Biodistribution and Semiquantitative Analysis of ^99mTc-HYNIC-PSMA-11in Prostate Cancer Patients: A Retrospective Study

Abstract

Aim:

To emphasize the feasibility of imaging prostate cancer patients under SPECT/CT with ^99mTc-HYNIC-PSMA-11 and evaluate the possibility in clinical application for screening patients for radioligand therapy, with the help of SUV-based biodistribution mapping.

Introduction:

In context to prostate specific membrane antigen’s (PSMA) overexpression in prostate cancer cells there has been a gradual increase in imaging prostate cancer, currently the second leading cause of cancer deaths in males and the sixth cause of cancer death worldwide, with the help of radiolabeled PSMA small molecule inhibitors over the past few years. Of the two modalities under Nuclear Medicine facility (positron emission tomography/CT [PET/CT] and single photon emission CT [SPECT/CT]), though PET/CT has high sensitivity for tumor sites, even at low PSA level less however, the availability of 18F based radiotracer for PET imaging necessitates the dependency on a cyclotron in the near vicinity for production or regular supply. Another option of Germanium 68/ Gallium 68 generator (⁶⁸Ge/⁶⁸Ga generator) can be explored but the cost involved becomes a limitation. These challenges led to the exploration of an alternative to help cater the needs of society with ^99mTc labeled PSMA inhibitor, which is very easily available and a cost effective solution for imaging prostate cancer workup.

Material and Method:

A retrospective analysis was conducted on 39 patients who had undergone ^99mTc-HYNIC-PSMA-11 study over a period of 6 months. The patients were further categorized based on spread of disease and divided in two categories: localized disease and oligometastatic disease. Imaging was done under Discovery NM/CT 670 DR model SPECT/CT. The planar and SPECT/CT images were analyzed using Xeleris DR Functional imaging (Version 4.1) workstation under Q. Volumetric MI evolution for oncology software and maximum and mean standardized uptake value (SUV_max, SUV_mean) were determined on skeletal and soft tissue regions for both the categories (localized and oligometastatic). Further, statistical analysis was conducted through an independent samples t-test to find the significance, if any.

Results:

The biodistribution of ^99mTc- HYNIC-PSMA-11 showed high uptake in parotid gland, submandibular gland, kidneys and urinary bladder, of which kidneys showed the highest uptake in all the scans. The SUV_max and SUV_mean comparison between localized and oligometastatic condition for various body regions did not show any statistically significant differences as indicted by the p-value.

Conclusion:

The biodistribution of ^99mTc- HYNIC-PSMA-11 is similar to 68Ga-PSMA-11 scan, hence can be used as a cost-effective alternate for imaging overexpression of PSMA in case of prostate cancer patients.

Introduction

Prostate cancer is currently the second leading cause of cancer deaths in males and the sixth cause of cancer death worldwide.[1,2,3] In spite of technical advancement in conventional imaging computed tomography (CT) and magnetic resonance imaging (MRI), there is limited sensitivity in the evaluation of recurrence cases, especially with low prostate-specific antigen (PSA) level and in cases of biochemical relapse.[4] In addition to this, CT does not help in predicting the outcome till at least 12 weeks after therapy.[5] However, in the recent years, molecular imaging with radiotracers under nuclear medicine facility is being used for targeting various aspects of tumor biology in vivo.[4] In context to this, prostate-specific membrane antigen’s (PSMA) overexpression in prostate cancer cells has gained special interest, and there has been a gradual increase in imaging prostate cancer with the help of radiolabeled PSMA small molecule inhibitors over the past few years.[6] PSMA is a transmembrane type II glycoprotein, consisting of 750 amino acids. It is known as folate hydrolase I or glutamate carboxypeptidase II also. It is expressed virtually by all prostate cancer and is considered to be an ideal biomarker for prostate cancer.[7,8,9,10,11] The normal physiological expression of PSMA is seen in the prostate, kidney, proximal small intestine, and salivary glands but is significantly overexpressed (100–1000 times) in prostate cancer tissues in comparison to benign prostate cells or normal tissue.[4,12,13,14] It is this overexpression, which has been researched and exploited for imaging and therapeutic outcomes to great success.[15,16,17,18,19] Radiolabeling with alpha and beta emitters has paved a new path for treatment, thus benefiting patients with prostate cancer.[20,21] Of the two modalities under nuclear medicine facility (Positron emission tomography/computed tomography (PET/CT) and Single photon emission computed tomography/computed tomography (SPECT/CT)), PET/CT has high sensitivity for tumor sites, even at low PSA level less than 1 ng/mL, and currently, there are five food and drug administration approved PET radiotracers as shown in Table 1.[4] However, the availability of ¹⁸F-based radiotracer for PET imaging necessitates the dependency on a cyclotron in the near vicinity for production or regular supply.[14] Another option could be procurement of long-lived Germanium-68/Gallium-68 generator (⁶⁸Ge/⁶⁸Ga generator) which could serve for 8–9 months, but at a price of approximately 4 million INR annually, including the consumables; second, there is limited radiotracer produced per elution. Both these factors limit it optimum utilization and increase the waitlist in high volume centers like ours. These challenges led to the exploration of an alternative to help cater the needs of society with ^99mTc-labeled PSMA inhibitor, which is very easily available and a cost-effective solution for imaging prostate cancer workup. Till date, many PSMA inhibitors have been radiolabeled with ^99mTc: ^99mTc-MIP-1404, ^99mTc-MIP 1405, ^99mTc-PSMA-IandS, and ^99mTc-EDDA/HYNIC-iPSMA.[22,23,24,25] We at our institute utilized HYNIC-PSMA-11 for radiolabeling with ^99mTc for imaging of prostate cancer patients and understanding its biodistribution in terms of standardized uptake value (SUV) in histologically confirmed prostate cancer patients retrospectively.

