Introduction

It is known that the uterus, mammary gland, placenta, liver, central nervous system, cardiovascular system and bones have high ERα content and prostate, testis, ovary, thyroid gland, skin, lung and muscle contain ERβ [1].

Prostate and breast cancers were reported that were most common cancer types [2]. Diethylstilbestrol (DES) is a synthetic nonsteroidal estrogen, being used to treat postmenopausal breast and advanced prostate cancer [3–5]. Diethylstilbestrol diphosphate (fosfestrol) is an inactive synthetic estrogen, used as a prodrug of the active DES that has phosphate substituted on both hydroxyl groups for localised, advanced and hormone refractory prostate cancer [6–10]. In other study the response of fosfestrol in bony metastases was very high [11, 12].

According to the College of American Pathologists, alkaline phosphatase enzyme is an important prognostic factor for patients with renal cell carcinoma. Although elevated enzyme activity usually prompts a search for bone and/or liver metastases, the analysis lacks specificity [13]. It is known that alkaline phosphatase enzyme is present in large amounts in bone in which it plays a role in mineralization [14–16]. This is why the synthesis compounds which undergo phosphating process (conjugation of phosphate group) come into question in the imaging and therapy of consisting alkaline phosphatase enzyme cancer types.

Although some works exist about 123/125/131I radiolabeled DES [17], 131/125I radiolabeled DES glucuronide derivatives [18] and also DES labeled with Auger emitters [19] we could not find any report about the radiolabeled phosphate derivatives of DES except Schomacker’s study [20].

In the present study 99mTc was used, which is a radionuclide with a half-life of 6.02 h and plays an important role in widespread applications of nuclear medicine [21, 22], but to date the authors are not aware of an existing study regarding 99mTc labeled phosphate derivative of DES, that is either provided with conjugation of phosphate group.

The aim of the present study was to synthesize phosphate derivative compound of DES [diethylstilbestrol diphosphate (DES-P)] and to label with 99mTc using tin chloride as reducing agent and to evaluate its in vivo biological activity in healthy male and female Wistar Albino rats.

Materials and methods

Na99mTcO4 was supplied by the Department of Nuclear Medicine of Ege University. DES was purchased from Sigma. All other chemicals were supplied from Merck Chemical Co and Aldrich Chemical Co. All animal experiments were carried out under the approval of to the relevant Institutional Animal Review Committee of Ege University, Turkey.

Equipments

Thin layer radio chromatograms (TLRC) and High-performance liquid chromatography (HPLC) chromatograms were obtained using a Cd(Te) detector equipped with a RAD 501 single-channel analyzer and HPLC instrument (LC-10ATvp quaternary pump and an auto sampler (SIL-20A HT)and 7.0 μm reversed-phase (RP)-C-18 column 250 × 21 mm I.D., Macherey–Nagel) in Ege University Institute of Nuclear Sciences. Liquid chromatography mass spectrometry (LC/MS/MS) chromatograms were taken using an API 4000 LCMSMS Triple Quadrapole. RadioTLC (Bioscan 2000) was in Ege University Institute of Nuclear Sciences.

Synthesis of DES-P

DES-P was synthesized according to the previous procedure with the Synkavit [23]. 1.39 g (0.10 mol) DES was dissolved in 4 mL pyridine and 5 mL POCl3 was added to this solution drop by drop and stirred during one night at room temperature. After the reaction completed the pyridine was evaporated then 10 mL of dichloromethane and 10 mL of water was added. Thus the product was extracted to organic phase. The resulting supernatant (the organic phase) was separated and pH was adjusted to 9 by 3 M LiOH solution and the product was eluted using Dowex 50 column anion exchange column. The eluate was evaporated in water bath. Then the crude product was washed by 8 mL ethanol and pH was adjusted to 10 by using 1 M NaOH. The mixture was purified by boiling with active carbon for 20 min and then the pure product was precipitated by evaporating ethanol.

Structural analysis

Liquid chromatography-mass spectrometry (LC/MS/MS)

The structure of DES-P was given in Fig. 1. LC/MS/MS spectra were taken using a Zivac TandemGold Triple Quadrupole LC/MS/MS. (m/z) values for DES, DES-P and some other fragments were presented in Fig. 2 and Table 1.

