Destruction of contrast microbubbles and the association with inertial cavitation

The destruction of insonified Sonazoid microbubbles and its association with inertial cavitation in vitro utilizing an active acoustic detector was investigated. The experimental observation indicated that contrast microbubbles could be damaged at moderate acoustic pressures of 0.6-1.6 MPa (0.4-1.0 in mechanical index, MI). A damaged bubble could be dissolved into the medium on the order of 1 ms, implying that the destruction at moderate pressures is a relatively slow (relative to inertial bubble collapse), nonviolent dissolution process following the disruption of encapsulating surface materials. Inertial cavitation events in the presence of contrast microbubbles were observed using multiple highly intense ultrasound (US) pulses (>1.6 MPa). This observation suggested that intense US might disintegrate contrast microbubbles, and fragments of disintegrated microbubbles could be activated by an upcoming highly intense imaging pulse. The above results imply that inertial cavitation is unlikely to take place in the presence of Sonazoid contrast microbubbles when exposed to diagnostic US with an MI <1.

Enhanced detection of blood flow in the normal canine prostate using an ultrasound contrast agent

RATIONALE AND OBJECTIVES

To evaluate the sonographic appearance of normal prostate vascularity in dogs before and after injection of a new ultrasound contrast agent, NC100100.

METHODS

Thirty-five intravenous injections of NC100100, in doses ranging from 0.00625 to 0.05 microL microbubbles/kg, were administered to seven anesthetized mongrel male dogs. Transrectal color Doppler imaging and power Doppler imaging were used to perform the assessment. The visibility of the vascular pattern of the prostate was assessed, including dynamics of contrast inflow, blood flow symmetry, and duration times.

RESULTS

Before contrast administration, the vascular pattern was poorly visualized in all cases. After contrast injection, the visibility of the vascular architecture improved significantly for both modalities. Independent of the imaging method used, higher doses tended to be more effective than lower doses. Contrast kinetics in the prostate vessels was demonstrated with a mean time from injection of the ultrasound contrast agent to enhancement of the Doppler signals in the subcapsular arteries (+/-1 SD) of 13+/-3 seconds, and the ultrasound contrast agent reached the central periurethral veins 3 to 6 seconds later. A spokelike radial pattern of internal prostatic vessels observed with enhanced ultrasound could also be seen on silicone microfil x-ray images. The Doppler enhancement persisted for a mean time ( +/-1 SD) of 904 seconds (approximately 15 minutes) +/- 225 seconds and tended to increase with increasing dose.

CONCLUSIONS

NC100100 significantly improves the detection of blood flow in the normal canine prostate and allows more accurate depiction of the vascular architecture of the prostate.

Factors Influencing the Therapeutic Efficacy of the PSMA Targeting Radioligand 212Pb-NG001

Simple Summary

Prostate-specific membrane antigen (PSMA) is a protein overexpressed in metastatic castration-resistant prostate cancer and a promising target for targeted radionuclide therapy. PSMA-targeted alpha therapy is of growing interest due to the high-emission energy and short range of alpha particles, resulting in a prominent cytotoxic potency. This study assesses the influence of various factors on the in vitro and in vivo therapeutic efficacy of the alpha particle generating PSMA-targeting radioligand ²¹²Pb-NG001.

Abstract

This study aimed to determine the influence of cellular PSMA expression, radioligand binding and internalization, and repeated administrations on the therapeutic effects of the PSMA-targeting radioligand ²¹²Pb-NG001. Cellular binding and internalization, cytotoxicity, biodistribution, and the therapeutic efficacy of ²¹²Pb-NG001 were investigated in two human prostate cancer cell lines with different PSMA levels: C4-2 (PSMA+) and PC-3 PIP (PSMA+++). Despite 10-fold higher PSMA expression on PC-3 PIP cells, cytotoxicity and therapeutic efficacy of the radioligand was only 1.8-fold better than for the C4-2 model, possibly explained by lower cellular internalization and less blood-rich stroma in PC-3 PIP xenografts. Mice bearing subcutaneous PC-3 PIP xenografts were treated with 0.2, 0.4, and 0.8 MBq of ²¹²Pb-NG001 that resulted in therapeutic indexes of 2.7, 3.0, and 3.5, respectively. A significant increase in treatment response was observed in mice that received repeated injections compared to the corresponding single dose (therapeutic indexes of 3.6 for 2 × 0.2 MBq and 4.4 for 2 × 0.4 MBq). The results indicate that ²¹²Pb-NG001 can induce therapeutic effects at clinically transferrable doses, both in the C4-2 model that resembles solid tumors and micrometastases with natural PSMA expression and in the PC-3 PIP model that mimics poorly vascularized metastases.

