Alkylating benzamides with melanoma cytotoxicity

Derivatives of nitrogen mustard anticancer agents with improved cytotoxicity

In previous studies, we demonstrated that esters of bendamustine containing a basic moiety are far more cytotoxic anticancer agents than their parent compound and that the substitution of the labile ester moiety by a branched ester or an amide markedly increases stability in the blood plasma. In the current study, we showed that this substitution was bioisosteric. Aiming at increased cytotoxicity, we introduced the same modification to related nitrogen mustards: 6-isobendamustine, chlorambucil, and melphalan. The synthesis was accomplished using the coupling reagents N,N'-dicyclohexylcarbodiimide or 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethylaminium tetrafluoroborate. Cytotoxicity against a panel of diverse cancer cells (carcinoma, sarcoma, and malignant melanoma) was assessed in a kinetic chemosensitivity assay. The target compounds showed cytotoxic or cytocidal effects at concentrations above 1 µM: a striking enhancement over bendamustine and 6-isobendamustine, both ineffective against the selected cancer cells at concentrations up to 50 µM, and a considerable improvement over chlorambucil, showing some potency only against the sarcoma cells. Melphalan was almost as effective as the target compounds-derivatization only provided a small improvement. The novel cytostatics are of interest as model compounds for analyzing a correlation between cytotoxicity and membrane transport and for the treatment of malignancies.

Comparative preclinical evaluation of 68Ga-NODAGA and 68Ga-HBED-CC conjugated procainamide in melanoma imaging

Malignant melanoma is the most aggressive form of skin cancer. The early detection of primary melanoma tumors and metastases using non-invasive PET imaging determines the outcome of this disease. Previous studies have shown that benzamide derivatives (e.g. procainamide) conjugated with PET radionuclides specifically bind to melanin pigment of melanoma tumors. ⁶⁸Ga chelating agents can have high influence on physiological properties of ⁶⁸Ga labeled bioactive molecules, as was experienced during the application of HBED-CC on PSMA ligand. The aim of this study was to assess this concept in the case of the melanin specific procaindamide (PCA) and to compare the melanin specificity of ⁶⁸Ga-labeled PCA using HBED-CC and NODAGA chelators under in vitro and in vivo conditions. Procainamide (PCA) was conjugated with HBED-CC and NODAGA chelators and was labeled with Ga-68. The melanin specificity of ⁶⁸Ga-HBED-CC-PCA and ⁶⁸Ga-NODAGA-PCA was investigated in vitro and in vivo using amelanotic (MELUR and A375) and melanin containing (B16-F10) melanoma cell lines. Tumor-bearing mice were prepared by subcutaneous injection of B16-F10, MELUR and A375 melanoma cells into C57BL/6 and SCID mice. 21±2days after tumor cell inoculation and 90min after intravenous injection of the ⁶⁸Ga-labelledlabeled radiopharmacons whole body PET/MRI scans were performed. ⁶⁸Ga-NODAGA-PCA and ⁶⁸Ga-HBED-CC-PCA were produced with excellent radiochemical purity (98%). In vitro experiments demonstrated that after 30 and 90min incubation time ⁶⁸Ga-NODAGA-PCA uptake of B16-F10 cells was significantly (p≤0.01) higher than the ⁶⁸Ga-HBED-CC-conjugated PCA accumulation in the same cell line. Furthermore, significant difference (p≤0.01 and 0.05) was found between the uptake of melanin negative and positive cell lines using ⁶⁸Ga-NODAGA-PCA and ⁶⁸Ga-HBED-CC-PCA. In vivo PET/MRI studies using tumor models revealed significantly (p≤0.01) higher ⁶⁸Ga-NODAGA-PCA uptake (SUVmean: 0.46±0.05, SUVmax: 1.96±0.25,T/M ratio: 40.7±4.23) in B16-F10 tumors in contrast to ⁶⁸Ga-HBED-CC-PCA where the SUVmean, SUVmax and T/M ratio were 0.13±0.01, 0.56±0.11 and 11.43±1.24, respectively. Melanin specific PCA conjugated with NODAGA chelator showed higher specific binding properties than conjugated with HBED-CC. The chemical properties of the bifunctional chelators used for ⁶⁸Ga-labeling of PCA determine the biological behaviour of the probes. Due to the high specificity and sensitivity ⁶⁸Ga-labeled PCA molecules are promising radiotracers in melanoma imaging.

