A New Route to Guanidines from Bromoalkanes

A DFT Investigation of the Reactivity of Guanidinium Salts in Tandem aza-Michael Addition/Intramolecular Cyclization

A proposed mechanism of the reaction of guanidinium chlorides with dimethyl acetylenedicarboxylate in a tandem aza-Michael addition reaction/intramolecular cyclization was investigated by DFT M06-2X and B3LYP computational approaches. The energies of the products were compared against the G3, M08-HX, M11, and wB97xD data or experimentally obtained product ratios. The structural diversity of the products was interpreted by the concurrent formation of different tautomers formed in situ upon deprotonation with a 2-chlorofumarate anion. A comparison of relative energies of the characteristic stationary points along the examined reaction paths indicated that the initial nucleophilic addition is energetically the most demanding process. The overall reaction is strongly exergonic, as predicted by both methods, which is primarily due to methanol elimination during the intramolecular cyclization step producing cyclic amide structures. Formation of a five-membered ring upon intramolecular cyclization is highly favored for the acyclic guanidine, while optimal product structure for the cyclic guanidines is based on a 1,5,7-triaza [4.3.0]-bicyclononane skeleton. Relative stabilities of the possible products calculated by the employed DFT methods were compared against the experimental product ratio. The best agreement was obtained for the M08-HX approach while the B3LYP approach provided somewhat better results than the M06-2X and M11 methods.

Guanidine cyclic diimides and their polymers

We report the formation and degradation of a unique guanidine cyclic diimide (GCDI) structure under mild conditions. Furthermore, the GCDI-based polymers can be readily synthesized from guanidine and dianhydride monomers.

N-Succinimidyl 3-((4-(4-[(18)F]fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(guanidinomethyl)benzoate ([(18)F]SFBTMGMB): a residualizing label for (18)F-labeling of internalizing biomolecules

Residualizing labeling methods for internalizing peptides and proteins are designed to trap the radionuclide inside the cell after intracellular degradation of the biomolecule. The goal of this work was to develop a residualizing label for the (18)F-labeling of internalizing biomolecules based on a template used successfully for radioiodination. N-Succinimidyl 3-((4-(4-[(18)F]fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(bis-Boc-guanidinomethyl)benzoate ([(18)F]SFBTMGMB-Boc2) was synthesized by a click reaction of an azide precursor and [(18)F]fluorohexyne in 8.5 ± 2.8% average decay-corrected radiochemical yield (n = 15). An anti-HER2 nanobody 5F7 was labeled with (18)F using [(18)F]SFBTMGMB ([(18)F]RL-I), obtained by the deprotection of [(18)F]SFBTMGMB-Boc2, in 31.2 ± 6.7% (n = 5) conjugation efficiency. The labeled nanobody had a radiochemical purity of >95%, bound to HER2-expressing BT474M1 breast cancer cells with an affinity of 4.7 ± 0.9 nM, and had an immunoreactive fraction of 62-80%. In summary, a novel residualizing prosthetic agent for labeling biomolecules with (18)F has been developed. An anti-HER2 nanobody was labeled using this prosthetic group with retention of affinity and immunoreactivity to HER2.

A practical, automated synthesis of meta-[(18)F]fluorobenzylguanidine for clinical use

Many neuroendocrine tumors, such as neuroblastoma (NB), arise from neural crest cells of the sympathetic nervous system. This nerve-like phenotype has been exploited for functional imaging using radioactive probes originally designed for neuronal and adrenal medullary applications. NB imaging with meta-[(123)I]iodobenzylguanidine ([(123)I]MIBG) is limited by the emissions of (123)I, which lead to poor image resolution and challenges in quantification of its accumulation in tumors. meta-[(18)F]Fluorobenzylguanidine ([(18)F]MFBG) is a promising alternative to [(123)I]MIBG that could change the standard of practice for imaging neuroendocrine tumors, but interest in this PET radiotracer has suffered due to its complex and inefficient radiosynthesis. Here we report a two-step, automated method for the routine production of [(18)F]MFBG by thermolysis of a diaryliodonium fluoride and subsequent acid deprotection. The synthesis was adapted for use on a commercially available synthesizer for routine production. Full characterization of [(18)F]MFBG produced by this route demonstrated the tracer's suitability for human use. [(18)F]MFBG was prepared in almost 3-fold higher yield than previously reported (31% corrected to end of bombardment, n = 9) in a synthesis time of 56 min with >99.9% radiochemical purity. Other than pH adjustment and dilution of the final product, no reformulation was necessary after purification. This method permits the automated production of multidose batches of clinical grade [(18)F]MFBG. Moreover, if ongoing clinical imaging trials of [(18)F]MFBG are successful, this methodology is suitable for rapid commercialization and can be easily adapted for use on most commercial automated radiosynthesis equipment.

