A Concise Synthetic Approach to Highly Reactive Click-to-Release Trans-Cyclooctene Linkers

An increase in the click-to-release reaction rate between cleavable trans-cyclooctenes (TCO) and tetrazines would be beneficial for drug delivery applications. In this work, we have developed a short and stereoselective synthesis route towards highly reactive sTCOs that serve as cleavable linkers, affording quantitative tetrazine-triggered payload release. In addition, the fivefold more reactive sTCO exhibited the same in vivo stability as current TCO linkers when used as antibody linkers in circulation in mice.

Click-to-Release: Cleavable Radioimmunoimaging with [89Zr]Zr-DFO-Trans-Cyclooctene-Trastuzumab Increases Tumor-to-Blood Ratio

One of the main challenges of PET imaging with 89Zr-labeled monoclonal antibodies (mAbs) remains the long blood circulation of the radiolabeled mAbs, leading to high background signals, decreasing image quality. To overcome this limitation, here we report the use of a bioorthogonal linker cleavage approach (click-to-release chemistry) to selectively liberate [89Zr]Zr-DFO from trans-cyclooctene-functionalized trastuzumab (TCO-Tmab) in blood, following the administration of a tetrazine compound (trigger) in BT-474 tumor-bearing mice. Methods: We created a series of TCO-DFO constructs and evaluated their performance in [89Zr]Zr-DFO release from Tmab in vitro using different trigger compounds. The in vivo behavior of the best performing [89Zr]Zr-TCO-Tmab was studied in healthy mice first to determine the optimal dose of the trigger. To find the optimal time for the trigger administration, the rate of [89Zr]Zr-TCO-Tmab internalization was studied in BT-474 cancer cells. Finally, the trigger was administered 6 h or 24 h after [89Zr]Zr-TCO-Tmab- administration in tumor-bearing mice to liberate the [89Zr]Zr-DFO fragment. PET scans were obtained of tumor-bearing mice that received the trigger 6 h post-[89Zr]Zr-TCO-Tmab administration. Results: The [89Zr]Zr-TCO-Tmab and trigger pair with the best in vivo properties exhibited 83% release in 50% mouse plasma. In tumor-bearing mice the tumor-blood ratios were markedly increased from 1.0 ± 0.4 to 2.3 ± 0.6 (p = 0.0057) and from 2.5 ± 0.7 to 6.6 ± 0.9 (p < 0.0001) when the trigger was administered at 6 h and 24 h post-mAb, respectively. Same day PET imaging clearly showed uptake in the tumor combined with a strongly reduced background due to the fast clearance of the released [89Zr]Zr-DFO-containing fragment from the circulation through the kidneys. Conclusions: This is the first demonstration of the use of trans-cyclooctene-tetrazine click-to-release chemistry to release a radioactive chelator from a mAb in mice to increase tumor-to-blood ratios. Our results suggest that click-cleavable radioimmunoimaging may allow for substantially shorter intervals in PET imaging with full mAbs, reducing radiation doses and potentially even enabling same day imaging.

Bioorthogonal chemistry

Bioorthogonal chemistry represents a class of high-yielding chemical reactions that proceed rapidly and selectively in biological environments without side reactions towards endogenous functional groups. Rooted in the principles of physical organic chemistry, bioorthogonal reactions are intrinsically selective transformations not commonly found in biology. Key reactions include native chemical ligation and the Staudinger ligation, copper-catalysed azide-alkyne cycloaddition, strain-promoted [3 + 2] reactions, tetrazine ligation, metal-catalysed coupling reactions, oxime and hydrazone ligations as well as photoinducible bioorthogonal reactions. Bioorthogonal chemistry has significant overlap with the broader field of 'click chemistry' - high-yielding reactions that are wide in scope and simple to perform, as recently exemplified by sulfuryl fluoride exchange chemistry. The underlying mechanisms of these transformations and their optimal conditions are described in this Primer, followed by discussion of how bioorthogonal chemistry has become essential to the fields of biomedical imaging, medicinal chemistry, protein synthesis, polymer science, materials science and surface science. The applications of bioorthogonal chemistry are diverse and include genetic code expansion and metabolic engineering, drug target identification, antibody-drug conjugation and drug delivery. This Primer describes standards for reproducibility and data deposition, outlines how current limitations are driving new research directions and discusses new opportunities for applying bioorthogonal chemistry to emerging problems in biology and biomedicine.