In Vivo Imaging of Experimental Melanoma Tumors using the Novel Radiotracer 68Ga-NODAGA-Procainamide (PCA)

Purpose: The most aggressive form of skin cancer is the malignant melanoma. Because of its high metastatic potential the early detection of primary melanoma tumors and metastases using non-invasive PET imaging determines the outcome of the disease. Previous studies have already shown that benzamide derivatives, such as procainamide (PCA) specifically bind to melanin pigment. The aim of this study was to synthesize and investigate the melanin specificity of the novel ⁶⁸Ga-labeled NODAGA-PCA molecule in vitro and in vivo using PET techniques.

Methods: Procainamide (PCA) was conjugated with NODAGA chelator and was labeled with Ga-68 (⁶⁸Ga-NODAGA-PCA). The melanin specificity of ⁶⁸Ga-NODAGA-PCA was tested in vitro, ex vivo and in vivo using melanotic B16-F10 and amelanotic Melur melanoma cell lines. By subcutaneous and intravenous injection of melanoma cells tumor-bearing mice were prepared, on which biodistribution studies and small animal PET/CT scans were performed for ⁶⁸Ga-NODAGA-PCA and ¹⁸FDG tracers.

Results: ⁶⁸Ga-NODAGA-PCA was produced with high specific activity (14.9±3.9 GBq/µmol) and with excellent radiochemical purity (98%<), at all cases. In vitro experiments showed that ⁶⁸Ga-NODAGA-PCA uptake of B16-F10 cells was significantly (p≤0.01) higher than Melur cells. Ex vivo biodistribution and in vivo PET/CT studies using subcutaneous and metastatic tumor models showed significantly (p≤0.01) higher ⁶⁸Ga-NODAGA-PCA uptake in B16-F10 primary tumors and lung metastases in comparison with amelanotic Melur tumors. In experiments where ¹⁸FDG and ⁶⁸Ga-NODAGA-PCA uptake of B16-F10 tumors was compared, we found that the tumor-to-muscle (T/M) and tumor-to-lung (T/L) ratios were significantly (p≤0.05 and p≤0.01) higher using ⁶⁸Ga-NODAGA-PCA than the ¹⁸FDG accumulation.

Conclusion: Our novel radiotracer ⁶⁸Ga-NODAGA-PCA showed specific binding to the melanin producing experimental melanoma tumors. Therefore, ⁶⁸Ga-NODAGA-PCA is a suitable diagnostic radiotracer for the detection of melanoma tumors and metastases in vivo.

Design, synthesis and evaluation of antiproliferative activity of melanoma-targeted histone deacetylase inhibitors

The clinical validation of histone deacetylase inhibition as a cancer therapeutic modality has stimulated interest in the development of new generation of potent and tumor selective histone deacetylase inhibitors (HDACi). With the goal of selective delivery of the HDACi to melanoma cells, we incorporated the benzamide, a high affinity melanin-binding template, into the design of HDACi to generate a new series of compounds 10a-b and 11a-b which display high potency towards HDAC1 and HDAC6. However, these compounds have attenuated antiproliferative activities relative to the untargeted HDACi. An alternative strategy furnished compound 14, a prodrug bearing the benzamide template linked via a labile bond to a hydroxamate-based HDACi. This pro-drug compound showed promising antiproliferative activity and warrant further study.

Targeting ALDH1 to decrease tumorigenicity, growth and metastasis of human melanoma