Table 1.

Food and Drug Administration-approved positron emission tomography radiotracers and its target

PET tracer	Target
68Ga PSMA ligands	PSMA
18F-FDG	Glucose metabolism
11C choline/18F choline	Cell membrane metabolism
18F sodium fluoride	Osteoblastic activity
18F fluciclovine	Amino acids

PET: Positron emission tomography, PSMA: Prostate-specific membrane antigen, FDG: Fluorodeoxyglucose

Materials and Methods

A retrospective analysis was conducted on 39 patients who had undergone ^99mTc-HYNIC-PSMA-11 study over a period of 6 months (June 2021 to November 2021). Out of 39 patients who had undergone SPET/CT imaging, 27 were newly diagnosed with prostate cancer and 12 were postoperative, of which one had undergone chemotherapy, 7 underwent hormonal therapy, and 4 underwent bilateral orchiectomy. The distribution is shown in Figure 1a and and Supplementary Table 1. The patients were further categorized based on the spread of disease and divided into two categories: localized disease and oligometastatic disease (presence of three to five metastatic lesions.[26,27,28,29,30,31] The study was approved by the ethical committee of our Institute (Project No 11000559, approval dated 11/05/22). The clinical characteristic of all patients is shown in Table 2. The radiotracer was prepared as per manufacturer’s instruction. The cold kit was stored at −20°C before usage. For radiolabeling process, briefly, sterile, nonpyrogenic sodium pertechnetate solution (0.5 mL), containing maximum of 45 millicurie (mCi) of ^99mTcO^4-activity was injected in the kit vial containing 25 microgram (μg) lyophilized powder of HYNIC-PSMA-11 and an equal volume of air withdrawn. Thereafter, vial was removed from the lead shield and immersed in water bath in vertical position for 10 min. Care was taken that the boiling water did not contact the aluminum cap of the vial. After the end of this process, vial was removed from water bath and placed in lead shield for 15 min to cool down. Quality control of the product was conducted with the help of instant thin layer chromatography (ITLC) on silica gel using different mobile phases: 0.1M sodium citrate (pH 5) was used as a solvent to determine the nonpeptide bound ^99mTc co-ligand and ^99mTcO^4-(retardation factor-R_f = 1), the mixture of methanol/1M ammonium acetate as a solution was used in a volume ratio of 1:1 for ^99mTc colloid (R_f = 0) detection. The percent of bound radiocomplex was calculated using the formula mentioned below:

Figure 1.

(a) Patient distribution, (b) standardized uptake value calculation prerequisites

Supplementary Table 1.

Patient distribution

Category	Number of patients
Newly diagnosed	27
Postoperative	12
Previously treated	8 (1-chemotherap;7 -hormonal therapy;4-bilateral orchidectomy)

Table 2.