Fluorogenic Bifunctional trans-Cyclooctenes as Efficient Tools for Investigating Click-to-Release Kinetics

The inverse electron demand Diels-Alder pyridazine elimination reaction between tetrazines and allylic substituted trans-cyclooctenes (TCOs) is a key player in bioorthogonal bond cleavage reactions. Determining the rate of elimination of alkylamine substrates has so far proven difficult. Here, we report a fluorogenic tool consisting of a TCO-linked EDANS fluorophore and a DABCYL quencher for accurate determination of both the click and release rate constants for any tetrazine at physiologically relevant concentrations.

Trans-Cyclooctene-Functionalized PeptoBrushes with Improved Reaction Kinetics of the Tetrazine Ligation for Pretargeted Nuclear Imaging

Tumor targeting using agents with slow pharmacokinetics represents a major challenge in nuclear imaging and targeted radionuclide therapy as they most often result in low imaging contrast and high radiation dose to healthy tissue. To address this challenge, we developed a polymer-based targeting agent that can be used for pretargeted imaging and thus separates tumor accumulation from the imaging step in time. The developed targeting agent is based on polypeptide-graft-polypeptoid polymers (PeptoBrushes) functionalized with trans-cyclooctene (TCO). The complementary ¹¹¹In-labeled imaging agent is a 1,2,4,5-tetrazine derivative, which can react with aforementioned TCO-modified PeptoBrushes in a rapid bioorthogonal ligation. A high degree of TCO loading (up to 30%) was achieved, without altering the physicochemical properties of the polymeric nanoparticle. The highest degree of TCO loading resulted in significantly increased reaction rates (77-fold enhancement) compared to those with small molecule TCO moieties when using lipophilic tetrazines. Based on computer simulations, we hypothesize that this increase is a result of hydrophobic effects and significant rearrangements within the polymer framework, in which hydrophobic patches of TCO moieties are formed. These patches attract lipophilic tetrazines, leading to increased reaction rates in the bioorthogonal ligation. The most reactive system was evaluated as a targeting agent for pretargeted imaging in tumor-bearing mice. After the setup was optimized, sufficient tumor-to-background ratios were achieved as early as 2 h after administration of the tetrazine imaging agent, which further improved at 22 h, enabling clear visualization of CT-26 tumors. These findings show the potential of PeptoBrushes to be used as a pretargeting agent when an optimized dose of polymer is used.

Evaluation of the inverse electron demand Diels-Alder reaction in rats using a scandium-44-labelled tetrazine for pretargeted PET imaging

BACKGROUND

Pretargeted imaging allows the use of short-lived radionuclides when imaging the accumulation of slow clearing targeting agents such as antibodies. The biotin-(strept)avidin and the bispecific antibody-hapten interactions have been applied in clinical pretargeting studies; unfortunately, these systems led to immunogenic responses in patients. The inverse electron demand Diels-Alder (IEDDA) reaction between a radiolabelled tetrazine (Tz) and a trans-cyclooctene (TCO)-functionalized targeting vector is a promising alternative for clinical pretargeted imaging due to its fast reaction kinetics. This strategy was first applied in nuclear medicine using an ¹¹¹In-labelled Tz to image TCO-functionalized antibodies in tumour-bearing mice. Since then, the IEDDA has been used extensively in pretargeted nuclear imaging and radiotherapy; however, these studies have only been performed in mice. Herein, we report the ⁴⁴Sc labelling of a Tz and evaluate it in pretargeted imaging in Wistar rats.

RESULTS

⁴⁴Sc was obtained from an in house ⁴⁴Ti/⁴⁴Sc generator. A 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-functionalized tetrazine was radiolabelled with ⁴⁴Sc resulting in radiochemical yields of 85-95%, a radiochemical purity > 99% at an apparent molar activity of 1 GBq/mmol. The ⁴⁴Sc-labelled Tz maintained stability in solution for up to 24 h. A TCO-functionalized bisphosphonate, which accumulates in skeletal tissue, was used as a targeting vector to evaluate the ⁴⁴Sc-labelled Tz. Biodistribution data of the ⁴⁴Sc-labelled Tz showed specific uptake (0.9 ± 0.3% ID/g) in the bones (humerus and femur) of rats pre-treated with the TCO-functionalized bisphosphonate. This uptake was not present in rats not receiving pre-treatment (< 0.03% ID/g).

CONCLUSIONS

We have prepared a ⁴⁴Sc-labelled Tz and used it in pretargeted PET imaging with rats treated with TCO-functionalized bisphosponates. This allowed for the evaluation of the IEDDA reaction in animals larger than a typical mouse. Non-target accumulation was low, and there was a 30-fold higher bone uptake in the pre-treated rats compared to the non-treated controls. Given its convenient half-life and the ability to perform positron emission tomography with a previously studied DOTA-functionalized Tz, scandium-44 (t_1/2 = 3.97 h) proved to be a suitable radioisotope for this study.