Cells with aldehyde dehydrogenase activity (ALDH+) are the most tumorigenic cells in many cancers, including melanoma, making ALDH a candidate therapeutic target. We examined the effects of chemical inhibition of ALDH1 on the response of human melanoma xenografts to chemotherapy and the effects of ALDH1A1 RNA silencing on melanoma growth and metastasis. Addition of ALDH1 inhibitors (e.g. diethylaminobenzaldehyde) to dacarbazine chemotherapy, not only reduced tumor growth in vivo, but also resulted in a significant decrease in the number of residual cells capable of tumorigenesis. shRNA depletion of ALDH1A1 in melanoma cells resulted not only in a significant delay in appearance of xenograft melanomas and reduction in growth, but also significantly decreased the number of metastases and metastatic burden after lateral tail vein injections in mice. In summary, ALDH1 inhibition in combinatorial therapy with dacarbazine reduced the number of residual tumorigenic cells post-therapy and ALDH1A1 depletion had marked inhibitory effects on both melanoma growth and metastasis. These findings suggest that ALDH1 inhibition may not only be able to provide a therapeutic advantage in melanoma treatment, but may also prevent rapid relapse after therapy, as residual tumorigenic cells are fewer and metastatic ability is diminished.

Critical role of STAT3 in melanoma metastasis through anoikis resistance

Anoikis is an anchorage-independent cell death. Resistance to anoikis is one of the key features of metastatic cells. Here, we analyzed the role of STAT3 in anoikis resistance in melanoma cells leading to metastasis. When grown under anchorage-independent conditions, significant proportion of cells resisted anoikis and these resistant cells had higher rate of migration and invasion as compared to the cells grown under anchorage-dependent conditions. The anoikis resistant cells also had significantly higher expression and phosphorylation of STAT3 at Y705 than the cells that were attached to the basement membrane. STAT3 inhibitors, AG 490 and piplartine (PL) induced anoikis in a concentration-dependent manner in anoikis resistant cells. Over-expression of STAT3 or treatment with IL-6 not only increased anoikis resistance, but also protected the cancer cells from PL-induced anoikis. On the other hand, silencing STAT3 decreased the potential of cancer cells to resist anoikis and to migrate. STAT3 knock-down cells and PL treated cells did not form tumors as well as failed to metastasize in SCID-NSG mice as compared to untreated anchorage-independent cells, which formed big tumors and extensively metastasized. In summary, our results for the first time establish STAT3 as a critical player that renders anoikis resistance to melanoma cells and enhance their metastatic potential.

Radiopharmaceutical therapy of patients with metastasized melanoma with the melanin-binding benzamide 131I-BA52

The performance of cytotoxic drugs is defined by their selectivity of uptake and action in tumor tissue. Recent clinical responses achieved by treating metastatic malignant melanoma with therapeutic modalities based on gene expression profiling showed that malignant melanoma is amenable to systemic treatment. However, these responses are not persistent, and complementary targeted treatment strategies are required for malignant melanoma.

METHODS

Here we provide our experience with different labeling procedures for the radioiodination of benzamides and report on initial dosimetry data and the first therapeutic application of (131)I-BA52, a novel melanin-binding benzamide in patients with metastatic malignant melanoma. Twenty-six adults with histologically documented metastasized malignant melanoma received a single dose of 235 ± 62 MBq of (123)I-BA52 for planar and SPECT/CT imaging. Nine patients were selected for radionuclide therapy and received a median of 4 GBq (minimum, 0.51 GBq; maximum, 6.60 GBq) of the β-emitting radiopharmaceutical (131)I-BA52.

RESULTS

A trimethyltin precursor-based synthesis demonstrated high radiochemical yields in the large-scale production of radioiodinated benzamides required for clinical application. (123)I-BA52 showed specific uptake and long-term retention in tumor tissue with low transient uptake in the excretory organs. In tumor tissue, a maximum dose of 12.2 Gy per GBq of (131)I-BA52 was calculated. The highest estimated dose to a normal organ was found for the lung (mean, 3.1 Gy/GBq). No relevant acute or mid-term toxicity was observed with the doses administered until now. Even though dosimetric calculations reveal that the doses applied in this early phase of clinical application can be significantly increased, we observed antitumor effects with follow-up imaging, and single patients of the benzamide-positive cohort of patients (3/5 of the patients receiving a dose > 4.3 GBq) demonstrated a surprisingly long survival of more than 2 y.

CONCLUSION

These data indicate that systemic radionuclide therapy using (131)I-BA52 as a novel approach for the therapy of malignant melanoma is of considerable potential. Future trials should be done to enhance the precision of dosimetry, validate the maximum tolerable dose, and evaluate the effectiveness of the treatment in a prospective manner.