Clinical characteristics of patients

Patient characteristics	Age (years)	Height (cm)	Weight (kg)	PSA level, ng/ml	Preinjected dose (mCi)	Postinjected dose (mCi)	Administered dose (mCi)
Mean±SD	67.1±9.13	165.7±8.98	67±12.50	42.80±64.30	16.32±2.65	0.57±0.25	15.75±2.62
Median (IQR)	66 (60–74.5)	166.5 (159.25–172.75)	70 (57.5–75)	15.93 (4.72–47)	16 (14.55–17.85)	0.53 (0.35–0.75)	15.65 (14.07–17.28)
Range	52–84	140–179	40–97	0.08–285	11.6–22.5	0.19–1.02	11.2–21.5

SD: Standard deviation, IOR: Interquartile range, PSA: Prostate-specific antigen

ITLC silica gel/0.1 M sodium citrate (pH = 5)

% ^99mTc co-ligand and ^99mTcO^4- = counts in solvent front ×100/total Counts [Equation-1]

ITLC silica gel/methanol: 1M ammonium acetate (1:1)

% ^99mTc-TcO₂ = counts at origin front × 100/total counts [Equation-2]

^99mTc-HYNIC-PSMA-11 = 100-(% ^99mTc co-ligand and ^99mTcO^4-+ % ^99mTcO₂) [Equation-3]

Imaging was done under the Discovery NM/CT 670 DR model SPECT/CT. Whole-body planar images were acquired 3 h’ postinjection, followed by a SPECT/CT (vertex to mid-thigh). The acquisition parameters were as mentioned in Table 3. SPECT components were calibrated monthly using standard ⁵⁷Co source for quantitative imaging. The planar and SPECT/CT images were analyzed using Xeleris DR Functional imaging (Version 4.1) workstation under Q. Volumetric MI evolution for oncology software and maximum and mean standardized uptake value (SUV_max, SUV_mean) were determined on skeletal (skull, cervical, scapula, humerus, ribs, sternum, thoracic, lumbar, pelvis, and femur) and soft tissue components (brain, parotid, submandibular, mediastinal blood pool, liver, spleen, small bowel, kidneys, urinary bladder, and prostate) by drawing a spherical voxel of interest (diameter of 10 mm) on different areas. The uptake intensity value was based on the measured activity concentration of tissues, normalized by patient weight and injected activity, as shown in Figure 1b. SPECT data were reconstructed using ordered subset expectation maximization with necessary correction (motion, scatter, attenuation, and resolution recovery). The SUV_max and SUV_mean were compared between localized and oligometastatic conditions for various body regions using an independent samples t-test. The statistical test was considered to be statistically significant if P < 0.05.

Table 3.

Parameters

Acquisition parameters	Specification
Planar
Region	Vertex to toe
Start position	H
Patient location	Feet first supine
Body contour	on
Mode	Continuous
Exposure time per pixel	100 s
Pallet velocity	24 cm/min
Zoom	0.92
Emission parameters
Patient location	Feet first supine
Overlap	6.19 cm
View angle	6°
CT range	Full
Table motion	Step and shoot
Matrix size	128×128
Zoom	1
Mode	Step and shoot
Time per step	10 s
Energy session	Tc99m SC (140.5 and 120)
Collimator	LEHR
COR correction	Yes
Uniformity map	Tc99m
Energy window	140.5%±10% keV
Scatter window	120%±5% keV
CT parameters
Scan type	Helical
Voltage	120 kV
Current	Auto mA
Minimum current	100 mA
Maximum current	350 mA
Noise index	12.35
Slice thickness	3.75 mm
SFOV	Large
DFOV	50
Matrix	512

CT: Computed tomography, SFOV: Scan field of view, DFOV: Display field of view, LEHR: Low energy high resolution, COR: Centre of rotation

Results

We detected twenty-one prostate cancer patients with localized disease and eighteen with oligometastatic disease using ^99mTc-HYNIC-PSMA-11, SPECT/CT imaging. Patients had a mean age of 67.1 ± 9.13 years, height of 165.7 ± 8.98 cm, and weight of 67 ± 12.50 kg. PSA Level mean value was 42.80 ± 64.30. The preinjected dose, postinjected dose, and net administered dose were in the range of 16.32 ± 2.65 mCi (603.84 ± 98.05MBq), 0.57 ± 0.25 mCi (21.09 ± 9.25MBq), and 15.75 ± 2.62 mCi (582.75 ± 96.94 MBq), respectively. The percentage purity of the prepared radiopharmaceutical was 96.76 ± 1.38 as evaluated from the ITLC graph [Figure 2a and b]. The biodistribution of ^99mTc-HYNIC-PSMA-11 showed high uptake in the parotid gland, submandibular gland, kidneys, and urinary bladder, of which kidneys showed the highest uptake in all the scans. Other organs, bowel, liver, and spleen showed relatively low level of uptake, and brain showed almost negligible activity. The SUV_max and SUV_mean comparison between localized and oligometastatic conditions for various body regions did not show any statistically significant differences as indicated by P value [Table 4]. Figure 3a and b demonstrate the CT, SPECT, fused SPECT/CT images and maximum intensity projection image of ^99mTc-HYNIC-PSMA-11 scan. [Figures 4a,b and 5a,b] shows the distribution of SUV_max and SUV_mean values for localized and oligometastatic patients on different skeletal and soft tissue areas.