Tetrazine- trans-Cyclooctene Chemistry Applied to Fabricate Self-Assembled Fluorescent and Radioactive Nanoparticles for in Vivo Dual Mode Imaging

Multimodal imaging agents combine two or more imaging modalities into one probe. Self-assembling fluorescent nanoparticles are a promising class of modular multimodal imaging probes as they can allow easy blending of imaging and targeting modalities. Our group recently developed a class of self-assembling and intrinsically fluorescent small molecule-based nanoparticles (SMNPs) with excellent optical properties. In this article, we describe the efficient radiolabeling of these SMNPs via a two-step bioconjugation strategy involving the inverse-electron-demand Diels-Alder ligation between a tetrazine (Tz)-tagged radiolabel and a trans-cyclooctene (TCO)-tagged fluorescent small molecule building block of the SMNPs. Studies in mice revealed that the SMNPs are well tolerated and could be monitored by both radioactivity and fluorescence, thereby demonstrating the potential of SMNPs in optical and dual-mode imaging in vivo. The work also testifies to the utility of the Tz-TCO conjugation chemistry for the labeling of self-assembled nanoparticles.

Click-to-Release from trans-Cyclooctenes: Mechanistic Insights and Expansion of Scope from Established Carbamate to Remarkable Ether Cleavage

The bioorthogonal cleavage of allylic carbamates from trans-cyclooctene (TCO) upon reaction with tetrazine is widely used to release amines. We disclose herein that this reaction can also cleave TCO esters, carbonates, and surprisingly, ethers. Mechanistic studies demonstrated that the elimination is mainly governed by the formation of the rapidly eliminating 1,4-dihydropyridazine tautomer, and less by the nature of the leaving group. In contrast to the widely used p-aminobenzyloxy linker, which affords cleavage of aromatic but not of aliphatic ethers, the aromatic, benzylic, and aliphatic TCO ethers were cleaved as efficiently as the carbamate, carbonate, and esters. Bioorthogonal ether release was demonstrated by the rapid uncaging of TCO-masked tyrosine in serum, followed by oxidation by tyrosinase. Finally, tyrosine uncaging was used to chemically control cell growth in tyrosine-free medium.

Chemical Control over T-Cell Activation in Vivo Using Deprotection of trans-Cyclooctene-Modified Epitopes

Activation of a cytotoxic T-cell is a complex multistep process, and tools to study the molecular events and their dynamics that result in T-cell activation in situ and in vivo are scarce. Here, we report the design and use of conditional epitopes for time-controlled T-cell activation in vivo. We show that trans-cyclooctene-protected SIINFEKL (with the lysine amine masked) is unable to elicit the T-cell response characteristic for the free SIINFEKL epitope. Epitope uncaging by means of an inverse-electron demand Diels–Alder (IEDDA) event restored T-cell activation and provided temporal control of T-cell proliferation in vivo.

Chemically triggered drug release from an antibody-drug conjugate leads to potent antitumour activity in mice

Current antibody-drug conjugates (ADCs) target internalising receptors on cancer cells leading to intracellular drug release. Typically, only a subset of patients with solid tumours has sufficient expression of such a receptor, while there are suitable non-internalising receptors and stroma targets. Here, we demonstrate potent therapy in murine tumour models using a non-internalising ADC that releases its drugs upon a click reaction with a chemical activator, which is administered in a second step. This was enabled by the development of a diabody-based ADC with a high tumour uptake and very low retention in healthy tissues, allowing systemic administration of the activator 2 days later, leading to efficient and selective activation throughout the tumour. In contrast, the analogous ADC comprising the protease-cleavable linker used in the FDA approved ADC Adcetris is not effective in these tumour models. This first-in-class ADC holds promise for a broader applicability of ADCs across patient populations.

Current antibody-drug conjugates (ADCs) target internalising receptors on cancer cells. Here, the authors report the development and in vivo validation of a non-internalising ADC with the capacity to target cancer cells and release its therapeutic cargo extracellularly via a chemical trigger.

Triggered Drug Release from an Antibody-Drug Conjugate Using Fast "Click-to-Release" Chemistry in Mice

The use of a bioorthogonal reaction for the selective cleavage of tumor-bound antibody-drug conjugates (ADCs) would represent a powerful new tool for ADC therapy, as it would not rely on the currently used intracellular biological activation mechanisms, thereby expanding the scope to noninternalizing cancer targets. Here we report that the recently developed inverse-electron-demand Diels-Alder pyridazine elimination reaction can provoke rapid and self-immolative release of doxorubicin from an ADC in vitro and in tumor-bearing mice.

