PSMA inhibitor-based radiopharmaceuticals for diagnosis and therapy of prostate cancer
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Abstract
Prostate cancer (PCa) is the second most common diagnosed cancer type among men worldwide. In relation to death rate, it is on the fifth place of all cancer types worldwide. When it is diagnosed early, the chances of cure are very good with a 5-year survival rate of 98%. Patients with advanced cancer often develop metastases, reducing the chances of cure drastically. Thus, an early diagnosis and therapy is crucial for an appropriate medical care.
The prostate-specific membrane antigen (PSMA) is a membrane-bound glycoprotein, which shows only a low expression in the healthy prostate but can be overexpressed up to 1000-fold by prostate cancer cells. Additionally, the expression correlates with the progress of the disease enabling classification of the stage of the disease. The discovery and exploration of PSMA led to an improvement of the patients care. Today, PSMA is used as target structure for the targeted diagnosis and therapy of prostate cancer and related metastases.
PSMA gained particular interest in the field of radiopharmacy. Here, addressing PSMA enables an early imaging and diagnosis of prostate cancer in the positron emission tomography (PET) or single-photon emission computed tomography (SPECT), even before morphological chances can be seen in computed tomography (CT) or magnetic resonance imaging (MRI). Besides this, PSMA enables the targeted enrichment of therapeutic radiopharmaceuticals. Many sensitive and highly potent PSMA radiopharmaceuticals were developed in the last years. Especially known among these are [68Ga]Ga PSMA-11 for PET, which is already approved by the FDA and [177Lu]Lu-PSMA-617 for therapy of PCa.
Despite the research already conducted and the clinically established radiopharmaceuticals, in PSMA-based diagnosis and therapy of PCa there are still fields and problems that need to be improved or solved. Examples are the unwanted off-target accumulation of PSMA ligands or the targeted drug delivery. The aim of this work is the development of novel PSMA ligands, improvement of already existing ligands and the provision of solutions and applications for the existing problems of the PSMA imaging and therapy. For this purpose, new designed radiopharmaceutical compounds were synthesized, radiolabelled with diagnostic and/or therapeutic radionuclides, evaluated regarding their in vitro properties and in part evaluated regarding in vivo behaviour.
The first section of this work sets its focus on the use of the hybrid chelators DATA5m and AAZTA5 as radiolabelling moiety. Here, different PSMA inhibitors were functionalised with the hybrid chelators. The DATA5m-based compounds DATA5m.SA.KuE and DATA5m.PSMA-617 were evaluated regarding their radiolabelling properties with gallium-68. In followed up in vitro binding affinity studies the PSMA binding behaviour of the two compounds were examined. AAZTA5.SA.KuE, which was synthesised and evaluated radiochemically in previous studies, was also included. In animal studies, the potential of [68Ga]Ga-DATA5m.SA.KuE as diagnostic radiopharmaceutical and [44Sc]Sc-/[177Lu]Lu-AAZTA5.SA.KuE as theranostic PSMA ligand were proven, as they show better kidney clearance than [68Ga]Ga PSMA 11, for example.
The second part deals with the problem of the PSMA-based therapy of PCa related bone metastases, since PSMA expression in bone metastases can be very heterogeneous. This makes the therapy with normal PSMA ligands more difficult. To solve this problem, the compound DOTA L Lys(SA.Pam) PSMA 617 was developed in this work. Beside a PSMA target vector, the compound contains an additional bisphosphonate structure as targeting unit. The bisphosphonate enables the enrichment in bone areas with increased metabolism, as it is found in bone metastases. This enables the enrichment in PCa related bone metastases via two independent mechanisms. After synthesis, the novel compound was successfully radiolabelled with the therapeutic nuclide lutetium 177 and it showed a high in vitro PSMA and bone binding potential. In in vivo studies with tumour-bearing mice, it showed promising enrichment in the tumour as well as in the bone. This makes [177Lu]Lu-DOTA-L-Lys(SA.Pam)-PSMA-617 to an interesting compound for therapy of PCa related bone metastases.
In the third part of this work, two drug delivery systems were developed, which enable the targeted transport of drugs to the prostate cancer cells via a PSMA targeting unit. For this purpose, BisHD-hbPG polymers were functionalised with a SA.KuE target vector and formed into liposomes. An additional 18F-label enables the evaluation of the in vivo behaviour of the PSMA liposomes in PET measurements. In this work, the functionalisation of the polymers with the PSMA inhibitor, the radiolabelling and the liposome formation were established successfully. The in vivo examinations are pending. As second delivery system, a small molecule drug delivery conjugate (SMDC) was synthesized and evaluated. The cytotoxic drug MMAE was conjugated via the enzymatically cleavable linker valine-citrulline and squaric acid to the PSMA targeting unit 617.KuE. In vitro examinations showed that the novel compound is able to transport the drug via PSMA targeting to the cancer cell and MMAE is released via degradation of the valine-citrulline linker. Nevertheless, the cytotoxicity of the compound is lower than the cytotoxicity of free MMAE and the PSMA affinity is not that good than that of PSMA-617. Animal studies displayed that the application of the novel compound can inhibit tumour growth. Here, optimisation of the structure is necessary to increase affinity and cytotoxicity.
The last part of this work deals with the optimisation of the lysine-urea-glutamate (KuE) structure, which is the targeting vector of the most common PSMA radiopharmaceuticals. It is literature-known that the structure lysine-glutamate-urea-glutamate (K-EuE) provides better pharmacological properties. Thus, both target vectors were conjugated to squaric acid and were evaluated regarding in vivo binding to PSMA along with the targeting unit of PSMA-617, KuE.617. All three structures had comparable affinities. Additionally, the influence of chelators on binding affinity should be examined. For this, each targeting unit was conjugated to the chelators DATA5m, AAZTA5 and DO2AGA and the PSMA affinity as determined. Here, it was particularly noticeable that the addition of a chelator led to a general degrease of affinity. This was especially evident with the targeting unit SA.K-EuE. It was shown that the SA.K-EuE targeting unit is not suitable for conjugation with chelators. The superiority of K-EuE over KuE as it is described in literature could not be confirmed for squaric acid-based compounds. Although the affinities of the chelator conjugated SA.KuE and 617.KuE got also lower but only to a lesser extent. Additionally, no clear trend could be observed which chelator or which targeting unit had the better affinity. This seems to be dependent of the respective combination. In further studies, spacer moieties should be introduced between chelator and targeting unit to evaluate if a further distance between the two units can reduce the affinity degreasing influence of the chelator.