Correlation of Somatostatin Receptor-2 Expression with Gallium-68-DOTA-TATE Uptake in Neuroblastoma Xenograft Models.

Peptide-receptor imaging and therapy with radiolabeled somatostatin analogs such as 68Ga-DOTA-TATE and 177Lu-DOTA-TATE have become an effective treatment option for SSTR-positive neuroendocrine tumors. The purpose of this study was to evaluate the correlation of somatostatin receptor-2 (SSTR2) expression with 68Ga-DOTA-TATE uptake and 177Lu-DOTA-TATE therapy in neuroblastoma (NB) xenograft models. We demonstrated variable SSTR2 expression profiles in eight NB cell lines. From micro-PET imaging and autoradiography, a higher uptake of 68Ga-DOTA-TATE was observed in SSTR2 high-expressing NB xenografts (CHLA-15) compared to SSTR2 low-expressing NB xenografts (SK-N-BE(2)). Combined autoradiography-immunohistochemistry revealed histological colocalization of SSTR2 and 68Ga-DOTA-TATE uptake in CHLA-15 tumors. With a low dose of 177Lu-DOTA-TATE (20 MBq/animal), tumor growth inhibition was achieved in the CHLA-15 high SSTR2 expressing xenograft model. Although, in vitro, NB cells showed variable expression levels of norepinephrine transporter (NET), a molecular target for 131I-MIBG therapy, low 123I-MIBG uptake was observed in all selected NB xenografts. In conclusion, SSTR2 expression levels are associated with 68Ga-DOTA-TATE uptake and antitumor efficacy of 177Lu-DOTA-TATE. 68Ga-DOTA-TATE PET is superior to 123I-MIBG SPECT imaging in detecting NB tumors in our model. Radiolabeled DOTA-TATE can be used as an agent for NB tumor imaging to potentially discriminate tumors eligible for 177Lu-DOTA-TATE therapy.

Synthesis and characterization of rhenium and technetium-99m labeled insulin.

A (99m)Tc-labeled insulin analogue was synthesized through a direct labeling method in which the [(99m)Tc(CO)(3)](+) core was combined with a protected insulin derivative (9) bearing a M(I) chelate linked to the first amino acid of the B-chain (B1). Regioselective labeling was achieved by careful control over the pH and the reaction time. Following a TFA-anisole mediated deprotection step (decay-corrected yield of 30 +/- 11%, n = 4), the identity of the final (99m)Tc-labeled product was confirmed by HPLC. Displacement of (125)I-insulin from the insulin receptor (IR) by the Re analogue 6 was similar to that of native insulin (17.8 nM vs 11.7 nM, respectively). The extent of autophosphorylation and Akt activation, as indicated by production of phospho-Akt (pAkt), showed no statistical difference between 6 and native insulin in both assays. These results support the use of the reported (99m)Tc-insulin derivative as a tracer for studying insulin biochemistry in vivo.

Expedient multi-step synthesis of organometallic complexes of Tc and Re in high effective specific activity. A new platform for the production of molecular imaging and therapy agents.

For over thirty years, instant labeling kits which involve no purification steps have been the only method used to prepare (99m)Tc radiopharmaceuticals for clinical studies. To address the limitations imposed by instant kits, which is hindering the development of molecularly targeted Tc- and Re-based imaging and therapy agents, a new strategy for the rapid multistep synthesis and purification of organometallic technetium-based molecular probes and corresponding rhenium-based therapeutic analogues was developed. Beginning with MO4(-) (M = (99m)Tc, (186/188)Re), the carbonyl precursor [M(CO)3(H2O)3](+) was synthesized in 3 min in quantitative yield in a microwave reactor. A dipicolyl ligand was added and the chelate complex was formed in high yield in 2 min using microwave heating at 150 degrees C. This was followed by a new purification strategy to remove unlabeled ligand which entailed using a copper resin/C18 solid phase extraction protocol giving the desired product in greater than 78% decay corrected yield (dcy). Conversion to the corresponding succinimidyl active ester was achieved following a 5 min microwave irradiation at 120 degrees C and C18 solid phase extraction purification in 60% dcy. A series of amides were prepared subsequently by microwave heating at 120 degrees C for 5 min producing the desired targets in greater than 86% dcy. The reported method represents a move away from traditional instant kits toward more versatile platform synthesis and purification technologies that are better suited for producing modern molecular imaging and therapy agents.

