SPECT/CT using [99mTc]-labeled anti-programmed death-ligand 1 (PD-L1) single-domain antibody (NM-01) to predict response to immune checkpoint inhibition in non-small cell lung cancer: preliminary results from the PD-L1 Expression in Cancer (PECan) study.

Introduction: Immune checkpoint inhibition with anti-programmed cell death protein 1 (PD-1) or programmed death-ligand 1 (PD-L1), alone or in combination with chemotherapy, is a standard of care in the management of non-small cell lung cancer (NSCLC). PD-L1 expression by immunohistochemistry (IHC) is an imperfect predictive biomarker and temporospatial heterogeneity is well documented. [^99mTc]-labeled anti-PD-L1 single-domain antibody (NM-01) single-photon emission computed tomography (SPECT) has been shown to correlate with PD-L1 expression by IHC. We present preliminary results of the PECan study (NCT04436406), in which we are assessing the relationship between PD-L1 expression measured by [^99mTc]NM-01 SPECT/CT and metabolic response to anti-PD-(L)1 therapy (determined by [¹⁸F]FDG-PET/CT in NSCLC).

Methods: Thoracic [^99mTc]NM-01 SPECT/CT was performed at 2h post injection of 740MBq [^99mTc]NM-01 before and after 9-weeks of anti-PD-1 pembrolizumab, alone or in combination with chemotherapy, in patients with advanced NSCLC. Tumor to blood pool ratio (T:BP) ROImax measurements were performed in primary and metastatic lesions. Intertumoral heterogeneity of PD-L1 expression was measured on [^99mTc]NM-01 SPECT/CT at baseline, defined as ≥50% difference in T:BP between primary and individual metastatic sites. PD-L1 tumor proportion score (TPS) by IHC was performed using diagnostic sampling with the Ventana PD-L1 SP263 assay.

The primary objective was to determine the relationship of baseline PD-L1 expression (measured by [^99mTc]NM-01 SPECT/CT and IHC) with metabolic response (defined as ≥30% decrease in SUVmax between baseline and 9-week [¹⁸F]FDG-PET/CT). Changes in PD-L1 expression between baseline and 9-week [^99mTc]NM-01 SPECT/CT were also correlated with [¹⁸F]FDG-PET/CT response.

Results: Ten patients (median 64 years; 6 male) with advanced NSCLC, scheduled for treatment with PD-L1 immunotherapy +/- combination chemotherapy, were included. Intertumoral heterogeneity of PD-L1 expression measured on baseline [^99mTc]NM-01 SPECT/CT was present in 6/10 patients.

Response to treatment as assessed by change in [¹⁸F]FDG-PET/CT SUVmax at 9-weeks (n=7) correlated with high baseline PD-L1 expression measured by [^99mTc]NM-01 T:BP (r=-0.71, p=0.037), but not with TPS measured by IHC (r=0.003, p=0.498). Primary tumor baseline [^99mTc]NM-01 T:BP ≥4.0 predicted 9-week [¹⁸F]FDG-PET/CT SUVmax response with 100% sensitivity and specificity. Metastasis [^99mTc]NM-01 T:BP ≥2.65 predicted metabolic response with 72% sensitivity and 100% specificity.

16/18 (89%) of the primary or metastatic lesions that showed [¹⁸]FDG-PET/CT response were associated with stable (n=4) or decreased (n=12) PD-L1 expression measured by change in [^99mTc]NM-01 T:BP following 9-weeks of anti-PD-L1 therapy.

Conclusions: In this preliminary analysis, baseline intertumoral heterogeneity of PD-L1 expression measured by [^99mTc]NM-01 SPECT/CT was frequently demonstrated in patients with NSCLC. Pretreatment [^99mTc]NM-01 SPECT/CT was able to predict early metabolic [¹⁸F]FDG-PET/CT response to anti-PD-1 therapy with high specificity. PD-L1 expression, measured by [^99mTc]NM-01 following anti-PD-1 therapy, demonstrated stability or a reduction in the majority of responding lesions.

[^99mTc]NM-01 SPECT/CT therefore has potential to add to immunotherapy management in NSCLC, with further investigation and studies ongoing.