Preparation of99mTc-N2S2 conjugates of chrysamine G, potential probes for the beta-amyloid protein of Alzheimer's disease

Recent progress in the development of metal complexes as β-amyloid imaging probes in the brain

In this review, we have focused on the recent progress in metal complexes that are able to bind to β-amyloid (Aβ) species. We have discussed various radioactive complexes of 99mTc, 68Ga, 64Cu, 89Zr, and 111In, which were designed as Aβ imaging agents for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging, non-radioactive Re and Ru complexes as Aβ sensors using luminescence methods, and Gd3+ complexes as contrast agents for magnetic resonance imaging (MRI).

Synthesis and evaluation of a 99mTc-BAT-phenylbenzothiazole conjugate as a potential in vivo tracer for visualization of amyloid β

We have conjugated S,S'-bis-trityl-N-BOC-N'-acetic acid-1,2-ethylenedicysteamine, a protected bis-amino-bis-thiol (BAT) tetraligand, with 2-(4'-aminophenyl)-1,3-benzothiazole, a derivative of thioflavin-T with known affinity for amyloid. The conjugate was efficiently labelled with (99m)Tc by heating of the protected precursor in diluted hydrochloric acid followed by neutralization and heating in the presence of (99m)Tc-tartrate. It was demonstrated that the (99m)Tc-BAT-phenylbenzothiazole conjugate binds in vitro to amyloid beta present in postmortem brain slices of Alzheimer's patients. Despite its high lipophilicity and neutral character, the radiolabelled conjugate did not cross the blood-brain barrier to a sufficient degree and therefore is not useful for detection of Alzheimer's disease. Further evaluation of this (99m)Tc-labelled tracer agent could elucidate its potential usefulness to visualize amyloid plaques in peripheral amyloidosis.

Dimethylamino-fluorenes: ligands for detecting beta-amyloid plaques in the brain

Formation of beta-amyloid (Abeta) plaques in the brain is a major contributing factor in the pathogenesis of Alzheimer's disease (AD). Detection of Abeta plaques in the brain will be potentially useful in early diagnosis and monitoring the progression of the disease. A series of novel Abeta aggregate-specific ligands based on fluorenes, which are simple and rigid tricyclic molecules, are synthesized and characterized. Starting with 2- or 3-aminofluorenes, 1a-1f, the amino group was converted to the N,N-dimethylamino group (2a-2f) in excellent yield. It was found that 7-iodo-2-N,N-dimethylaminofluorene (2f) showed an extremely high binding affinity to preformed Abeta40 aggregates (K(i) = 0.9 nM). In vitro autoradiography study using brain sections obtained from transgenic mice (Tg2576) with [(125)I]2f showed exquisitely high specific binding to Abeta plaques. The same section also displayed an equivalent labeling when stained by Thioflavin-S, a commonly used fluorescent dye for Abeta plaques. When [(125)I]2f was injected intravenously into normal mice, it exhibited an excellent brain uptake. Taken together the data suggest that [(125)I]2f may be useful as an in vivo imaging agent to detect Abeta plaques in the brain.

Imaging Abeta plaques in living transgenic mice with multiphoton microscopy and methoxy-X04, a systemically administered Congo red derivative

The identification of amyloid deposits in living Alzheimer disease (AD) patients is important for both early diagnosis and for monitoring the efficacy of newly developed anti-amyloid therapies. Methoxy-X04 is a derivative of Congo red and Chrysamine-G that contains no acid groups and is therefore smaller and much more lipophilic than Congo red or Chrysamine-G. Methoxy-X04 retains in vitro binding affinity for amyloid beta (Abeta) fibrils (Ki = 26.8 nM) very similar to that of Chrysamine-G (Ki = 25.3 nM). Methoxy-X04 is fluorescent and stains plaques, tangles, and cerebrovascular amyloid in postmortem sections of AD brain with good specificity. Using multiphoton microscopy to obtain high-resolution (1 microm) fluorescent images from the brains of living PSI/APP mice, individual plaques could be distinguished within 30 to 60 min after a single i.v. injection of 5 to 10 mg/kg methoxy-X04. A single i.p. injection of 10 mg/kg methoxy-X04 also produced high contrast images of plaques and cerebrovascular amyloid in PSI/APP mouse brain. Complementary quantitative studies using tracer doses of carbon- 11-labeled methoxy-X04 show that it enters rat brain in amounts that suggest it is a viable candidate as a positron emission tomography (PET) amyloid-imaging agent for in vivo human studies.