Synthesis and screening of mono- and di-aryl technetium and rhenium metallocarboranes. A new class of probes for the estrogen receptor.

A series of mono and diaryl rhenium(I)-carborane derivatives were prepared using microwave heating and screened for their affinity for two isoforms of the estrogen receptor (ER). The rhenacarborane derivative [(RR'C 2B9H9)Re(CO)3](-) (R = p-PhOH, R' = H), which was generated by taking advantage of a recently discovered cage isomerization process, and the neutral nitrosated analogue [(RR'C2B9H9)Re(CO)2(NO)] (R = p-PhOH, R' = H) showed the highest affinities of the compounds screened. As a result, the (99m)Tc analogue of one of the leads was produced in high yield (84%) and specific activity in a manner that is suitable for routine production in support of future preclinical and molecular imaging studies.

Assaying small-molecule-receptor interactions by continuous flow competitive displacement chromatography/mass spectrometry.

Small-molecule screening techniques that employ mass spectrometry detection have been highly successful. However, the inability of conventional techniques, such as frontal affinity chromatography-mass spectrometry (FAC-MS), to easily identify weak binding molecules (i.e., Kd >or= 1 microM) using small amounts of target protein (subpicomole levels) represents a significant impediment to the widespread use of the method in the routine screening of low-abundance membrane receptors. This limitation is particularly notable in the early stages of the drug discovery process, as weak binding molecules can serve as useful leads for targets with no known ligand or when existing tight binding ligands have little therapeutic value. Competitive assay methods involving the displacement of an indicator ligand offer a more sensitive alternative, as the ability to generate an appreciable signal through various methods, including transient overconcentrations of indicator compounds, provides an unambiguous means for identifying weak affinity ligands. In this work we describe a continuous flow competitive assay based on the principles of FAC-MS that can be widely used to identify and characterize weak affinity ligands using low levels of the nicotinic acetylcholine receptor from Torpedo californica (nAChR). The validity of the assay is shown through the ability to identify nicotine (Kd approximately 1 microM) with columns containing <2 pmol of binding sites. Multiple injections of nicotine on a single column produce reproducible peaks in the signal of the indicator compound, epibatidine (Kd approximately 2 nM) showing minimal degradation in signal intensity between trials. The intensity of the peaks is dependent on the concentration of nicotine being injected, and binding curves can be generated through multiple injections on the same column. We investigate and optimize various parameters, including assay speed and concentrations, and demonstrate an automated assay format with the potential for use as a high-throughput screening tool. The ability to screen for weak binders of more pharmacologically relevant membrane receptors in a high-throughput screening format is discussed.

Immobilized enzyme reactor chromatography: optimization of protein retention and enzyme activity in monolithic silica stationary phases.