Diabody Pretargeting with Click Chemistry In Vivo

Radioimmunotherapy and nuclear imaging (immuno-PET/SPECT) of cancer with radiometal-labeled antibody fragments or peptides is hampered by low tumor-to-kidney ratios because of high renal radiometal retention. Therefore, we developed and evaluated a pretargeting strategy using click chemistry in vivo to reduce kidney uptake and avoid unwanted radiation toxicity. We focused on the bioorthogonal reaction between a trans-cyclooctene (TCO)-functionalized TAG72 targeting diabody, AVP04-07, and a low-molecular-weight radiolabeled tetrazine probe that was previously shown to have low kidney retention and relatively fast renal clearance.

METHODS

AVP04-07 diabodies were functionalized with TCO tags, and in vitro immunoreactivity toward bovine submaxillary mucin and tetrazine reactivity were assessed. Next, pretargeting biodistribution studies were performed in LS174T tumor-bearing mice with AVP04-07-TCO(n) (where n indicates the number of TCO groups per diabody) and radiolabeled tetrazine to optimize the TCO modification grade (0, 1.8, or 4.7 TCO groups per diabody) and the (177)Lu-tetrazine dose (0.1, 1.0, or 10 Eq with respect to the diabody). Radiolabeled tetrazine was injected at 47 h after diabody injection, and mice were euthanized 3 h later. A pretargeting SPECT/CT study with (111)In-tetrazine was performed with the optimized conditions.

RESULTS

Immunoreactivity for native AVP04-07 was similar to that for TCO-functionalized AVP04-07, and the latter reacted efficiently with radiolabeled tetrazine in vitro. The combination of the pretargeting component AVP04-07 functionalized with 4.7 TCO groups and 1 Eq of (177)Lu-tetrazine with respect to the diabody showed the most promising biodistribution. Specifically, high (177)Lu-tetrazine tumor uptake (6.9 percentage injected dose/g) was observed with low renal retention, yielding a tumor-to-kidney ratio of 5.7. SPECT/CT imaging confirmed the predominant accumulation of radiolabeled tetrazine in the tumor and low nontumor retention.

CONCLUSION

Pretargeting provides an alternative radioimmunotherapy and nuclear imaging strategy by overcoming the high renal retention of low-molecular-weight radiometal tumor-homing agents through the separate administration of a tumor-homing agent and a radioactive probe with fast clearance.

Bioresponsive probes for molecular imaging: concepts and in vivo applications

Molecular imaging is a powerful tool to visualize and characterize biological processes at the cellular and molecular level in vivo. In most molecular imaging approaches, probes are used to bind to disease-specific biomarkers highlighting disease target sites. In recent years, a new subset of molecular imaging probes, known as bioresponsive molecular probes, has been developed. These probes generally benefit from signal enhancement at the site of interaction with its target. There are mainly two classes of bioresponsive imaging probes. The first class consists of probes that show direct activation of the imaging label (from "off" to "on" state) and have been applied in optical imaging and magnetic resonance imaging (MRI). The other class consists of probes that show specific retention of the imaging label at the site of target interaction and these probes have found application in all different imaging modalities, including photoacoustic imaging and nuclear imaging. In this review, we present a comprehensive overview of bioresponsive imaging probes in order to discuss the various molecular imaging strategies. The focus of the present article is the rationale behind the design of bioresponsive molecular imaging probes and their potential in vivo application for the detection of endogenous molecular targets in pathologies such as cancer and cardiovascular disease.

Trans-cyclooctene tag with improved properties for tumor pretargeting with the diels-alder reaction