Figure 2.

(a and b) Instant thin layer chromatography under 0.1M sodium citrate and under methanol/1M ammonium acetate, respectively

Table 4.

The standardized uptake value_max and standardized uptake value_mean values for localized and oligometastatic conditions in skeletal and soft tissue regions

Body regions	Localized		Oligometastasis		P
	SUVmax	SUVmean	SUVmax	SUVmean	SUVmax	SUVmean
Skeletal
Skull	1.23±0.74	0.50±0.24	1.29±0.56	0.55±0.24	0.751	0.502
Cervical	1.60±0.81	0.98±0.50	1.63±0.87	0.94±0.58	0.918	0.833
Scapula	1.06±0.56	0.43±0.22	1.53±0.88	0.53±0.34	0.060	0.317
Humerus	0.56±0.30	0.18±0.13	0.66±0.34	0.20±0.08	0.295	0.628
Ribs	1.39±0.82	0.56±0.31	1.81±1.53	0.63±0.32	0.313	0.475
Sternum	1.19±0.35	0.42±0.17	1.19±0.45	0.45±0.16	0.971	0.601
Thoracic	1.16±0.54	0.52±0.28	1.31±0.52	0.56±0.30	0.382	0.632
Lumbar	1.30±0.38	0.58±0.20	1.30±0.64	0.55±0.24	0.982	0.629
Pelvis	1.45±0.95	0.45±0.19	1.66±1.16	0.46±0.31	0.549	0.894
Femur	0.82±0.53	0.34±0.22	0.69±0.40	0.28±0.19	0.387	0.365
Soft tissue
Brain	0.46±0.38	0.14±0.13	0.42±0.27	0.12±0.09	0.743	0.508
Parotid	21.7±8.77	15.8±6.29	21.1±7.55	15.5±6.04	0.812	0.870
Submandibular	18.8±6.97	11.5±4.70	20.3±8.57	12.8±5.80	0.560	0.449
Mediastinal blood pool	2.26±0.92	1.28±0.96	2.63±1.04	1.13±0.53	0.754	0.535
Liver	5.51±2.42	3.81±1.90	5.94±2.40	3.99±1.83	0.582	0.774
Spleen	6.48±3.19	4.30±2.38	7.63±3.47	4.74±2.23	0.291	0.551
Small bowel	9.85±5.56	5.79±3.51	9.81±5.68	5.76±4.30	0.984	0.986
Kidneys	46.3±22.5	32.7±15.3	45.5±21.1	32.7±15.9	0.907	0.998
Urinary activity	58.6±55.1	43.2±40.2	47.8±23.5	34.4±18.9	0.422	0.381
Prostate	22±14	9.68±7.64	29.7±24.9	19.9±16.6	0.279	0.061

SUV: Standardized uptake value

Figure 3.

(a and b) Computed tomography (CT), single-photon emission CT (SPECT), fused SPECT/CT, and MIP images of ^99mTc-HYNIC-PSMA-11 scan

Figure 4.

(a and b) The distribution of maximum standardized uptake value and mean standardized uptake value values separately for localized disease and oligometastatic disease, on different skeletal regions: (1) skull; (2) cervical; (3) scapula; (4) humerus; (5) ribs; (6) sternum; (7) thoracic; (8) lumbar; (9) pelvis; (10) femur

Figure 5.

(a and b) The distribution of maximum standardized uptake value and mean standardized uptake value values for localized disease and oligometastatic disease on different soft tissue area: (1) brain; (2) parotid; (3) submandibular; (4) mediastinal blood pool; (5) liver; (6) spleen; (7) small bowel; (8) kidneys; (9) urinary activity; (10) prostate

