Towards Improving the Efficacy of PSMA-Targeting Radionuclide Therapy for Late-Stage Prostate Cancer—Combination Strategies

The lutetium-177 (¹⁷⁷Lu)-labeled radiopharmaceutical [¹⁷⁷Lu]Lu-PSMA-617 (market name Pluvicto™) has been approved by the US Food and Drug Administration (FDA) [1] and European Medicines Agency (EMA) [2] in 2022 as a last-line therapy for metastatic castration-resistant prostate cancer (mCRPC). Patients can be treated with [¹⁷⁷Lu]Lu-PSMA-617 after progression on anti-hormonal therapy with androgen receptor pathway inhibitors (ARPIs) and taxane chemotherapy. The mode of action of [¹⁷⁷Lu]Lu-PSMA-617 begins with specific binding of the compound to the prostate-specific membrane antigen (PSMA) [3], a transmembrane protein that is overexpressed in prostate cancer with expression levels rising with higher progression [4, 5]. Upon binding, the pharmaceutical is internalized by PSMA-expressing cells leading to its accumulation and dispersion throughout the cytoplasm [6]. The radiation emitted from ¹⁷⁷Lu damages the DNA and other macromolecules, ultimately causing cell death. ¹⁷⁷Lu predominantly emits beta-minus particle radiation which has a relatively low linear energy transfer (LET: amount of energy deposited across a unit of traveled particle track) of 0.2 keV/µm [7]. Still, compared to gamma radiation, beta particles emitted from ¹⁷⁷Lu have very low tissue penetration ability traveling a mean distance of 0.67 mm [8], thereby damaging only tumor cells in their proximity with limited off-target irradiation of neighboring healthy tissues. The radiation causes single-strand breaks (SSBs) and double-strand breaks (DSBs) in the DNA, either by direct ionization of DNA or indirectly via the formation of highly reactive hydroxyl radicals [9, 10]. These properties render beta-emitting radiopharmaceuticals valuable for the specific targeting of extensively metastasized prostate cancer.

The international, randomized, prospective phase III VISION trial completed the clinical testing phase of [¹⁷⁷Lu]Lu-PSMA-617, which led to its FDA and EMA approval for mCRPC. The trial assessed the efficacy of the novel treatment comparing [¹⁷⁷Lu]Lu-PSMA-617 plus standard of care (SOC) and SOC only ([11]; NCT03511664). Permitted SOC included ARPIs (e.g., abiraterone, enzalutamide) and excluded, e.g., chemotherapy and immunotherapy due to unknown risks in combination with [¹⁷⁷Lu]Lu-PSMA-617. In the radioligand therapy (RLT) arm, patients received a median of five cycles, each at a dose of 7.4 gigabecquerel (GBq) at 6-week intervals. The median overall survival (OS) was significantly better in the RLT arm as compared to SOC only (15.3 months vs. 11.3 months). Patients treated with RLT had a median progression-free survival (PFS) of 8.7 months and those treated with SOC 3.4 months. Of note, a substudy of the trial characterized outcomes depending on the patients’ baseline PSMA expression intensity according to positron emission tomography/ computed tomography (PET/CT) imaging. Patients in the highest quartile (whole-body mean standard uptake value (SUV) ≥ 10.2) had a PFS of 14.1 months and those in the lowest quartile (whole body mean SUV < 6.0) 5.8 months. This analysis shows that with low-PSMA expression patients do still benefit from PSMA-targeting RLT with ¹⁷⁷Lu ([¹⁷⁷Lu]Lu-PSMA RLT) [12].

The Australian multicenter, randomized, phase II trial “TheraP” ([13]; NCT03392428) enrolled patients previously receiving docetaxel and in most cases ARPIs. The study compared the efficacy of cabazitaxel, one of the standard subsequent treatment regimens, to [¹⁷⁷Lu]Lu-PSMA-617. RLT was administered every 6 weeks for up to six cycles and cabazitaxel once every 3 weeks for up to ten cycles. The primary endpoint was prostate-specific antigen response (PSA: serum marker for biochemical response) and was defined as the proportion of patients achieving a ≥ 50% PSA decline. The response rates were 66% in the RLT arm and 37% in the cabazitaxel arm.

While [¹⁷⁷Lu]Lu-PSMA-617 represents a highly specific and effective drug with a favorable safety profile, patients eventually relapse. The OS and PFS are prolonged by 4–5 months when compared to SOC. Some patients do not respond in the first place. Therefore, there is a need for improvements that aim at achieving a response in non-responders or overcome acquired resistance. This review compiles recent studies dealing with rescuing efficacy after failure, promising combination therapies for both [¹⁷⁷Lu]Lu-PSMA RLT-naïve and -refractory patients, and treatment options after [¹⁷⁷Lu]Lu-PSMA-617 therapy (Table 1).