Synthesis and evaluation of 4-[18F]fluoropropoxy-3-iodobenzylguanidine ([18F]FPOIBG): A novel 18F-labeled analogue of MIBG

INTRODUCTION

Radioiodinated meta-iodobenzylguanidine (MIBG), a norepinephrine transporter (NET) substrate, has been extensively used as an imaging agent to study the pathophysiology of the heart and for the diagnosis and treatment of neuroendocrine tumors. The goal of this study was to develop an (18)F-labeled analogue of MIBG that like MIBG itself could be synthesized in a single radiochemical step. Towards this end, we designed 4-fluoropropoxy-3-iodobenzylguanidine (FPOIBG).

METHODS

Standards of FPOIBG and 4-fluoropropoxy-3-bromobenzylguanidine (FPOBBG) as well as their tosylate precursors for labeling with (18)F, and a tin precursor for the preparation of radioiodinated FPOIBG were synthesized. Radiolabeled derivatives were synthesized by nucleophilic substitution and electrophilic iododestannylation from the corresponding precursors. Labeled compounds were evaluated for NET transporter recognition in in vitro assays using three NET-expressing cell lines and in biodistribution experiments in normal mice, with all studies performed in a paired-label format. Competitive inhibition of [(125)I]MIBG uptake by unlabeled benzylguanidine compounds was performed in UVW-NAT cell line to determine IC50 values.

RESULTS

[(18)F]FPOIBG was synthesized from the corresponding tosylate precursor in 5.2 ± 0.5% (n = 6) overall radiochemical yields starting with aqueous fluoride in about 105 min. In a paired-label in vitro assay, the uptake of [(18)F]FPOIBG at 2h was 10.2 ± 1.5%, 39.6 ± 13.4%, and 13.3 ± 2.5%, in NET-expressing SK-N-SH, UVW-NAT, and SK-N-BE(2c) cells, respectively, while these values for [(125)I]MIBG were 57.3 ± 8.1%, 82.7 ± 8.9%, and 66.3 ± 3.6%. The specificity of uptake of both tracers was demonstrated by blocking with desipramine. The (125)I-labeled congener of FPOIBG gave similar results. On the other hand, [(18)F]FPOBBG, a compound recently reported in the literature, demonstrated much higher uptake, albeit less than that of co-incubated [(125)I]MIBG. IC50 values for FPOIBG were higher than those obtained for MIBG and FPOBBG. Unlike the case with [(18)F]FPOBBG, the heart uptake [(18)F]FPOIBG in normal mice was significantly lower than that of MIBG.

CONCLUSION

Although [(18)F]FPOIBG does not appear to warrant further consideration as an (18)F-labeled MIBG analogue, analogues wherein the iodine in it is replaced with a chlorine, fluorine or hydrogen might be worth pursuing.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

An (18)F-labeled analogue of the well-known radiopharmaceutical MIBG could have significant impact, potentially improving imaging of NET related disease in cardiology and in the imaging of neuroendocrine tumors. Although (18)F-labeled analogues of MIBG have been reported including LMI1195, we undertook this work hypothesizing that based on its greater structural similarity to MIBG, FPOIBG might be a better analogue than LMI1195.