IBOX(2-(4'-dimethylaminophenyl)-6-iodobenzoxazole): a ligand for imaging amyloid plaques in the brain

It is well known that overproduction and accumulation of beta-amyloid (Abeta) plaques in the brain is a key event in the pathogenesis of Alzheimer's disease (AD). Previously it was demonstrated that [125I]TZDM, 2-(4'-dimethylaminophenyl)-6-iodobenzothiazole, a thioflavin derivative, was an effective ligand with good in vitro and in vivo binding characteristics. To further improve the initial uptake and washout rate from the brain, important properties for in vivo imaging agents, a novel radioiodinated ligand, 2-(4'-dimethylaminophenyl)-6-iodobenzoxazole ([125I]IBOX, 3), for detecting Abeta plaques in the brain, was synthesized and evaluated. The new iodinated ligand, IBOX, is based on an isosteric replacement of a sulfur atom of TZDM by an oxygen, by which the molecular weight is reduced while the lipophilicity of the iodinated ligand is increased. Partition coefficients (P.C.) of these two ligands were 70 and 124 for TZDM and IBOX, respectively. In vitro binding study indicated that the isosteric displacement yielded a new ligand with equal binding potency to Abeta(1-40) aggregates (K(i) = 1.9 and 0.8 nM for TZDM and IBOX, respectively). Autoradiography of postmortem brain sections of a confirmed AD patient by [125I]IBOX showed excellent labeling of plaques similar to that observed with [125I]TZDM. More importantly, in vivo biodistribution of [125I]IBOX in normal mice displayed superior peak brain uptake (2.08% at 30 min vs 1.57% at 60 min dose/brain for [125I]IBOX and [125I]TZDM, respectively). In addition, the washout from the brain was much faster for [125I]IBOX as compared to [125I]TZDM. Based on the data presented for [125I]IBOX, it is predicted that the brain trapping of this new radioiodinated ligand in the Abeta containing regions will be more favorable than that of the parent compound, [125I]TZDM. Further evaluation of [125I]IBOX is warranted to confirm the Abeta plaque labeling properties in vivo.

Evaluation of 99mTc-MAMA-chrysamine G as an in vivo probe for amyloidosis

To date, systemic amyloidosis is diagnosed histologically using Congo red staining or in vivo using iodine-123 labelled serum amyloid P component (123I-SAP) scintigraphy. We developed 99mTc-MAMA-CG, a 99mTc-labelled derivative of the lipophilic Congo red analogue chrysamine G (CG), as a possible alternative to 123I-SAP. In vivo 99mTc-MAMA-CG scintigraphy, performed in chickens with spontaneous joint amyloidosis, resulted as soon as 10 min after injection in scintigraphic images showing uptake of activity in amyloid-loaded organs (liver, joints). One of these chickens was studied also with 123I-SAP resulting in scintigraphic images revealing 123I-SAP binding to amyloid deposits in the liver. However, up to 11 h after injection no radioactivity was visible in the amyloid positive joints. In vitro autoradiography, performed on sections of chicken joints with Enterococcus faecalis induced amyloid arthropathy (chjAA), demonstrated the failure of 99mTc-MAMA-CG to bind significantly to amyloid deposits in the presence of 10 microM Congo red The specificity of 99mTc-MAMA-CG localisation was also established by the absence of 99mTc-MAMA-CG binding in non-amyloidotic organs in vitro and in vivo. 99mTc-MAMA-CG did not show any sign of acute toxicity. These findings establish the usefulness of 99mTc-MAMA-CG as a non-invasive in vivo diagnostic probe in chickens with amyloid arthropathy and suggest that it may also be applicable to human amyloidosis.

In vitro affinity of 99Tcm-labelled N2S2 conjugates of chrysamine G for amyloid deposits of systemic amyloidosis

To date, systemic amyloidosis is diagnosed histologically in vitro using Congo red staining or in vivo using iodine-123 serum amyloid P component (123I-SAP) scintigraphy. 99Tcm-labelled derivatives of chrysamine G (CG), a lipophilic analogue of Congo red, were synthesized as potential tracer agents for direct and quantitative scintigraphic evaluation of amyloid deposits. To determine the affinity of 99Tcm-MAMA-CG, 99Tcm-Me4MAMA-CG and 99Tcm-MAMA-CG diethyl ester for amyloid, in vitro autoradiography was performed on sections of human kidney biopsy cylinders from kidneys with amyloid deposits (types AA, Alambda and Akappa) or control kidney tissue after incubation with the respective tracer agents. The binding of 99Tcm-MAMA-CG and its tetramethyl derivative was higher to kidney biopsy material with amyloid deposits of the AA, Alambda or Akappa type compared with control kidney tissue. This higher binding was prevented by the presence of 10 microM Congo red in the incubation medium. The diethyl ester of 9Tcm-MAMA-CG did not demonstrate increased binding to Congo red-positive kidney tissue. In conclusion, 99Tcm-MAMA-CG and 99Tcm-Me4MAMA-CG localize specifically to amyloid deposits in human kidney tissue, suggesting that these tracer agents may be applicable as specific targeting agents for diagnostic purposes in clinical amyloidosis.