Targeted radionuclide therapy with alpha particle emitters might have advantages over therapy with beta emitters under certain circumstances. Alpha emitters have a higher LET of around 80 keV/µm and shorter particle range of up to 100 µm as compared to beta emitters [10]. These properties are associated with induction of a higher number and longer persistence of DNA DSBs and more pronounced cytotoxicity [9, 14]. The highly damaging radiation not only affects tumors but also the physiologically PSMA-expressing salivary glands limiting the applicable dose for alpha therapy [15]. Nevertheless, targeted alpha therapy represents an attractive option for patients that develop resistance to the beta-emitting [¹⁷⁷Lu]Lu-PSMA RLT or are not responsive in the first place.

Several (mostly retrospective) studies assessed the outcomes of sequencing PSMA-targeting RLT with actinium-225 (²²⁵Ac) after failure of [¹⁷⁷Lu]Lu-PSMA RLT or of an administration of both in tandem. Unfortunately, not all studies provided information on the quality of the patients’ responses to prior [¹⁷⁷Lu]Lu-PSMA RLT, the knowledge of which would have been helpful in rating the study outcomes. Three studies [16, 19, 20¹⁷⁷Lu]Lu-PSMA RLT but then developed resistance and those that did not respond from the beginning. The responses were rated based on changes in PSA levels. However, the criteria for an insufficient response, that justified the patients’ eligibility for alpha or tandem therapy, were somewhat different across studies. Therefore, the available information is given for each study presented in the following paragraphs.

When ionizing radiation as monotherapy lacks efficacy, radiosensitizers help to overcome this limitation, ideally, by lowering the threshold for toxicity in cancer cells but not in normal cells. For [¹⁷⁷Lu]Lu-PSMA RLT of mCRPC, radiosensitizers facilitating apoptosis, inhibiting mitosis, and inducing DNA damage are currently under clinical investigation. However, there is hardly any data on the combined application in the [¹⁷⁷Lu]Lu-PSMA-617-refractory setting. Larger studies address the benefits of such combinations in [¹⁷⁷Lu]Lu-PSMA RLT-naïve patients.

The prospective single-center phase I/II LuPIN trial investigated 56 mCRPC patients that were allocated into three groups, each receiving different doses of the radiosensitizer idronoxil together with [¹⁷⁷Lu]Lu-PSMA-617 [40•]. Idronoxil is a derivative of the isoflavone genistein, a compound isolated from soy with anticancer properties. It was found to be cytotoxic to prostate cancer cells triggering apoptosis in vitro [41]. Compared to patients treated with [¹⁷⁷Lu]Lu-PSMA-617 in the VISION trial, patients in the LuPIN trial exhibited a better 50% PSA decline rate (61% vs. 46%) and a better median OS (19.7 months vs. 15.3 months). Adverse effects were mainly of grade 1 and the different doses of idronoxil did not produce significantly different outcomes or toxicity.

A combination of [¹⁷⁷Lu]Lu-PSMA-617 with taxane chemotherapy is also under evaluation. A case study reported the successful use of low-dose docetaxel and [¹⁷⁷Lu]Lu-PSMA RLT in a mCRPC patient [42]. In the mCRPC setting, treatment with docetaxel and beta-emitting RLT was also analyzed in a phase I study that enrolled 15 patients evaluating the PSMA-targeting antibody [¹⁷⁷Lu]Lu-J591 ([43]; NCT00916123). No dose-limiting toxicity was identified. Recently, the LuCAB trial was launched to assess the safety and efficacy of cabazitaxel in addition to [¹⁷⁷Lu]Lu-PSMA-617 in heavily pretreated mCRPC (NCT05340374).