Imaging of 212Pb in mice with a clinical SPECT/CT

INTRODUCTION

212Pb is a promising radionuclide for targeted alpha therapy. Here, the feasibility of visualising the tumour uptake and biodistribution of 212Pb-NG001 in mice with a clinical SPECT/CT scanner was investigated.

METHODS

A mouse phantom with 212Pb was imaged with a clinical- and a preclinical SPECT/CT scanner. Different acquisition and reconstruction settings were investigated on the clinical system (Siemens Symbia Intevo Bold). Two athymic nude mice carrying PC-3 PIP prostate cancer tumours of 235-830 μl received 1.44 MBq of 212Pb-NG001 and were imaged 2, 6, and 24 h post-injection on the clinical SPECT/CT with a Medium Energy collimator and a 40% energy window centred on 79 keV. All acquisition times were 30 min, except the mouse imaging 24 h post-injection which was 60 min. After the final imaging, the organs were harvested and measured on a gamma counter to give an indication of how much activity was present in organs of interest at the last imaging time point.

RESULTS

Four volumes in the mouse phantom of ~ 300 μl with 246-303 kBq/ml of 212Pb were distinguishable on images acquired with the clinical SPECT/CT with a high number of reconstruction updates. With the preclinical SPECT, the same volumes were easily distinguished with 49 kBq/ml of 212Pb. Clinical SPECT/CT images of the mice revealed uptake in tumours and bladders 2 h after injection and in tumours containing down to approximately 15 kBq/ml at 6 and 24 h after injection.

CONCLUSION

Although the preclinical scanner should be used preferentially in biodistribution studies in mice, the clinical SPECT/CT confirmed uptake in small volumes (e.g. ~ 300 μl volume with ~ 250 kBq/ml). Regardless of system, the resolution and sensitivity limits should be carefully determined, otherwise false negative or too low uptakes can be wrongly interpreted.

Targeted alpha therapy with the 224Ra/212Pb-TCMC-TP-3 dual alpha solution in a multicellular tumor spheroid model of osteosarcoma

Osteosarcoma patients with overt metastases at primary diagnosis have a 5-year survival rate of less than 20%. TP-3 is a murine IgG2b monoclonal antibody with high affinity for an epitope residing on the p80 osteosarcoma cell surface membrane antigen. The tumor-associated antigen p80 is overexpressed in osteosarcomas, and has very low normal tissue expression. We propose a novel dual alpha targeting solution containing two radionuclides from the same decay chain, including the bone-seeking ²²⁴Ra, and cancer cell-surface seeking ²¹²Pb-TCMC-TP-3 for the treatment of osteoblastic bone cancers, circulating cancer cells and micrometastases. In this in vitro study, the cytotoxic effects of ²¹²Pb-TCMC-TP-3 (single alpha solution) and ²²⁴Ra/²¹²Pb-TCMC-TP-3 (dual alpha solution) were investigated in a multicellular spheroid model mimicking micrometastatic disease in osteosarcoma. OHS spheroids with diameters of 253 ± 98 μm treated with 4.5, 2.7, and 3.3 kBq/ml of ²¹²Pb-TCMC-TP-3 for 1, 4, and 24 h, respectively, were disintegrated within 3 weeks. The ²¹²Pb-TCMC-TP-3 induced a 7-fold delay in spheroid doubling time compared to a 28-times higher dose with the non-specific ²¹²Pb-TCMC-rituximab. The ²²⁴Ra/²¹²Pb-TCMC-TP-3 completely disintegrated spheroids with diameters of 218-476 μm within 3 and 2 weeks after 4 and 24 h incubation with 5 kBq/ml, respectively. Treatment with 1 kBq/ml of ²²⁴Ra/²¹²Pb-TCMC-TP-3 for 24 h caused an 11.4-fold reduction in spheroid viability compared with unconjugated ²²⁴Ra/²¹²Pb. The single and dual alpha solutions with TP-3 showed cytotoxicity in spheroids of clinically relevant size, which warrant further testing of the dual alpha solution using in vivo osteosarcoma models.