrast, no organ invasion or metastases were demonstrated by subcutaneous implantation. In vivo, AIPA and IPA displayed high affinity for pancreatic tumors. Radioactivity uptake into a tumor at 60 and 240 min amounted to 7.2+/-2.1% and 10.7+/-2.5% I.D./g for AIPA and 13.3+/-3.5% and 15.2+/-3.8% I.D./g for IPA in heterotopic tumors as compared with 11.8+/-3.2% and 15.2+/-2.4% I.D./g for AIPA and 16.7+/-3.5% and 22.5+/-4.3% I.D./g for IPA in orthotopically implanted tumors. In comparison, the FDG uptake was 10.8+/-2.9% and 2.5+/-0.6% I.D./g into heterotopic tumors and 12.5+/-3.8% and 3.5+/-1.2% I.D./g into the orthotopic ones at 60 and 240 min postinjection. The FDG uptake markedly increased (>400%) in the area of inflammation, whereas accumulation of AIPA and IPA in inflammation remained moderate and comparable with that determined in muscle. In summary, the orthotopic implantation model, more than the heterotopic one, reflects more closely the clinical course of the disease, thus representing the appropriate in vivo model for preclinical studies. The specific and high-level targeting of AIPA and IPA to human pancreatic tumor xenografts, with marked tumor-to-background ratios, indicate that AIPA and IPA are interesting candidates as radiotracers for noninvasive imaging of pancreatic cancer. IPA has the advantage of relatively low renal uptake and thus presents as the most promising candidate.

Characterization of uptake of 3-[131I]iodo-α-methyl-L-tyrosine in human monocyte-macrophages

The radiopharmaceutical 3-[(123)I]iodo-alpha-methyl-L-tyrosine ([(123)I]IMT) can be used to study amino acid transport by single-photon emission tomography (SPET). In order to evaluate the potential contribution of [(123)I]IMT accumulation in macrophages to overall uptake values measured in neoplastic lesions in vivo, we studied the mechanisms governing the uptake of this tracer by human monocyte-macrophages (HMMs). HMMs were isolated from healthy human donors by density gradient centrifugation using Ficoll methods. The human glioblastoma cell line U-138 MG (GLIOs) was obtained from American Type Culture Collection. Using multiwell dishes, cells were incubated in phosphate buffered saline or an equivalent sodium-free buffer with 50 kBq [(131)I]IMT per well. [(131)I]IMT uptake was quantified as % injected dose per mass of protein within each culture well. Several natural and artificial amino acids were used as potential transport inhibitors both in sodium-containing and sodium-free medium. [(131)I]IMT uptake was significantly lower in HMMs than in GLIOs (34 +/- 2 %/mg (40 min) vs. 507 +/- 50 %/mg at 30 minutes of incubation, respectively; p < 0.01). Endotoxin (LPS) significantly increased [(131)I]IMT uptake in HMMs by a factor of approximately 2. Transport into non-stimulated HMMs was exclusively sodium-independent and inhibitable by BCH, but not by MeAIB. Under LPS stimulation exclusively, there was in addition also a sodium-dependent inhibition of [(131)I]IMT uptake by L-arginine and MeAIB, albeit to a minor extent. [(131)I]IMT accumulation in HMMs is mainly mediated via an L-like amino acid transport system and increases on HMM activation by LPS. LPS may induce an additional Na(+)-dependent transport system in HMMs. The considerably lower [(131)I]IMT uptake in HMMs than in GLIOs suggests that overall uptake values of this tracer measured by SPET in tumors are not significantly affected by [(123)I]IMT accumulation in macrophages within the neoplastic lesion.

p-[123I]iodo-L-phenylalanine for detection of pancreatic cancer: basic investigations of the uptake characteristics in primary human pancreatic tumour cells and evaluation in in vivo models of human pancreatic adenocarcinoma