Inhibitors of the enzyme poly(ADP-ribose) polymerase (PARP) are valuable drugs targeting the DNA damage response (DDR) for the treatment of advanced prostate cancer. In particular, phase III studies on the use of PARP inhibitors have been published in mCRPC patients as mono- and combination therapies using olaparib, niraparib, talazoparib, and rucaparib. As of today, only olaparib is approved as mono- and combination therapy with abiraterone by the EMA, and olaparib and rucaparib are approved by the FDA [44‐46]. PARP is involved in the early phase of DNA SSB repair, specifically in damage site recognition and protection of SSBs from cellular nucleases [47, 48]. With PARP inhibited, SSBs can transform to much more toxic DSBs upon initiation of replication. Since ¹⁷⁷Lu-labeled RLT induces SSBs, combining RLT with PARP inhibitors might achieve a synergistic effect on tumor control, especially in DNA repair-deficient prostate cancer. However, data on efficacy of combining the two treatments for prostate cancer are very limited. On preclinical level, PARP inhibitors (olaparib, talazoparib) did not add to the DNA-damaging effect of [¹⁷⁷Lu]Lu-PSMA RLT in vitro. Only veliparib augmented the amount of DNA DSBs [49]. In an in vivo therapy study on PC-3 PIP xenograft-bearing mice, the combination of PARP inhibitors and [¹⁷⁷Lu]Lu-PSMA RLT did not improve tumor control [49]. The study sequenced [¹⁷⁷Lu]Lu-PSMA RLT first, followed by PARP inhibitors. It is possible that a reverse treatment sequence that weakens the DNA SSB repair capacity prior to [¹⁷⁷Lu]Lu-PSMA RLT might have led to a different outcome. On the clinical level, a phase I study combining [¹⁷⁷Lu]Lu-PSMA-617 and olaparib (LuPARP, NCT03874884) for genetically unselected mCRPC is being conducted and is estimated to be completed by December 2023.

Curiously, DNA-damaging pharmaceuticals are being investigated for their potential to modulate PSMA expression levels. High PSMA expression is associated with mutations in DDR genes such as BRCA2 or ATM [5] suggesting a role of PSMA in DNA damage repair. Remarkably, a recent work [39] identified the topoisomerase-2 inhibitors daunorubicin and mitoxantrone as effective inducers of PSMA protein expression in prostate cancer cell cultures and a patient-derived xenograft. These effects, in turn, might potentiate PSMA RLT efficacy.

The efficacy of [¹⁷⁷Lu]Lu-PSMA RLT of mCRPC is influenced by the extent of PSMA expression of tumors. Patients with low PSMA expression in mCRPC are less likely to experience a PSA response after PSMA RLT [25]. In case of low PSMA expression, PFS and OS are still extended more than with SOC [12] but outcomes for these patients might be improved with measures aiming at increasing PSMA levels. PSMA expression is suppressed by androgens and rises upon androgen deprivation [26, 27]. Furthermore, there is evidence suggesting that androgen blockade with ARPIs leads to a PSMA upregulation in mCRPC, specifically [30, 31], which might allow a synergistic tumor control in combination with PSMA RLT. However, evidence of an improved RLT efficacy following pretreatment with ARPIs is lacking so far.

Radiation-mediated cell death can induce tumor immunogenicity due to release of cell debris with tumor antigens and their uptake by antigen presenting cells and presentation to T cells [7]. Furthermore, sublethal irradiation activates the DNA sensing cGAS-STING pathway, which induces immunostimulatory type I interferon production. These radiation-mediated effects might enhance the efficacy of immunotherapy with, e.g., immune checkpoint inhibitors. Direct immunomodulatory effects of focal irradiation with external beam radiotherapy are well described. Emerging preclinical evidence suggests that RLT can also alter the tumor microenvironment and that the combination of RLT and immune checkpoint inhibition has a synergistic inhibitory effect on cancer growth [50, 51].

Czernin et al. [52] showed in a preclinical xenograft mouse model for CRPC that combining [²²⁵Ac]Ac-PSMA-617 and an anti-programmed death 1 (PD-1) antibody provides an advantage for tumor growth control and survival. As an alternative strategy to boost an antitumor immune response, [²²⁵Ac]Ac-PSMA-617 was administered to the same mouse model together with an agonist of the STING system [53]. The combination treatment completely suppressed tumor growth. The authors even reported a persisting immune memory upon rechallenge of the mice with the cancer cells.