Discussion

The high expression of PSMA in cases of prostate cancer has paved the pathway for radiolabeling PSMA inhibitors as a promising molecular imaging agent for prostate cancer patients.[32,33] Both PET and SPECT based PSMA inhibitors have been radiolabeled in nuclear medicine for imaging of prostate cancer patients. Coming to PET/CT, two main radiopharmaceuticals have been exploited to a greater extent: ¹⁸F-PSMA-1007 and ⁶⁸Ga‐PSMA‐11. Each has its advantages and disadvantages. ¹⁸F-PSMA-1007 comes with an advantage of being injected in higher quantities, giving a high target to background ratio in comparison to ⁶⁸Ga‐PSMA‐11 and having hepatic clearance, which helps in better diagnostic efficacy of local recurrence in the bladder area, but is confined only to centers near medical cyclotron facility for easy availability.[34,35,36] On the other hand, ⁶⁸Ga-based radiopharmaceuticals can be easily prepared in-house once a generator (approximately 4 million INR annually) is procured and can serve for a continuous period of 8–9 months, with multiple elution per day, however has renal clearance and short half-life of 68 min.[34] To have a balanced approach of image quality (high target to background ratio) and affordable cost (in comparison to ⁶⁸Ga generator), ^99mTc-based radiopharmaceutical serves to be one of the best solutions in spite of its renal clearance, which can be taken care due to delayed imaging postinjection. Confining only to PSMA inhibitors with SPECT-based radiotracers, many radiotracers have been evaluated in the recent past and found be very sensitive in detecting PSMA-positive lesions.[22,24,32,37,38,39] Indeed, ⁶⁸Ga-PSMA imaging is now considered the gold standard molecular imaging method for prostate cancer management,[40] and several international guidelines recommend PSMA PET/CT for patients with biochemical relapse after primary therapy;[4,14] nevertheless, ^99mTc-based PSMA imaging has shown to be helpful for disease work at a reduced cost for prostate cancer in comparison to PET-based radiotracers.[41] Moreover, it can help in the evaluation of visceral and bony lesions both in one single study.[42,43] This can be a solution to most of the national and international treatment guidelines, wherein bone imaging has been incorporated for prostate cancer patient workup.[44,45,46,47] This single imaging can avoid multiple study and be one-stop solution. This property was exploited by Su et al., wherein the study reported the superiority of ^99mTc-PSMA SPECT/CT imaging in the detection of bone metastases in comparison to bone scan and MRI.[38] Another added benefit is the reduced radiation exposure in a single SPECT/CT imaging with ^99mTc-labeled PSMA inhibitors in comparison with PET imaging agents or multiple radiotracer studies. The retrospective analysis conducted at our end on ^99mTc-HYNIC-PSMA-11 imaging showed the biodistribution pattern in consensus with the study conducted by Zhang et al.[48] Secondly, the images obtained, give confidence in using the SPECT radiotracer as a potential substitute of ⁶⁸Ga-PSMA PET/CT.[43,49] The study analyzed at our end was not compared with any other study (conventional or SPECT/PET based radiotracer); however, earlier studies with ^99mTc-PSMA inhibitor imaging, like the one conducted by Albalooshi et al.,[43] reported equal sensitivity of ⁶⁸GaPSMA PET/CT and ^99mTc PSMA SPECT/CT in the detection of bone metastases. In addition to this, the SUV values calculated on our study can give an additional information/risk stratification on the aggressiveness of the disease, as has been reported by Li et al.,[14] wherein the SUV_max value showed positive correlation with increasing PSA level and hence the detection rate. We do accept the improved resolution of PET/CT in comparison to SPECT/CT, especially to lesions in the prostate bed, in multiregional metastatic cases; however, the patient management does not change in such scenario, as reported by Fallahi et al.[49] Second, in comparison to PET/CT based study, ^99mTc based study (with half-life of 6 h) has an advantage of being done with a delay of at least 3 h, to help increase the target to background ratio and overcome its limitation.[50] Hence, we foresee a wider potential in developing country like ours, wherein the cost of setting up a PET/CT facility, with its dependence on regular availability of medical cyclotron-produced radiopharmaceutical or supply of high cost ⁶⁸Ge/⁶⁸Ga generators are a financial limitation. Second, SPECT/CT and its radiopharmaceuticals can indeed be a low-cost solution for investigating prostate cancer patients, making the small- and medium-sized medical institutions viable.[22,37,38]

Limitation

The study did not include widespread metastatic disease patients. Second, it is a retrospective analysis done at a single center and lacking histological validation of SPECT-positive lesions. A prospective study with large sample size should be done, and one can evaluate the relationship of PSA score with ^99mTc-HYNIC-PSMA-11 imaging if any.

Conclusion

The biodistribution of ^99mTc-HYNIC-PSMA-11 is similar to ⁶⁸Ga-PSMA-11. Hence, ^99mTc-HYNIC-PSMA-11 scan can be used as a cost-effective alternative for imaging overexpression of PSMA in case of prostate cancer patients, especially in institutes having only SPECT/CT or facilities devoid of in-house gallium generator. Second, ^99mTc-HYNIC-PSMA-11 can also be used to evaluate the eligibility of patients for radioligand therapy.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.