Radioimmunotherapy (RIT) of solid tumors is hampered by low tumor-to-nontumor (T/NT) ratios of the radiolabeled monoclonal antibodies resulting in low tumor doses in patients. Pretargeting technologies can improve the effectiveness of RIT in cancer therapy by increasing this ratio. We showed that a pretargeting strategy employing in vivo chemistry in combination with clearing agents, proceeds efficiently in tumor-bearing mice resulting in high T/NT ratios. A dosimetry study indicated that the chemical pretargeting technology, which centered on the bioorthogonal Diels-Alder click reaction between a radiolabeled tetrazine probe and a trans-cyclooctene-oxymethylbenzamide-tagged CC49 antibody (CC49-TCO(1)), can match the performance of clinically validated high-affinity biological pretargeting approaches in mice ( Rossin J Nucl Med. 2013 , 54 , 1989 - 1995 ). Nevertheless, the increased protein surface hydrophobicity of CC49-TCO(1) led to a relatively rapid blood clearance and concomitant reduced tumor uptake compared to native CC49 antibody. Here, we present the in vivo evaluation of a TCO-oxymethylacetamide-tagged CC49 antibody (CC49-TCO(2)), which is highly reactive toward tetrazines and less hydrophobic than CC49-TCO(1). CC49-TCO(2) was administered to healthy mice to determine its blood clearance and the in vivo stability of the TCO. Next, pretargeting biodistribution and SPECT studies with CC49-TCO(2), tetrazine-functionalized clearing agent, and radiolabeled tetrazine were carried out in nude mice bearing colon carcinoma xenografts (LS174T). CC49-TCO(2) had an increased circulation half-life, a 1.5-fold higher tumor uptake, and a 2.6-fold improved in vivo TCO stability compared to the more hydrophobic TCO-benzamide-CC49. As a consequence, and despite the 2-fold lower reactivity of CC49-TCO(2) toward tetrazines compared with CC49-TCO(1), administration of radiolabeled tetrazine afforded a significantly increased tumor accumulation and improved T/NT ratios in mice pretargeted with CC49-TCO(2). In conclusion, the TCO-acetamide derivative represents a large improvement in in vivo Diels-Alder pretargeting, possibly enabling application in larger animals and eventually humans.

In vivo biodistribution of radiolabeled MMP-2/9 activatable cell-penetrating peptide probes in tumor-bearing mice

Matrix metalloproteinases (MMPs) play a pivotal role in cancer progression and present therefore an interesting biomarker for early diagnosis, staging and therapy evaluation. Consequently, MMP-specific molecular imaging probes have been proposed for noninvasive visualization and quantification of MMP activity. An interesting approach is MMP-2/9 activatable cell-penetrating peptides (ACPP) that accumulate in the tumor tissue after activation. However, a recent study revealed that probe activation occurred already in the vasculature followed by nonspecific tumor targeting. In the latter study, biodistribution was determined 6 and 24 h post-ACPP injection. An alternative explanation could still be that the kinetics of tumor-specific activation is faster than that of blood activation plus subsequent nonspecific uptake in tumor. The aim of this study was to assess if tumor-specific ACPP activation occurs in mice with MMP-2/9 positive subcutaneous HT-1080 tumors at 3 h post-injection. As control, we studied the MMP-2/9 sensitive ACPP in mice bearing subcutaneous BT-20 tumors with low MMP-2/9 expression to test if probe cleavage correlates with tumoral MMP expression. Ex vivo biodistribution showed no improved tumoral ACPP activation in HT-1080 tumor-bearing mice at 3 h post-injection compared with previous reported data collected at 24 h post-injection. Furthermore, tumoral uptake and relative tumoral activation for ACPP were similar in both BT-20 and HT-1080 tumor-bearing mice. In conclusion, this study suggests that tumoral ACPP uptake in these tumor models originates from probe activation in the vasculature instead of tumor-specific MMP activation. Novel ACPPs that target tissue-specific proteases without nonspecific activation may unleash the full potential of the elegant ACPP concept.

Diels-Alder reaction for tumor pretargeting: in vivo chemistry can boost tumor radiation dose compared with directly labeled antibody

Current pretargeting systems use noncovalent biologic interactions, which are prone to immunogenicity. We previously developed a novel approach based on the bioorthogonal reaction between a radiolabeled tetrazine and an antibody-conjugated trans-cyclooctene (TCO). However, the tumor-to-blood ratio was low due to reaction with freely circulating antibody-TCO.

METHODS

Here we developed 2 tetrazine-functionalized clearing agents that enable rapid reaction with and removal of a TCO-tagged antibody (CC49) from blood. Next, we incorporated this approach into an optimized pretargeting protocol in LS174T-bearing mice. Then we compared the pretargeted (177)Lu-labeled tetrazine with (177)Lu-labeled CC49. The biodistribution data were used for mouse and human dosimetry calculations.

RESULTS

The use of a clearing agent led to a doubling of the tetrazine tumor uptake and a 125-fold improvement of the tumor-to-blood ratio at 3 h after tetrazine injection. Mouse dosimetry suggested that this should allow for an 8-fold higher tumor dose than is possible with nonpretargeted radioimmunotherapy. Also, humans treated with CC49-TCO-pretargeted (177)Lu-tetrazine would receive a dose to nontarget tissues 1 to 2 orders of magnitude lower than with directly labeled CC49.