N-Succinimidyl guanidinomethyl iodobenzoate protein radiohalogenation agents: influence of isomeric substitution on radiolabeling and target cell residualization

INTRODUCTION

N-succinimidyl 4-guanidinomethyl-3-[(*)I]iodobenzoate ([(*)I]SGMIB) has shown promise for the radioiodination of monoclonal antibodies (mAbs) and other proteins that undergo extensive internalization after receptor binding, enhancing tumor targeting compared to direct electrophilic radioiodination. However, radiochemical yields for [(131)I]SGMIB synthesis are low, which we hypothesize is due to steric hindrance from the Boc-protected guanidinomethyl group ortho to the tin moiety. To overcome this, we developed the isomeric compound, N-succinimidyl 3-guanidinomethyl-5-[(131)I]iodobenzoate (iso-[(131)I]SGMIB) wherein this bulky group was moved from ortho to meta position.

METHODS

Boc2-iso-SGMIB standard and its tin precursor, N-succinimidyl 3-((1,2-bis(tert-butoxycarbonyl)guanidino)methyl)-5-(trimethylstannyl)benzoate (Boc2-iso-SGMTB), were synthesized using two disparate routes, and iso-[*I]SGMIB synthesized from the tin precursor. Two HER2-targeted vectors - trastuzumab (Tras) and a nanobody 5F7 (Nb) - were labeled using iso-[(*)I]SGMIB and [(*)I]SGMIB. Paired-label internalization assays in vitro with both proteins, and biodistribution in vivo with trastuzumab, labeled using the two isomeric prosthetic agents were performed.

RESULTS

When the reactions were performed under identical conditions, radioiodination yields for the synthesis of Boc2-iso-[(131)I]SGMIB were significantly higher than those for Boc2-[(131)I]SGMIB (70.7±2.0% vs 56.5±5.5%). With both Nb and trastuzumab, conjugation efficiency also was higher with iso-[(131)I]SGMIB than with [(131)I]SGMIB (Nb, 33.1±7.1% vs 28.9±13.0%; Tras, 45.1±4.5% vs 34.8±10.3%); however, the differences were not statistically significant. Internalization assays performed on BT474 cells with 5F7 Nb indicated similar residualizing capacity over 6h; however, at 24h, radioactivity retained intracellularly for iso-[(131)I]SGMIB-Nb was lower than for [(125)I]SGMIB-Nb (46.4±1.3% vs 56.5±2.5%); similar results were obtained using Tras. Likewise, a paired-label biodistribution of Tras labeled using iso-[(125)I]SGMIB and [(131)I]SGMIB indicated an up to 22% tumor uptake advantage at later time points for [(131)I]SGMIB-Tras.

CONCLUSION

Given the higher labeling efficiency obtained with iso-SGMIB, this residualizing agent might be of value for use with shorter half-life radiohalogens.

In silico design, synthesis, and screening of novel deoxyhypusine synthase inhibitors targeting HIV-1 replication

The human enzyme deoxyhypusine synthase (DHS) is an important host cell factor that participates in the post-translational hypusine modification of eukaryotic initiation factor 5A (eIF-5A). Hypusine-modified eIF-5A plays a role in a number of diseases, including HIV infection/AIDS. Thus, DHS represents a novel and attractive drug target. So far, four crystal structures are available, and various substances have been tested for inhibition of human DHS. Among these inhibitors, N-1-guanyl-1,7-diaminoheptane (GC7) has been co-crystallized in the active site of DHS. However, despite its potency, GC7 is not selective enough to be used in drug applications. Therefore, new compounds that target DHS are needed. Herein we report the in silico design, chemical synthesis, and biological evaluation of new DHS inhibitors. One of these inhibitors showed dose-dependent inhibition of DHS in vitro, as well as suppression of HIV replication in cell cultures. Furthermore, the compound exhibited no cytotoxic effects at active concentrations. Thus, this designed compound demonstrated proof of principle and represents a promising starting point for the development of new drug candidates to specifically interfere with DHS activity.

N-substituted 2-aminoimidazole inhibitors of MRSA biofilm formation accessed through direct 1,3-bis(tert-butoxycarbonyl)guanidine cyclization

Antibiotic resistance is a significant problem and is compounded by the ability of many pathogenic bacteria to form biofilms. A library of N-substituted derivatives of a previously reported 2-aminoimidazole/triazole (2-AIT) biofilm modulator was constructed via α-bromoketone cyclization with 1,3-bis(tert-butoxycarbonyl)guanidine, followed by selective substitution. Several compounds exhibited the ability to inhibit biofilm formation by three strong biofilm forming strains of methicillin resistant Staphylococcus aureus (MRSA). Additionally, a number of members of this library exhibited synergistic activity with oxacillin against planktonic MRSA. Compounds with this type of dual activity have the potential to be used as adjuvants with conventional antibiotics.