Synthesis and evaluation of novel radioiodinated nicotinamides for malignant melanoma

INTRODUCTION

A series of iodonicotinamides based on the melanin-binding iodobenzamide compound N-2-diethylaminoethyl-4-iodobenzamide was prepared and evaluated for the potential imaging and staging of disseminated metastatic melanoma.

METHODS

[(123)I]Iodonicotinamides were prepared by iododestannylation reactions using no-carrier-added iodine-123 and evaluated in vivo by biodistribution and competition studies and by single photon emission computed tomography (SPECT) imaging in black and albino nude mice bearing B16F0 murine melanotic and A375 human amelanotic melanoma tumours, respectively.

RESULTS

The iodonicotinamides displayed low-affinity binding for sigma(1)-sigma(2) receptors (K(i)>300 nM). In biodistribution studies in mice, N-(2-(diethylamino)ethyl)-5-[(123)I]iodonicotinamide ([(123)I]1) exhibited the fastest and highest uptake of the nicotinamide series in the B16F0 tumour at 1 h ( approximately 8% ID/g), decreasing slowly over time. No uptake was observed in the A375 tumour. Clearance from the animals by urinary excretion was more rapid for N-alkyl-nicotinamides than for piperazinyl derivatives. At 1 h postinjection, the urinary excretion was 66% ID for [(123)I]1, while the gastrointestinal tract amounted to 17% ID. Haloperidol was unable to reduce the uptake of [(123)I]1 in pigmented mice, indicating that this uptake was likely due to an interaction with melanin. SPECT imaging of [(123)I]1 in black mice bearing the B16F0 melanoma indicated that the radioactivity was predominately located in the tumour and eyes. No specific localisation was observed in nude mice bearing A375 amelanotic tumours.

CONCLUSION

These findings suggest that [(123)I]1, which displays high tumour uptake with rapid clearance from the body, could be a promising imaging agent for the detection of melanotic tumours.

Role of acidic cell organelles in the higher nonmelanoma retention of melanoma markers based on N-(2-dialkylaminoethyl)benzamides and the cytotoxicity of alkylating benzamides

Melanoma markers based on both N-(2-dialkylaminoethyl)benzamides and lysosomotropic agents comprise a N-(2-dialkylaminoethyl)aminocarbamoyl pharmacophore, suggesting that benzamides and lysosomotropic probes should show affinity to melanoma and acidic cell organelles. We prepared novel fluorescent N-(2-dialkylaminoethyl)benzamides to prove this presumption. Lysosomotropic probes showed a melanin affinity comparable to benzamides. Lysosomal markers and benzamides colocalized in acidic organelles. Various nonmelanoma cell lines showed equal benzamide uptake and retention compared with melanoma cells. In nonmelanoma cells the amount of retained benzamides correlates with the number of acidic cell organelles. Benzamides almost completely failed to accumulate in melanoma cells with neutralized acidic organelles but normal melanin content. In melanoma retention of benzamides, acidic cell organelles are the main determinant. N-(2-dialkylaminoethyl)benzamides are lysosomotropic probes with high accumulation in nonmelanoma tumors with many acidic cell organelles. Alkylating benzamides were reported previously to show a melanoma unselective, in general enhanced cytotoxicity. Alkylating benzamides can act as lysosomotropic detergents or as DNA alkylators. The ability of alkylating benzamides to disrupt the membrane of lysosomes and cause liberation of lysosomal-trapped fluorescent dyes was demonstrated by fluorescence microscopy. Whether they act as an alkylating agent or a lysosomotropic detergent in a specific cell line is dependent on the amount of acidic cell organelles. In cell lines with small amounts of acidic cell organelles alkylating benzamides act as alkylating agents, whereas in cell lines with many acidic cell organelles they act as lysosomotropic detergents. In cell lines with high amounts of acidic cell organelles they do not reach the nucleus.