Our group recently reported on the application of protein-doped monolithic silica columns for immobilized enzyme reactor chromatography, which allowed screening of enzyme inhibitors present in mixtures using mass spectrometry for detection. The enzyme was immobilized by entrapment within a bimodal meso/macroporous silica material prepared by a biocompatible sol-gel processing route. While such columns proved to be useful for applications such as screening of protein-ligand interactions, significant amounts of entrapped proteins leached from the columns owing to the high proportion of macropores within the materials. Herein, we describe a detailed study of factors affecting the morphology of protein-doped bioaffinity columns and demonstrate that specific pH values and concentrations of poly(ethylene glycol) can be used to prepare essentially mesoporous columns that retain over 80% of initially loaded enzyme in an active and accessible form and yet still retain sufficient porosity to allow pressure-driven flow in the low muL/min range. Using the enzyme gamma-glutamyl transpeptidase (gamma-GT), we further evaluated the catalytic constants of the enzyme entrapped in capillary columns with different silica morphologies as a function of flowrate and backpressure using the enzyme reactor assay mode. It was found that the apparent activity of the enzyme was highest in mesoporous columns that retained high levels of enzyme. In such columns, enzyme activity increased by approximately 2-fold with increases in both flowrate (from 250 to 1000 nL/min) and backpressure generated (from 500 to 2100 psi) during the chromatographic activity assay owing to increases in k(cat) and decreases in K(M), switching from diffusion controlled to reaction controlled conditions at ca. 2000 psi. These results suggest that columns with minimal macropore volumes (<5%) are advantageous for the entrapment of soluble proteins for bioaffinity and bioreactor chromatography.

Inhibitor screening using immobilized enzyme reactor chromatography/mass spectrometry.

We describe the coupling of capillary-scale monolithic enzyme reactor columns directly to a tandem mass spectrometer for screening of enzyme inhibitors. A two-channel nanoLC system is used to continuously infuse substrate or substrate/inhibitor mixtures through the column, allowing continuous variation of inhibitor concentration by simply altering the ratio of flow from the two pumps. In the absence of inhibitor, infusion of substrate leads to formation of product, and both substrate and product ions can be simultaneously monitored in a quantitative manner by MS/MS. The presence of inhibitor leads to a decrease in product and an increase in substrate concentration in the column eluent. Knowing the product/substrate ratio and the total analyte concentration (P + S), the concentration of product eluting, and hence the relative enzyme activity, can be determined. Both IC50 and KI values can then be obtained by direct MS detection of the effect of inhibitors on relative activity. Inhibitor screening is demonstrated using reusable, sol-gel derived, monolithic capillary columns containing adenosine deaminase, directly interfaced to ESI-MS/MS. On-column enzyme activity was assessed by monitoring inosine and adenosine elution. It is shown that the method can be used for automated screening of the effects of compound mixtures on ADA activity and to determine the KI value of the known inhibitor, erythro-9-(2-hydroxy-3-nonyl)adenine, even when the compound is present within a mixture.

Entrapment of highly active membrane-bound receptors in macroporous Sol-Gel derived silica.

The immobilization of membrane-associated proteins remains a challenging task. Herein, we report on the entrapment of two classes of membrane-bound receptors into sol-gel derived silica. Both nicotinic acetylcholine receptor (nAChR), a ligand-gated ion channel, and dopamine D(2Short) receptor (D2R), a G-protein coupled receptor, were entrapped into a series of sol-gel derived nanocomposite materials. In cases where the silica had a bimodal pore size distribution wherein both mesopores and macropores were present, the two receptors showed 40-80% of solution activity over periods of at least 1 month. Furthermore, the dissociation constants of entrapped nAChR and D2R for binding to known agonists and antagonists were very close to the values obtained for free receptors in solution. These results indicate that membrane-bound receptors entrapped into bimodal meso/macroporous silica should provide a useful platform for the development of bioanalytical devices such as bioaffinity columns or microarrays, which could aid in diagnosis and high-throughput drug screening.

Bridging the gap between in vitro and in vivo imaging: isostructural Re and 99mTc complexes for correlating fluorescence and radioimaging studies.