Meta-iodobenzylguanidine derivatives containing a second guanidine moiety

Radioiodinated meta-iodobenzylguanidine (MIBG) is used in the diagnosis and therapy of various neuroendocrine tumors. To investigate whether an additional guanidine function in the structure of MIBG will yield analogues that may potentially enhance tumor-to-target ratios, two derivatives-one with a guanidine moiety and another with a guanidinomethyl group at the 4-position of MIBG-were prepared. In the absence of any uptake-1 inhibiting conditions, the uptake of 4-guanidinomethyl-3-[(131)I]iodobenzylguanidine ([(131)I]GMIBG) by SK-N-SH cells in vitro was 1.7+/-0.1% of input counts, compared to a value of 40.3+/-1.4% for [(125)I[MIBG suggesting that guanidinomethyl group at the 4-position negated the biological properties of MIBG. On the other hand, 4-guanidino-3-[(131)I]iodobenzylguanidine ([(131)I]GIBG) had an uptake (5.6+/-0.3%) that was 12-13% that of [(125)I]MIBG (46.1+/-2.7%), and the ratio of uptake by control over DMI-treated (nonspecific) cultures was higher for [(131)I]GIBG (20.9+/-0.3) than [(125)I]MIBG itself (15.0+/-2.7). The exocytosis of [(131)I]GIBG and [(125)I]MIBG from SK-N-SH cells was similar. The uptake of [(131)I]GIBG in the mouse target tissues, heart and adrenals, as well as in a number of other tissues was about half that of [(125)I]MIBG. These results suggest that substitution of guanidine functions, especially a guanidinomethyl group, in MIBG structure may not be advantageous.

Phase-transfer-catalyzed alkylation of guanidines by alkyl halides under biphasic conditions: a convenient protocol for the synthesis of highly functionalized guanidines

An operationally straightforward and efficient method for the alkylation of carbamate-protected guanidines with various alkyl halides and mesylates is described. This protocol proceeds via deprotonation of the acidic N-carbamate hydrogen of the guanidine under biphasic conditions using a catalytic amount of a tetrabutylammonium salt as a phase-transfer catalyst. In this manner, highly functionalized guanidines can be obtained. The reaction is tolerant of a wide range of functional groups on both the alkyl halide and guanidine component. In addition, the reaction is sufficiently mild such that simple aqueous workup and filtration through a short silica gel column yields the substituted guanidines in high purity. In conjunction with the EDCI-mediated guanylation of disubstituted thioureas with amines, phase-transfer catalyzed alkylation of guanidines via a one-pot, three-component synthesis of substituted guanidines was achieved.

TFA-sensitive arylsulfonylthiourea-assisted synthesis of N,N'-substituted guanidines

An efficient synthesis of N,N'-substituted guanidine derivatives was developed via an aromatic sulfonyl-activated thiourea intermediate. The use of certain aromatic sulfonamides, such as PbfNH(2), as the key reagent to incorporate a TFA-labile guanidine protection group greatly facilitates solid-phase synthesis of N,N'-substituted guanidine compounds.

Synthesis of ring- and side-chain-substituted m-iodobenzylguanidine analogues

With the goal of developing MIBG analogues with improved targeting properties especially for oncologic applications, several radioiodinated ring- and side-chain-substituted MIBG analogues were synthesized. Except for 3-[(131)I]iodo-4-nitrobenzylguanidine and N-hydroxy-3-[(131)I]iodobenzylguanidine, the radioiodinated analogues were prepared at no-carrier-added levels from their respective tin precursors. The radiochemical yields generally were in the range of 70-90% except for 3-amino-5-[(131)I]iodobenzylguanidine for which a radiochemical yield of about 40% was obtained. While the silicon precursor N(1),N(2)-bis(tert-butyloxycarbonyl)-N(1)-(4-nitro-3-trimethylsilylbenzyl)guanidine did not yield 3-[(131)I]iodo-4-nitrobenzylguanidine, its deprotected derivative, N(1)-(4-nitro-3-trimethylsilylbenzyl)guanidine was radioiodinated in a modest yield of 20% providing 3-[(131)I]iodo-4-nitrobenzylguanidine. Exchange radioiodination of 3-iodo-4-nitrobenzylguanidine gave 3-[(131)I]iodo-4-nitrobenzylguanidine in 80% radiochemical yield. No-carrier-added [(131)I]NHIBG was prepared from its silicon precursor N(1)-hydroxy-N(3)-(3-trimethylsilylbenzyl)guanidine in 85% radiochemical yield.