CONCLUSION

The in vivo performance of chemical pretargeting falls within the range of results obtained for the clinically validated pretargeting approaches in mice, with the advantage of potentially allowing for fractionated radiotherapy as a result of a lower likelihood of immunogenicity. These findings demonstrate that biologic pretargeting concepts can be translated to rapid bioorthogonal chemical approaches with retained potential.

Highly reactive trans-cyclooctene tags with improved stability for Diels-Alder chemistry in living systems

One of the challenges of pretargeted radioimmunotherapy, which centers on the capture of a radiolabeled probe by a preinjected tumor-bound antibody, is the potential immunogenicity of biological capturing systems. A bioorthogonal chemical approach may circumvent this drawback, but effective in vivo chemistry in mice, larger animals, and eventually humans, requires very high reagent reactivity, sufficient stability, and retained selectivity. We report here that the reactivity of the fastest bioorthogonal reaction, the inverse-electron-demand-Diels-Alder cycloaddition between a tetrazine probe and a trans-cyclooctene-tagged antibody, can be increased 10-fold (k2 = 2.7 × 10(5) M(-1) s(-1)) via the trans-cyclooctene, approaching the speed of biological interactions, while also increasing its stability. This was enabled by the finding that the trans-cyclooctene tag is probably deactivated through isomerization to the unreactive cis-cyclooctene isomer by interactions with copper-containing proteins, and that increasing the steric hindrance on the tag can impede this process. Next, we found that the higher reactivity of axial vs equatorial linked TCO can be augmented by the choice of linker. The new, stabilized, and more reactive tag allowed for improved tumor-to-nontumor ratios in pretargeted tumor-bearing mice.

Synthesis of phosphine and antibody-azide probes for in vivo Staudinger ligation in a pretargeted imaging and therapy approach

The application of intact monoclonal antibodies (mAbs) as targeting agents in nuclear imaging and radioimmunotherapy is hampered by the slow pharmacokinetics of these molecules. Pretargeting with mAbs could be beneficial to reduce the radiation burden to the patient, while using the excellent targeting capacity of the mAbs. In this study, we evaluated the applicability of the Staudinger ligation as pretargeting strategy using an antibody-azide conjugate as tumor-targeting molecule in combination with a small phosphine-containing imaging/therapeutic probe. Up to 8 triazide molecules were attached to the antibody without seriously affecting its immunoreactivity, pharmacokinetics, and tumor uptake in tumor bearing nude mice. In addition, two (89)Zr- and (67/68)Ga-labeled desferrioxamine (DFO)-phosphines, a (177)Lu-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-phosphine and a (123)I-cubyl phosphine probe were synthesized and characterized for their pharmacokinetic behavior in nude mice. With respect to the phosphine probes, blood levels at 30 min after injection were <5% injected dose per gram tissue, indicating rapid blood clearance. In vitro Staudinger ligation of 3.33 μM antibody-azide conjugate with 1 equiv of radiolabeled phosphine, relative to the azide, in aqueous solution resulted in 20-25% efficiency after 2 h. The presence of 37% human serum resulted in a reduced ligation efficiency (reduction max. 30% at 2 h), while the phosphines were still >80% intact. No in vivo Staudinger ligation was observed in a mouse model after injection of 500 μg antibody-azide, followed by 68 μg DFO-phosphine at t = 2 h, and evaluation in blood at t = 7 h. To explain negative results in mice, Staudinger ligation was performed in vitro in mouse serum. Under these conditions, a side product with the phosphine was formed and ligation efficiency was severely reduced. It is concluded that in vivo application of the Staudinger ligation in a pretargeting approach in mice is not feasible, since this ligation reaction is not bioorthogonal and efficient enough. Slow reaction kinetics will also severely restrict the applicability of Staudinger ligation in humans.

Abstract 4552: Pretargeted radioimmunoimaging and -therapy in tumor-bearing mice using a bioorthogonal reaction

A major challenge of radioimmunotherapy (RIT) of cancer is to enhance the nuclear radiation dose delivered to the tumor while minimizing the dose in healthy tissues. Monoclonal antibodies (mAb) circulate for a long time and only slowly accumulate in the tumor, which, when used for RIT, leads to dose-limiting side effects in healthy organs. This efficacy-limiting factor can be circumvented by pretargeting, which involves tumor targeting of a mAb followed by binding of a small radiolabeled probe to the tumor-bound mAb. The superior image contrast and the ability to administer higher (therapeutic) radiation doses compared to directly labeled mAbs is offset by the drawbacks of the current biological pretargeting systems, involving either immunogenicity issues or extensive re-engineering of the parent mAb. To address this, we designed a novel pretargeting approach based on the bio-orthogonal chemical inverse-electron-demand Diels Alder reaction, employing a trans-cyclooctene-conjugated mAb and a radiolabeled tetrazine derivative, and we evaluated the in vitro stability and reactivity, and tumor targeting in mice bearing colorectal xenografts.