Fig. 1
figure 1

Molecular structure of DES-P

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Fig. 2
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LC/MS/MS spectra (m/z) values for DES, DES-P and some other fragments

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Table 1 LC/MS/MS spectra (m/z) values for DES, DES-P and some different fragments and proposed structures of selected fragments
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Nuclear magnetic resonance (NMR)

1H-NMR (in D2O) and 13C-NMR (in Pyridine-D5) spectra were taken in order to identify the chemical structure of DES-P. Both of the NMR spectra were obtained by using the facility of Erzurum Ataturk University, Faculty of Science and Department of Chemistry. Theoretical (ACD/LogP Algorithm Computer Programme) spectra were used for comparison with experimental NMR spectra (Tables 2, 3).

Table 2 Experimental and theoretical δ (ppm) values of 13C-NMR (Pyridine-D5) for DES-P
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Table 3 Experimental and theoretical δ (ppm) values of 1H-NMR (D2O) for DES-P
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Spectrofotometric analysis of DES-P

Spectrofotometric analysis was performed to show the act of the alkaline phosphatase (ALP) enzyme on DES-P. ALP was dissolved with bidistilled water and aliquoted (100 μL), aliquots were stored at −80 °C until use. Different concentrations (1–10−9 μM) of DES solution were prepared, measured at 210 nm and calibration curve was drawn. Hundred microliters of ALP was activated with incubation at 37 °C for 5 min. After activation substrate (1 μM DES-P) with pH 10 buffer was added quickly and spectrofotometric measurements until 15 min incubation were done with multiplate reader (Thermo Varioscan). Results of 15 min measurements graphed in Fig. 3.

Fig. 3
figure 3

Spectrophotometric analysis of ALP enzyme and DES-P

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Radiolabeling of DES-P with 99mTc

The stock solution of DES-P (500 μg/0.50 mL bidisitle water) and SnCl2 (1 mg/1 mL bidistillated water) were prepared. Then 0.10 mL DES-P and 100 μL SnCl2 were mixed in a tube and pH was 7. One mCi (37 MBq)/100 μL of 99mTcO4 − were added into the mixture and incubated 20 min at room temperature. R f value and % radiolabeling yield of 99mTc labeled DES-P (99mTc-DES-P) were determined by Thin Layer Radio Chromatography (TLRC) method.

Quality control studies for 99mTc-DES-P

Thin layer radio chromatography (TLRC) studies

TLRC was performed with ITLC-SG (Merck-5554) using 1.5 × 10 cm size plates. Five μL aliquots of the samples (99mTcO4 −, Reduced 99mTc (Red. 99mTc) and 99mTc-DES-P) were applied to the plates 0.5 cm from the edge and placed in a tank containing the mobile phase, Acid Citrate Dextrose (ACD) and physiological serum (0.90% NaCl). Then, the TLRC plates were counted by TLC Scanner (BioscanAR 2000). R f values of each component determined by the TLRC method. Table 4 shows R f values of 99mTcO4 −, Red. 99mTc and 99mTc-DES-P.

Table 4 R f values of 99mTcO4 − , Red. 99mTc and 99mTc-DES-P
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High performance liquid radio chromatography (HPLRC) studies

A low pressure gradient HPLC system [LC-10ATvp quaternary pump and SPD-10A/V UV detector and an auto sampler (SIL-20A HT) and 5-μm RP-C18 column (250 × 4.6 mm I.D., Macharey-Nagel)] was used for analytical experiments. The column was eluted with 65% methanol and bidistilled water with 0.1% TFA (v/v) at 0.8 mL/min. The eluted components were detected at 240 nm by UV detector SPD-10A/V. Figure 4 shows chromatograms of 99mTcO4 − , Red. 99mTc and 99mTc-DES-P.

Fig. 4
figure 4

HPLRC chromatogram of 99mTcO4 −, Red. 99mTc and 99mTc-DES-P

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Biodistribution studies on healthy Albino Wistar rats

Experiments with animals were approved by the Institutional Animal Review Committee of Ege University. 99mTc labeled DES-P was sterilized by passing through a 0.22 μm membrane filter. The activity was approximately 400 μCi (0.2 mL)/each rat. Then, it was injected into the tail vein of 9 male and 9 female healthy Wistar Albino rats (1 μg/each rat), which were 24 weeks old and with a weighed between 110 and 150 g. Receptor blocked (99mTc-Rec-DES-P) experiments were done by 10 μg of DES for each rats 15 min before the injection of 99mTc-DES-P.