Pancreatic cancer is associated with the worst 5-year survival rate of any human cancer. This high mortality is due, in part, to difficulties in establishing early and accurate diagnosis. Because most tumours share the ability to accumulate amino acids more effectively than normal tissues and any other pathology, assessment of amino acid transport in tumour cells using radiolabelled amino acids has become one of the most promising tools for tumour imaging. This study investigated the potential of p-[(123)I]iodo-L-phenylalanine (IPA) for detection of pancreatic cancer by single-photon emission tomography. IPA affinity for pancreatic tumour was investigated in human pancreatic adenocarcinoma PaCa44 and PanC1 cells, followed by analysis of the underlying mechanisms of tracer accumulation in neoplastic cells. Thereafter, IPA was evaluated for targeting of pancreatic tumours using SCID mice engrafted with primary human pancreatic adenocarcinoma cells, as well as in acute inflammation models in immunocompetent mice and rats. IPA accumulated intensively in human pancreatic tumour cells. Radioactivity accumulation in tumour cells following a 30-min incubation at 37 degrees C/pH 7.4 varied from 41% to 58% of the total loaded activity per 10(6) cells. The cellular uptake was temperature and pH dependent and predominantly mediated by specific carriers for neutral amino acids, namely the sodium-independent and L-leucine-preferring (L-system) transporter and the alanine-, serine- and cysteine-preferring (ASC-system) transporter. Protein incorporation was less than 8%. Biodistribution studies showed rapid localization of the tracer to tumours, reaching 10%+/-2.5% to 15%+/-3% of the injected dose per gram (I.D./g) in heterotopic tumours compared with 17%+/-3.5% to 22%+/-4.3% I.D./g in the orthotopic tumours, at 60 and 240 min post injection of IPA, respectively. In contrast, IPA uptake in the gastrointestinal tract and areas of inflammation remained moderate and decreased with time. Excellent tumour detection was obtained by gamma camera imaging. The specific and high-level targeting of IPA to tumour and the negligible uptake in the gastrointestinal tract and areas of inflammation indicate that p-[(123)I]iodo-L-phenylalanine is a promising tracer for differential diagnosis of pancreatic cancer.

Pyrrolobenzimidazoles Linked to Heterocycles and Peptides. Design of DNA Base Pair Specific Phosphate Hydrolyzing Agents and Novel Cytotoxic Agents

Work in this laboratory has been involved with the design of aziridinyl quinone-based cancer drugs (PBIs) capable of both recognizing the DNA major groove and cleaving the phosphate backbone upon reduction to the hydroquinone. The hydroquinone species recognizes the major groove of DNA at single base pairs by Hoogsteen-type hydrogen bonding. The present study extends recognition beyond a single base pair by adding amino acid residues to the 3-amino center of the PBI system. Thus, extension with proline or proline-glycine results in phosphate cleavage at 5'-AA-3' with insignificant N(7) guanine alkylation. Molecular models were used to validate the observed sequence specificity. This report also describes the design of PBIs not capable of DNA alkylation. Removal of major groove interactions by methylation or the presence of steric bulk prevented DNA alkylation reactions upon reductive activation. From these studies it was concluded that DNA alkyation was not necessary for PBI cytostatic and cytotoxic activity. For example, linkage of two phenylalanines to the PBI results in highly selective cytostatic and cytotoxic activity against melanoma, although this compound is a weak DNA alkylator.

3-[(123)I]Iodo-alpha-methyl-L-tyrosine: uptake mechanisms and clinical applications

3-[(123)I]Iodo-alpha-methyl-L-tyrosine (IMT) is an artificial amino acid which has gained considerable interest in Nuclear Medicine in the last two decades. Although the tracer is not incorporated into proteins it exhibits high uptake in brain tumors and appears to be a valuable tool especially for the diagnostic evaluation and therapy planning of patients with cerebral gliomas. In this paper the present knowledge of the uptake mechanisms and the clinical applications of IMT are reviewed and the clinical perspectives discussed.

3-[123I]Iodo-L-alpha-methyl tyrosine transport into human fibroblasts and comparison with Ewing's sarcoma cells

The cellular transport systems and the transport kinetics of [123I]IMT uptake into non-malignant extracranial cells were characterized for the first time. Human fibroblasts were chosen as non-malignant extracranial cells as they are found ubiquitous in the body. [123I]IMT is exclusively transported into fibroblasts via the sodium independent system L. An apparent Michaelis constant K(m) = 116.2 +/- 18.9 microM and a maximum transport velocity V(max) = 191.6 +/- 13.9 pmol x (10(6) cells)(-1) x min(-1) were calculated for the sodium-independent transport. These results were compared with those determined in two malignantly transformed extracranial cell lines, the human Ewing's sarcoma cell lines VH-64 and CADO-ES-1.