Alkylating benzamides with melanoma cytotoxicity: experimental chemotherapy in a mouse melanoma model

The in-vivo antineoplastic potential of the alkylating N-(2-dialkylaminoethyl)benzamides BZA1 and BZA2, novel melanoma targeted anticancer drugs, was evaluated in a mouse melanoma model with nude mice bearing subcutaneous SkMel28, B16 or WM266-4. The maximal tolerated dose (MTD) for the intraperitoneal application of both agents was found to be 24 mg/kg. Treatment was initiated with an intraperitoneal injection of 8 mg/kg of BZA1 or BZA2 on days 0, 2 and 4 in the case of B16 melanoma on days 0, 1 and 2 after the onset of the experiment, when the mean tumor diameter ranged within 4-6 mm. The experiment was terminated when the mean tumor diameter in the control group had reached a value of 12 mm. Tumor growth delay of these agents was compared with dacarbazine (3x250 mg/kg), chlorambucil (3x5 mg/kg) and an untreated control group. Significant tumor growth delay was observed under BZA1, BZA2 and dacarbazine treatment compared with the untreated control group in all three evaluated melanomas with insignificant differences among BZA1, BZA2 and dacarbazine. The insignificant effect of chlorambucil and the strong improvement on growth delay achieved with BZA1 and BZA2 demonstrated melanoma targeting characteristics of the N-(2-dialkylaminoethyl)benzamide structure element. Dacarbazine was more effective in the in-vivo antineoplastic assay compared with the in-vitro cytotoxicity studies, probably due to hepatic bioactivation. Similar side effect intensity of BZA2 and dacarbazine was observed, whereas BZA1 was more toxic. BZA2 might represent an alternative antimelanoma drug, especially in patients not responding to dacarbazine.

Polyamine-Substituted Gadolinium Chelates: A New Class of Intracellular Contrast Agents for Magnetic Resonance Imaging of Tumors

A new class of intracellular contrast agents (CA) for magnetic resonance imaging has been developed, based on Gd(DTPA) with two positively charged amide-linked substituents. Uptake of Gd(DTPA) into cultured tumor cell lines (B16 mouse melanoma, MH3924A Morris hepatoma) was below the detection limit while CA with the melanin-binding pharmacophore 2-(diethylamino)ethylamine reached intracellular concentrations of ca. 0.03 fmol/cell (ca. 20 microM) for melanoma and 0.02 fmol/cell for hepatoma (24 h at 10 microM CA). With the polyamine substituents bis(2-aminoethyl)amine or spermidine, CA uptake increased up to 3-fold for melanoma (0.083 fmol/cell) and 9-fold for hepatoma (0.18 fmol/cell). Uptake of polyamine-substituted CA was reduced by the polyamine transport inhibitor benzyl viologen. Molar relaxivities for three Gd-DTPA-polyamine complexes were in the range 5.6-6.9 for the free complex in solution and 7.7-23.5 s-1 mM-1 for Morris hepatoma cell pellets. T1-weighted magnetic resonance imaging at 2.35 T of rats with MH3924A tumors showed contrast enhancement in tumor at 1 and 24 h postinjection of polyamine-substituted CA.

Alkylating benzamides with melanoma cytotoxicity: role of melanin, tyrosinase, intracellular pH and DNA interaction

N-(2-Dialkylaminoethyl)benzamides have been shown to selectively accumulate in melanoma metastases with high uptake capacity. Therefore, this class of compound has previously been evaluated as a transporter for cytostatic drugs. It has been demonstrated that this significant targeting effect improves the cytotoxicity against melanoma cells. Although these agents are not accumulated by non-melanoma cells, they have been found to be toxic. In order to identify mechanistic reasons for this effect, we investigated the DNA and melanin binding affinities of a selection of four benzamide-drug conjugates, together with their parental cytostatics. An investigation of the influence of the melanin content on the cytotoxicity of these substances in B16 melanoma and Morris hepatoma (MH3924A) cells was performed, together with their influence on melanosomal pH and tyrosinase activity. The suppression of melanin formation with phenylisothiourea and the saturation of melanin binding sites with chloroquine were also investigated. These experiments demonstrated high DNA binding and low melanin affinity, in accordance with the toxicity against tumour cells. Melanin has a concentration-dependent scavenging effect, thereby reducing cytotoxicity. These compounds lead to an increase in the acidic pH of melanosomes, resulting in an increase in tyrosinase activity. The consequence of this reaction chain is an amplification of the scavenging effect for the benzamide-drug conjugates. These effects may be considered as limiting factors for the targeting characteristics of this class of compound, necessitating further modifications to the carrier system.