A bifunctional ligand that is capable of forming Re and 99mTc complexes as complementary fluorescent and radioactive probes was developed. The tridentate bis(quinoline) amine ligand, which is referred to as the SAACQ system, was prepared in a single step from Fmoc protected lysine in high yield. Reaction of the SAACQ ligand with [Re(CO)3Br3]2- resulted in the formation of the SAACQ-(Re(CO)3)+complex which exhibits favorable fluorescence properties including a long lifetime and a large Stoke's shift. Because the SAACQ ligand is derived from an amino acid, it can readily be linked to or incorporated within peptides as a means of targeting the probe to specific receptors. To demonstrate this feature, the SAACQ ligand and the SAACQ-Re complex were incorporated into fMLFG, a peptide that binds to the formyl peptide receptor (FPR). Uptake of the fMLF[(SAACQ-Re(CO)3)+]G conjugate into human leukocytes in vitro was visualized by fluorescence microscopy, and the observed distribution of the peptide was similar to that of a well-established fluorescent FPR probe. The corresponding Tc complex, fMLF[(SAACQ-99mTc(CO)3)+]G, was prepared in excellent yield from [99mTc(CO)3(OH2)3]+, which affords the opportunity to correlate the results of the microscopy experiments with in vivo radioimaging studies because the probes are isostructural.

Screening of inhibitors using enzymes entrapped in sol-gel-derived materials.

In recent years, a number of new methods have been reported that make use of immobilized enzymes either on microarrays or in bioaffinity columns for high-throughput screening of compound libraries. A key question that arises in such methods is whether immobilization may alter the intrinsic catalytic and inhibition constants of the enzyme. Herein, we examine how immobilization within sol-gel-derived materials affects the catalytic constant (kcat), Michaelis constant (KM), and inhibition constant (KI) of the clinically relevant enzymes Factor Xa, dihydrofolate reductase, cyclooxygenase-2, and gamma-glutamyl transpeptidase. These enzymes were encapsulated into sol-gel-derived glasses produced from either tetraethyl orthosilicate (TEOS) or the newly developed silica precursor diglyceryl silane (DGS). It was found that the catalytic efficiency and long-term stability of all enzymes were improved upon entrapment into DGS-derived materials relative to entrapment in TEOS-based glasses, likely owing to the liberation of the biocompatible reagent glycerol from DGS. The KM values of enzymes entrapped in DGS-derived materials were typically higher than those in solution, whereas upon entrapment, kcat values were generally lowered by a factor of 1.5-7 relative to the value in solution, indicating that substrate turnover was limited by partitioning effects or diffusion through the silica matrix. Nonetheless, the apparent KI value for the entrapped enzyme was in most cases within error of the value in solution, and even in the worst case, the values differed by no more than a factor of 3. The implications of these findings for high-throughput screening are discussed.

Ion sensing and inhibition studies using the transmembrane ion channel peptide gramicidin A entrapped in sol-gel-derived silica.

The development of new, targeted drugs relies heavily on innovative technologies that allow for high-throughput screening of drug libraries against biologically relevant targets, particularly membrane-associated receptors. Therefore, immobilization of natural receptors is of the utmost importance to allow for screening of small molecule libraries. Herein, we describe the immobilization of liposomes containing the transmembrane peptide ion-channel gramicidin A into sol-gel-derived silicate materials. Steady-state fluorescence measurements of the intrinsic tryptophan residues of reconstituted gramicidin A in phospholipid vesicles consisting of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) were obtained in solution and following entrapment in diglyceryl silane (DGS)-derived silicate to examine the effects of entrapment on the conformation of the ion channel. Only minor deviations were observed in the fluorescence properties of gramicidin following entrapment in DGS-derived silicate. DOPC vesicles containing a 50 microM internal solution of the potential sensitive fluorescent dye safranine O were used to study ion flux through the membrane ion channel. The dependence of ion flux on both ion concentration and amount of gramicidin embedded in the membrane were examined before and after entrapment in sol-gel-derived silicate. It was found that ion channel activity upon entrapment in DGS-derived silicate mirrored very closely that observed in solution. Moreover, the ability to inhibit ion flux through gramicidin A due to blockage by calcium ions was retained after the immobilization procedure. The implications for development of drug-screening and -sensing platforms are discussed.