The rats were sacrificed at 30, 90 and 240 min post injection sodium pentobarbital (200 mg/kg) by intraperitoneal via, and tissues of interest were removed. Radioactivity was counted by a Cd(Te) detector equipped with a RAD 501 single channel analyzer. The percentage of injected dose per gram of tissue weight (%ID/g tissue) was determined.

Statistical analysis

Differences in the mean values of the measured activities were evaluated statistically by the SPSS 13 program (Univariate Variance Analyses and Pearson Correlation). Probability values <0.05 were considered significant. Pearson correlation was carried out among different organs for 99mTc-DES-P.

Imaging studies on healthy female and male Albino Wistar rats

The imaging studies were performed on healthy female and male Albino Wistar rats (n = 4) in two groups (receptor blocked and unblocked). The static scintigrams were obtained using a gamma camera (Diacan Instruments) in Celal Bayar University, School of Medicine, Department of Nuclear Medicine. 99mTc-DES-P was sterilized by passing through a 0.22 μm membrane filter and injected into the tail vein of female and male Albino Wistar rats. A supplemental dose of alfazine and alfamine was used. The injected mass of 99mTc-DES-P was 5 μg/rats and the activity was approximately 3.7 MBq (100 μCi). For receptor blocked group 15 min before the injection of 99mTc-DES-P, 50 μg of DES was injected to the rats. The scintigrams were obtained from anterior projection after different time intervals up to 2 h following the administration of 99mTc-DES-P.

Results

Results of structural analysis

LC/MS/MS results

LC/MS/MS spectra confirmed expected molecular structure. According to LS/MS/MS results, the molecular fragments at m/z 267.00, 427.60 belong to DES and DES-P which could be shown by the structures in Fig. 2 and Table 1, respectively.

NMR results

Tables 2 and 3 represent experimental and theoretical δ (ppm) values of 13C-NMR (Pyridine-D5) and 1H-NMR (D2O), respectively, for DES-P. Experimental data were in agreement with the theoretical data, as expected. Both 13C and 1H NMR data showed that carbon no 18, 19, 27 and 28 confirmed phosphate group of DES-P.

Spectrofotometric analysis results of DES-P

The equation obtained from the calibration curve of the DES was y = 0.52x + 0.29 (R 2 = 0.87). The absorbance values of the ALP enzyme incubated with DES-P were 0.99, 2.25 and 2.25 for 0., 5.–15 min, respectively. It’s seen that enzyme is cleaved the DES-P in 5 min.

Results of radiolabeling and quality control studies

TLRC

Radiolabeling yield of 99mTc-DES-P was found to be 99.00% ± 0.17 (n = 13). As seen Table 4, R f values of 99mTcO4 −, Red. 99mTc and 99mTc-DES-P were 0.93, 1.00 and 0.07, respectively for mobile phase 1.

HPLRC

HPLRC chromatograms confirmed that a phosphate derivative of DES [Diethylstilbestrol diphosphate (DES-P)] was labeledd with 99mTc successfully. The retention time (Rt) values of 99mTcO4 −, Red. 99mTc and 99mTc-DES-P were different from each other (4.29, 2.16 and 0.11) as seen in Fig. 4.

Results of biodistribution studies

Biodistribution of 99mTc-DES-P as % injected dose/g tissue (%ID/g tissue) for male and female rats is given in Figs. 5 and 6 and Tables 5 and 6. When receptor blocked, male rats have higher uptake values than females in period time.

Fig. 5
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The ratio of the injected dose per gram (% ID/g) value of ER-rich tissues in male rats to % ID/g of muscle (Bg, background) for 99mTc-DES-P and 99mTc-Rec-DES-P

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Fig. 6
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The ratio of the injected dose per gram (% ID/g) value of ER-rich tissues in female rats to % ID/g of muscle (Bg, background) for 99mTc-DES-P and 99mTc-Rec-DES-P

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Table 5 The biodistribution (in % dose/g) of receptor blocked and unblocked studies with 99mTc-DES-G for various selected tissues in male rats (n = 3)
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Table 6 The biodistribution (in % dose/g) of receptor blocked and unblocked studies with 99mTc-DES-G for various selected tissues in female rats (n = 3)
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99mTc-DES-P was eliminated through the kidneys and accumulated in bladder in both male and female rats for receptor unblocked and blocked studies. Small intestines uptakes in male rats were significantly higher (p < 0.05) than females. Uterus uptake was ~26 times less when receptor blocked within 240 min post injection. Similarly small and large intestines uptakes were decreased 26 and 19 times when ER receptors blocked.

No significant radioactivity uptakes were seen in brain of both sexes.