A Polar Substituent-Containing Acylation Agent for the Radioiodination of Internalizing Monoclonal Antibodies: N-Succinimidyl 4-Guanidinomethyl-3-[131I]iodobenzoate ([131I]SGMIB)

The objective of this study was to develop an acylation agent for the radioiodination of monoclonal antibodies that would maximize retention of the label in tumor cells following receptor- or antigen-mediated internalization. The strategy taken was to add a polar substituent to the labeled aromatic ring to impede transport of labeled catabolites across lysosomal and cell membranes after antibody degradation. Preparation of unlabeled N-succinimidyl 4-guanidinomethyl-3-iodobenzoate (SGMIB) was achieved in six steps from 3-iodo-4-methylbenzoic acid. Preparation of 4-guanidinomethyl-3-[131I]iodobenzoic acid from the silicon precursor, 4-(N1,N2-bis-tert-butyloxycarbonyl)guanidinomethyl-3-trimethylsilylbenzoic acid proceeded in less than 5% radiochemical yield. A more successful approach was to prepare [131I]SGMIB directly from the tin precursor, N-succinimidyl 4-(N1,N2-bis-tert-butyloxycarbonyl)guanidinomethyl-3-trimethylstannylbenzoate, which was achieved in 60-65% radiochemical yield. A rapidly internalizing anti-epidermal growth factor receptor variant III antibody L8A4 was labeled using [131I]SGMIB in 65% conjugation efficiency and with preservation of immunoreactivity. Paired-label in vitro internalization assays demonstrated that the amount of radioactivity retained in cells after internalization for L8A4 labeled with [131I]SGMIB was 3-4-fold higher than that for L8A4 labeled with 125I using either Iodogen or [125I]SIPC. Catabolite assays documented that the increased retention of radioiodine in tumor cells for antibody labeled using [131I]SGMIB was due to positively charged, low molecular weight species. These results suggest that [131I]SGMIB warrants further evaluation as a reagent for labeling internalizing antibodies.

Radiolabeled guanine derivatives for the in vivo mapping of O(6)-alkylguanine-DNA alkyltransferase: 6-(4-[(18)F]Fluoro-benzyloxy)-9H-purin-2-ylamine and 6-(3-[(131)I]Iodo-benzyloxy)-9H-purin-2-ylamine

Two radiolabeled analogues of 6-benzyloxy-9H-purin-2-ylamine (O(6)-benzylguanine; BG) potentially useful in the in vivo mapping of O(6)-alkylguanine-DNA alkyltransferase (AGT) were synthesized. Fluorine-18 labeling of the known 6-(4-fluoro-benzyloxy)-9H-purin-2-ylamine (FBG; 6) was accomplished by the condensation of 4-[(18)F]fluorobenzyl alcohol with 2-aminopurin-6-yltrimethylammonium chloride (4) or 2-amino-6-chloropurine in average decay-corrected radiochemical yields of 40 and 25%, respectively. Unlabeled 6-(3-iodo-benzyloxy)-9H-purin-2-ylamine (IBG; 7) was prepared from 4 and 3-iodobenzyl alcohol. Radioiodination of 9, prepared from 7 in two steps, and subsequent deprotection gave [(131)I]7 in about 70% overall radiochemical yield. The IC(50) values for the inactivation of AGT from CHO cells transfected with pCMV-AGT were 15 nM for IBG and 50 nM for FBG. The binding of [(18)F]6 and [(131)I]7 to purified AGT was specific and saturable with both exhibiting similar IC(50) values (5-6 microM).