The in vitro stability and reactivity of 111In-labeled tetrazine and anti-TAG72 mAb CC49 functionalized with trans-cyclooctene (TCO) moieties through lysine residue conjugation were monitored in PBS, serum and blood. In vivo pretargeting was performed in LS174T-tumored mice using 125I-labeled CC49-TCO and 111In-labeled tetrazine and assessed by dual isotope biodistribution and SPECT imaging with a nanoSPECT/CT.

When 111In-tetrazine was administered to mice 1 day after CC49-TCO, the chemically-tagged tumors reacted rapidly with 111In-tetrazine, resulting in pronounced radioactivity localization throughout the tumor and good tumor contrast, as demonstrated by SPECT/CT imaging of live mice 3 h post injection: 4.18%ID/g, tumor-to-muscle ratio (T/M)=13.1. In mice treated with unmodified CC49, the tumor could not be discriminated from the surrounding tissue (0.28 %ID/g, T/M=0.5). Mice treated with TCO-modified rituximab, which lacks specificity for TAG72, showed the expected retention of 111In-tetrazine in blood and non-target organs, and a much reduced tumor accumulation (1.02 %ID/g, T/M=2.1). Corresponding biodistribution experiments revealed a remarkable 52-57% reaction yield between TCO and tetrazine moieties present in tumor and blood.

We have demonstrated the first use of a chemical reaction between two exogenous moieties in living animals for the non-invasive imaging of low-abundance targets in clinically relevant conditions. The inverse-electron-demand Diels Alder reaction has the potential to improve the state of the art of pretargeted RIT and can be applied to a range of antibodies due to its universal and straightforward conjugation chemistry. The validation for pre-targeted RIT in LS174T-tumored mice is underway.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4552.

A doxorubicin prodrug activated by the staudinger reaction

Much effort has been devoted to prodrug systems that effect drug release at the tumor through enzymatic action. To widen the scope of prodrug therapy, the use of the selective Staudinger reaction as prodrug activator, instead of relying on enzymatic action, was investigated. Doxorubicin was conjugated to a p-azidobenzyl trigger that is cleaved after reacting with the chemical activator, triphenylphosphine. The prodrug activation was confirmed in water, cell growth medium, and serum, using HPLC and LCMS. Next, this approach was tested in a cell proliferation assay with A431 human vulvar skin squamous carcinoma cells. The doxorubicin prodrug was shown to exhibit a 176-fold higher IC50 of 15.1 microM vs 0.086 microM for the parent drug, doxorubicin. Addition of triphenylphosphine (5 x 60 microM in 72 h) to the prodrug in cell culture effected the complete recovery of the activity of the parent drug as evidenced by an IC50 value of 0.074 microM. Furthermore, high levels of triphenylphosphine were tolerated well by the cells. The demonstrated usefulness of the Staudinger reaction in cell culture and its in vivo potential opens up new avenues for prodrug therapy.

Combinatorial discovery of new asymmetric cis platinum anticancer complexes is made possible with solid-phase synthetic methods

To efficiently access asymmetric cis platinum (II) complexes for biological evaluation, a new solid-phase synthesis was designed. This synthesis was used for the preparation of a small library of platinum compounds. Several compounds from this library revealed promising activity during a cytotoxicity screen. Two active compounds were, therefore, synthesised on a larger scale and tested more extensively against a larger panel of cell-lines, confirming their high potential as antitumour compounds. The work presented illustrates how a combination of a new methodology and established techniques can speed up the search for platinum complexes with improved cytotoxic profiles compared to cisplatin.

Automated parallel solid-phase synthesis and anticancer screening of a library of peptide-tethered platinum(II) complexes

The automated parallel solid-phase synthesis of a 36-member library of peptide-tethered platinum(II) complexes is described. The identity and quality of each product were confirmed by mass spectrometry and (1)H NMR. Subsequently, each compound was screened for in vitro anticancer activity by treating the A2780 (human ovarian carcinoma) cell line with two concentrations of the drugs (100 and 10 microM) in quadruplicate. The reduction of cell proliferation induced by the drugs at these concentrations was determined with the MTT colorimetric assay (MTT = 3-(4',5'-dimethylthiazol-2'-yl)-2,5-diphenyltetrazolium bromide) and compared to cisplatin. Even though no very active library members could be identified, five apparently most active (8[1], 8[4], 8[10], 8[13], and 8[24]) and two inactive complexes (8[33] and 8[34]) were purified using gel permeation chromatography and fully characterized by NMR spectroscopy ((1)H, (195)Pt) and MS. The IC(50) values of these complexes and cisplatin in A2780 cells were subsequently determined using the MTT assay in a conventional manner. All seven complexes have an IC(50) above 100 microM, confirming the results generated by the assay at 100 and 10 microM of the crude reaction products.