Initial evaluation of the feasibility of single photon emission tomography with p-[123 I]iodo-L-phenylalanine for routine brain tumour imaging

p-[123I]iodo-L-phenylalanine (IPA) is a recently described radiopharmaceutical which is highly accumulated in gliomas. The present investigation was designed to evaluate the feasibility of single photon emission tomography (SPET) with IPA to image brain tumours under routine clinical conditions. Using a dual- and a triple-headed SPET camera, whole-body kinetic and brain SPET, as well as plasma, urinary and dosimetric analysis were determined in four patients with gliomas after intravenous injection of IPA. Results obtained by IPA SPET were retrospectively compared with histopathology, magnetic resonance imaging and positron emission tomography with [18F]fluorodeoxyglucose. Tumour lesions were clearly demonstrated by IPA SPET at 30 min, 1h and 4.5h post-injection, even in patients with low grade gliomas. In patients with glioblastoma, excellent visualization of the tumour was possible even at 7h p.i., indicative of the high retention of the radiopharmaceutical in cerebral gliomas. Analysis of the radioactivity in plasma and urine attested to the high in vivo stability of IPA. Blood clearance was rapid (> 65% after 10 min) and IPA was excreted predominantly by the kidneys, the urinary radioactivity excretion ranging from 27% at 1h to 54% of injected doses at 5h p.i. The average effective dose for adults was estimated to be 0.0152mSv*MBq(-1), leading to an effective dose of 3.8mSv in a typical brain SPET investigation with 250 MBq IPA. This result strongly suggests that IPA is a potentially valuable brain tumour imaging agent for widespread clinical studies with SPET. Its high specific tumour uptake and retention even in low grade gliomas represent a major advantage compared to presently available SPET radiopharmaceuticals. Moreover, the radiation dose estimates indicate that clinical use of IPA will result in acceptable radiation dose levels in humans.

Clinical value of iodine-123-alpha-methyl-L-tyrosine single-photon emission tomography in the differential diagnosis of recurrent brain tumor in patients pretreated for glioma at follow-up

PURPOSE

To assess the clinical potential of iodine-123-alpha-methyl-L-tyrosine (IMT) and single-photon emission tomography (SPET) in the differential diagnosis of recurrences in patients pretreated for gliomas at follow-up.

PATIENTS AND METHODS

Seventy-eight patients were examined after primary therapy over 36 months. Histopathologic diagnoses of all patients was known at first treatment; magnetic resonance and/or computed tomography examination was performed in addition to IMT-SPET. Cerebral SPET images were acquired 20 minutes after intravenous application of 190 +/- 10 MBq of IMT. SPET images were classified as positive or negative for recurrent tumor visually and by calculating the ratios between tracer accumulation in the lesion and the unaffected contralateral regions of reference using region of interest. Final diagnoses were based on prospective clinicopathologic findings obtained independently of IMT-SPET.

RESULTS

IMT-SPET detected all high-grade recurrent gliomas (grade 4; sensitivity, 100%). A difference could be demonstrated in grade 2 and 3 recurrences (sensitivity, 84% and 92%, respectively). Moreover, benign posttherapeutic lesions (postoperative scars, radiation necrosis) were correctly diagnosed as negative for tumor recurrence. In general, IMT uptake in grade 2 (1.45 +/- 0.24) was significantly lower than that in grades 3 (1.70 +/- 0.41) and 4 (1.88 +/- 0.32). However, it was difficult to evaluate tumor grade only from the IMT accumulation in individual cases.

CONCLUSION

IMT-SPET seems highly useful for detecting and delineating recurrent gliomas and differentiating between benign posttherapeutic lesions and malignant tumor tissue. It may be a valuable clinical tool to diagnose recurrences in patients pretreated for gliomas at follow-up. However, it showed limitations in determining histologic tumor grade.