Figure 5 demostrates the ratios of the injected doses per gram (% ID/g) to % ID/g of muscle (Bg, background) for ER-rich tissues in male rats. Prostate uptake was ~3.50 times less than from 90 min to 240 min postinjection (p < 0.05).

The ratio of the injected dose per gram (% ID/g) for ER-rich tissues in female rats to % ID/g of muscle for 99mTc-DES-P and 99mTc-Rec-DES-P can be seen in Fig. 6.

The uterus/muscle ratios when estrogen receptors unblocked are 0.89, 1.55, 26.25 in 30, 90 and 240 min, respectively. These results indicate that the 99mTc-DES-P has been uptaken significantly by the uterus tissue within 240 min.

Bone to muscle ratios for 99mTc-DES-P/99mTc-Rec-DES-P are given in Fig. 7. The bone/muscle ratios of female rats were significantly higher than the male rats’ the bone/muscle ratios at 90 min and 240 min. Female rats’ the bone/muscle ratio presented 15.14 times higher than male rats’ the bone/muscle ratio at 90 min postinjection particularly.

Fig. 7
figure 7

Bone to muscle (Bg, background) % ID/g ratio of 99mTc-DES-G and 99mTc-Rec-DES-G for female and male rats

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Results of imaging studies

Ratio of the bony regions’ ROI values of 99mTc-DES-P/99mTc-Rec-DES-P for female and male rats is given in Fig. 8. Figures 9 and 10 show the static scintigrams of female and male rats corresponding to 30, 60, 90 and 120 min after the administration of 99mTc-DES-P. When Figs. 7 and 8 were taken into consideration, uptake of bony regions was similar within 60 and 120 min. Particularly, the uptake of bony regions for female rats was higher than male rats’ uptake in these time period. As seen in Figs. 9 and 10 there were high accumulation in the kidneys, liver and spleen. This accumulation lowered within 120 min. Furthermore, posterior vertebrate accumulation was noteworthy. The primary excretion route for both female and male rats was kidneys.

Fig. 8
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Ratio of the ROI values of 99mTc-DES-P/99mTc-Rec-DES-P for female and male rats

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Fig. 9
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Static images of the 99mTc-DES-P and 99mTc-Rec-DES-P for female rats at 30, 60, 90, 120 min after injection

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Fig. 10
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Static images of the 99mTc-DES-P and 99mTc-Rec-DES-P for male rats at 30, 60, 90, 120 min s after injection

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Discussion

Fischer calculated the highest radiolabeling yields for 123/125/131I radiolabeled DES [17]. Yilmaz’ results were 93% and 81% for 131I labeled DES and 131I labeled DES-glucuronide, respectively [18]. In our study, radiolabeling yield of 99mTc-DES-P was found to be 99.00% ± 0.17 (n = 13).

99mTc-DES-P was eliminated through the kidneys and accumulated in the bladder in both male and female rats. The uptakes for the both sexes could be due to the conjugation of phosphate group. Hepatobiliary excretion was also observed. In literature, it is known that fosfestrol is metabolized to DES by phosphatase mainly in the liver and DES is eliminated into urine or feces [24].

Glucuronide conjugates of steroids were excreted in the bile and hence, in the intestines [25]. Some glucuronide derivatives DES-P [18], TAM-G [26], and other aglycon glucuronides [27–34] were reported in the literature. They have high intestinal system and urinary bladder uptakes.

In our study prostate uptake of 99mTc-DES-P was elevated in 90 min. Schomacker et al. reported that 123I radiolabeled diethylstilbestrolphosphate (DESP) was accumulated in ER rich tissues of prostate from 10 min up to 2 h post injection [20].

99mTc-DES-P has also a significant uptake by the uterus tissue within 240 min. Yilmaz reported that 131I radiolabeled DESG have receptor affinity on uterus, ovaries and breast tissues according to biodistribution studies with female Wistar Albino rats [18].

Conclusion

The results showed that DES-P could be successfully radiolabeled with 99mTc. The biodistribution studies showed that DES-P is ER specific. The bones uptake increased within 90 min. postinjection. It may potentially be proposed as a 99mTc labeled imaging agent for alkaline phosphatase enzyme rich tumors and and also estrogen receptor rich tissues such as uterus, prostate and bones.

Further studies with animal models with tumors and cell culture experiments should be done in order to explore if 99mTc-DES-P may be used as an imaging agent, early tumor detection and metastasis tumors.