Whether [¹⁷⁷Lu]Lu-PSMA RLT can act synergistically with immunotherapy needs to be clarified. Several clinical trials are registered. A phase Ib trial evaluated the safety and potential of a single [¹⁷⁷Lu]Lu-PSMA-617 dose to induce immune priming in order to increase efficacy of pembrolizumab in 18 patients with chemo-naïve mCRPC ([54]; NCT03805594). For the trial, patients were subjected to various sequencing strategies of [¹⁷⁷Lu]Lu-PSMA-617 and pembrolizumab. A complete or partial response was achieved in 8/18 (44%) patients. Radiographic PFS was 6.5 months and a rather low 50% PSA decline rate of 28% was reached. Another phase I/II trial (NCT03658447, PRINCE) evaluated the combined administration of up to six cycles of [¹⁷⁷Lu]Lu-PSMA-617, 6-weekly, and 3-weekly pembrolizumab for up to 35 cycles in 37 mCRPC patients, most of whom have had prior chemotherapy. A 50% PSA decrease was found in 28/37 (76%) patients and an imaging-based partial response in 7/10 (70%). The median radiographic PFS was 11.2 months, respectively [55•]. Of note, the combination treatment was more effective in this small study than the pembrolizumab monotherapy evaluated by the phase II KEYNOTE-199 trial that also included mCRPC patients with a history of chemotherapy (NCT02787005). An objective response rate of 5%, a radiographic PFS of 2.1 months and a 50% PSA decline rate of 7% were reported for patients with measurable disease per RECIST v1.1 after treatment with the monotherapy [56]. A more recently initiated randomized trial tests the combination of ipilimumab and nivolumab plus [¹⁷⁷Lu]Lu-PSMA-617 compared to [¹⁷⁷Lu]Lu-PSMA-617 monotherapy (EVOLUTION, NCT05150236). Unfortunately, so far, the use of pembrolizumab in prostate cancer has led to disappointing results in a series of phase III studies in metastatic hormone-sensitive prostate cancer and different stages of mCRPC. Four large randomized controlled trials combining pembrolizumab with chemotherapy, PARP inhibitors and novel second-generation hormonal agents were closed early for futility.

Combining RLT with ionizing radiation from an external source may provide a benefit with regard to escalation of the radiation dose to the tumor. Since the two sources of radiation have differing organs at risk, their simultaneous application can increase damage to tumor tissue while sparing healthy tissues. A preclinical study in mice bearing androgen-independent PC-3 PIP xenografts found an additive effect of the combination of proton beam radiotherapy and [¹⁷⁷Lu]Lu-PSMA-617 on tumor control [37].

Interestingly, it was shown that exposure to fractionated external beam irradiation increased the protein levels of PSMA in prostate cancer cell cultures and a patient-derived xenograft [38]. Cell lines were both androgen-dependent and -independent and the xenograft was mCRPC-derived. Six fractions of 2 Gray induced PSMA protein expression by approximately two-fold to ten-fold. If such effects can be reproduced in patients, irradiating tumors prior to administration of [¹⁷⁷Lu]Lu-PSMA RLT might enhance the uptake and internalization of the radiopharmaceutical.

Clinical data on the combination of external irradiation and [¹⁷⁷Lu]Lu-PSMA-targeting inhibitors is, so far, very limited. Two case reports exist describing the combined application in heavily pretreated mCRPC patients with brain metastasis [39]. Considering the multitude of tumors in late-stage prostate cancer, the feasibility of external irradiation might be limited to only larger tumors.

[¹⁷⁷Lu]Lu-PSMA RLT has recently been approved as a last-line therapy for mCRPC after failure of ARPIs and chemotherapy with taxanes. Treatment options for non-responders and patients with acquired resistance are, hence, limited. For the post-[¹⁷⁷Lu]Lu-PSMA RLT setting, [²²⁵Ac]Ac-PSMA-617/ [¹⁷⁷Lu]Lu-PSMA-617 tandem therapy represents a promising alternative eventually restoring response. Administration of targeted alpha monotherapy can also rescue the efficacy of RLT in patients refractory to treatment with beta-emitting radioligands. It is important to highlight that half of initial [¹⁷⁷Lu]Lu-PSMA RLT non-responders showed improvements with [²²⁵Ac]Ac-PSMA RLT but the studies that separately report outcomes according to the prior response history are very few, of small size and retrospective. A new, intriguing option for improving [¹⁷⁷Lu]Lu-PSMA RLT in patients naïve to the radiopharmaceuticals might be the combination with the immune checkpoint inhibitor pembrolizumab with its potential for synergistically remodeling the tumor microenvironment. Whether addition of ARPIs to upregulate PSMA expression, specifically enzalutamide, is able to enhance [¹⁷⁷Lu]Lu-PSMA RLT efficacy, needs to be clarified. Since many mCRPC patients are already on continued ARPIs when commencing [¹⁷⁷Lu]Lu-PSMA RLT, the number of patients that could potentially benefit from this strategy is limited. The options for therapeutics to combine with [¹⁷⁷Lu]Lu-PSMA-617 that were investigated or are currently under evaluation in clinical studies are summarized in Fig. 1.