Platinum complexes of diaminocarboxylic acids and their ethyl ester derivatives: the effect of the chelate ring size on antitumor activity and interactions with GMP and DNA

A number of new Pt(II) complexes is described having the general formula PtCl(2)(LL), where LL is a chelating diamine ligand. Ligands LL were chosen as D,L-2,3-diaminopropionic acid and its ethyl ester, and D,L-2,4-diaminobutyric acid and its ethyl ester. The compounds were characterized using analytical and spectroscopic methods. The influence of the size of the chelate ring and its functionalization on the biological properties was studied. It was demonstrated by circular dichroism (CD) that the effects on the secondary structure of DNA induced by the four complexes are different. The interaction takes place at the N7 position of the purine bases, as shown by NMR studies. The platinum complexes of 2,3-diaminopropionic acid and 2,4-diaminobutyric acid are able to form intrastrand adducts with DNA and to distort the double helix by changing the base stacking. The ethyl ester derivatives uncoil the DNA from the B form to the C form. The interactions with 5'-GMP and DNA were compared with their antitumor activity. The platinum complexes of diaminocarboxylic acids exhibit cytotoxic activity in the A431, HeLa, and HL-60 cell lines in a dose- and time-dependent manner.

The interaction of peptide-tethered platinum(II) complexes with DNA

The sequence specificity and intensity of DNA damage induced by six peptide-tethered platinum complexes was compared to cisplatin and Pt(en)Cl(2). DNA damage was investigated in pUC19 plasmid and in intact HeLa cells, and quantitatively analyzed using a Taq DNA polymerase/linear amplification assay. The DNA sequence specificity of the peptide-platinum compounds was found to be very similar to cisplatin and Pt(en)Cl(2), with runs of consecutive guanines being the most intensely damaged sites. The observed reactivity of the peptide-platinum complexes towards plasmid DNA was lower compared to cisplatin and Pt(en)Cl(2), with the glycine-tethered complex 3 and the phenylalanine-tethered complex 4 producing the highest relative damage intensity, followed by (in decreasing order) lysine-tethered (5), arginine-tethered (6), serine-tethered (7) and glutamate-tethered (8). The reactivity of the peptide-platinum complexes towards cellular DNA was also lower compared to cisplatin and Pt(en)Cl(2). For most investigated complexes, the relative damage intensities were found to be similar in cells compared to plasmid DNA, but were greatly reduced for 3 and 4. The lysine-tethered 5 complex produced the highest DNA damage intensity in cells followed by (in decreasing order) 6, 7, 3, 4 and 8.

Extending solid-phase methods in inorganic synthesis: the first dinuclear platinum complex synthesised via the solid phase

A method for obtaining potentially anti-tumour active dinuclear platinum coordination compounds via solid-phase inorganic synthesis is described for the first time.

(Aminoethanol)dichloroplatinum(II) complexes: influence of the hydroxyethyl moiety on 5'-GMP and DNA binding, intramolecular stability, the partition coefficient and anticancer activity

The influence of tethered hydroxyl groups on the binding behavior of the three (aminoethanol)dichloroplatinum complexes, dichloro(N,N'-bis(2-hydroxyethyl)ethylenediamine)-platinum(II) (1), dichloro(N-(2-hydroxyethyl)ethylenediamine)platinum(II) (2) and cis-dichlorobis(2-hydroxyethylamine)platinum(II) (3) towards 5'-GMP and DNA was investigated by 1H NMR and r(b) measurements, respectively. At pH 7.2, the sequence of reactivity with 5'-GMP is 1>2>>3. Complex 3 reacts very slowly with 5'-GMP and DNA and the amount and lifetime of the intermediate 5'-GMP monoadduct are much larger than for 1 and 2. At pH 5.5, the reaction of 3 with 5'-GMP is markedly accelerated and very small amounts of monoadduct are observed, indicating a pH-dependent ability of the pendant hydroxyl group to interact with the platinum moiety. In addition, the effect of the hydroxyethyl functionality on octanol/water partitioning and in vitro anticancer activity was studied. No correlation between lipophilicity and anticancer activity was detected. Furthermore, the lipophilicity and anticancer activity could not be directly correlated to 5'-GMP or DNA binding activity.