The feasibility of [¹⁸F]EF5-PET/CT to image hypoxia in ovarian tumors: a clinical study

Ovarian cancer (OC) is the most lethal gynecological malignancy, and the majority of patients are diagnosed at an advanced stage [1]. Although OC is initially chemosensitive, most women experience multiple and finally chemoresistant relapses. The survival odds have not markedly improved despite extensive research, and completeness of surgery is still a major prognostic factor [2].

The presence of hypoxic regions in solid tumors is associated with a poor prognosis for many cancer types [3‐5]. Hypoxia-mediated chemoresistance is also the greatest clinical challenge in OC [6, 7].

There is a considerable need for non-invasive imaging of tumor hypoxia since it provides additional information, which could be integrated into strategies of treatment [8]. The method can improve therapeutic outcomes by predicting chemoresistance and selecting potentially treatment-resistant tumors for targeted surgery.

¹⁸F-nitroimidazolpentafluoropropylacetamide ([¹⁸F]EF5) is one of the extensively investigated and clinically tested tracers of tissue hypoxia [9, 10]. ¹⁸F]EF5 belongs to the nitroimidazole group and has considerable membrane permeability and capability to accumulate in viable hypoxic, though not in apoptotic or necrotic cells [11‐13].

Since there is no systematic data evaluating the eligibility of hypoxia-imaging among patients with ovarian malignancy, we conducted the prospective clinical study to evaluate the feasibility of [¹⁸F]EF5-PET/CT scan in identifying hypoxic lesions in ovarian tumors.

Fifteen patients were enrolled. EF5-PET/CT suggested hypoxia in 46% (7/15) of the patients. In those patients the mean HSV was 87 [Cl95% (9.5–238)] cm³ and proportionally 31 [Cl95% (2–47)]% of MTV.

Patients’ clinical and imaging characteristics are presented in Table 1. The distribution of [¹⁸F]EF5-avid lesions was variable. In 7 of the 15 (46%) patients, [¹⁸F]EF5PET/CT was suggestive for hypoxia. All of these patients had [¹⁸F]EF5-accumulation inside the primary tumor and 4 also had [¹⁸F]EF5-avid metastatic lesions. Seven patients had omental metastases, and 3 had [¹⁸F]EF5-accumulation in the omental cake.

All of the [¹⁸F]EF5-avid tumors also had increased uptake of [¹⁸F]FDG; however, only a weak statistical correlation between [¹⁸F]EF5 and [¹⁸F]FDG-uptake in tumors was detected (Spearman’s rho 0.33, p = 0.057). SUVmax values of EF5 and FDG uptake in the malignant primary tumors and metastases of 13 patients are presented in Fig. 1.

Seven patients had FDG-avid peritoneal carcinosis, a finding typical of OC. Only 1 patient had [¹⁸F]EF5-accumulation in the peritoneal carcinosis (thickness < 1 cm). The distribution of the disease, MTV, and HSV values are presented in Fig. 2. None of the patients had [¹⁸F]EF5-avid metastases when the primary tumor PET-finding was not suggestive of hypoxia. There was no statistical difference between [¹⁸F]EF5-uptake in the primary tumor and metastases (p = 0.72).

Our study included two patients with non-malign tumors (patient nr 8 and 11), which presented no [¹⁸F]EF5-accumulation.

The physiological uptake of [¹⁸F]EF5 in the gall bladder/bile, small intestine, and urinary bladder was notably higher than in the tumors (Fig. 3).

A demonstrative EF5- and FDG-PET/CT images of a patient with advanced ovarian cancer are presented in Fig. 4.

Hypoxia is a common phenomenon in cancer with 50–60% of solid tumors containing hypoxic regions [15]. While hypoxia has a well-established role in promoting hematogenous metastases of cancer cells [16], the hematogenous spread is rare in OC at the time of diagnosis [17]. It should also be noted that the role of hypoxia in the transcoelomic spread to the peritoneum and omentum (common to OC) has not been widely investigated. On the basis of our study, [¹⁸F]EF5-PET/CT suggested hypoxia in half of the patients and the distribution of [¹⁸F]EF5-uptake was variable. [¹⁸F]EF5-uptake was detected mainly inside the ovarian tumor and less often in metastases. One preclinical study suggested a hypoxic environment to induce omental/peritoneal metastases [18]. Another study [19] which included two OC patients detected EF5-uptake and severe hypoxia in a peritoneal carcinosis biopsied laparoscopically promptly after the injection of EF5. Our cases with widespread peritoneal carcinosis typically had several [¹⁸F]FDG-avid areas but only one patient had [¹⁸F]EF5-avid peritoneal lesion.

The previous hypoxia imaging studies are conducted mostly on solid and locally advanced tumors [3, 5, 14, 20, 21], while half of our patients had disseminated cancer in the abdominal cavity hampering the comparison of our results to the other studies. However, [¹⁸F]EF5-uptake values in our study were comparable to previously reported [¹⁸F]EF5 SUVmax values in head and neck tumor studies, where the tumor hypoxia was confirmed with immunohistochemical staining [3, 14, 20]. Our study suggests that hypoxia is not a common phenomenon OC in transcoelomic metastases.

According to our study, the feasibility of [¹⁸F]EF5 PET/CT in ovarian cancer imaging in clinical practice appears to be limited. An uptake of [¹⁸F]EF5 in tumors was relatively weak and as the tumors were also [¹⁸F]FDG-avid, the visual estimation of [¹⁸F]-EF5-PET/CT was strongly lead by [¹⁸F]FDG-PET/CT. The avidity to both [¹⁸F]EF5 and [¹⁸F]FDG can be explained with Warburg’s effect [22], where cancer cells rely widely on glycolysis and reduce their respiration regardless of tissue oxygenation level. Unlike OC, head and neck and lung cancer are isolated tumors that are not surrounded by physiologically EF5-affine tissues. Our study is prospective, and two tumors eventually appeared to be benign. Nevertheless, we consider it important to present them especially as they showed no EF5 uptake.

Previously, two excretory paths of highly lipophilic [¹⁸F]EF5-tracer have been demonstrated [23, 24]. In the latter, it was assumed that due to slow tracer biliary excretion, only small amounts of activity would be seen in the small intestine. However, our study revealed an excessive [¹⁸F]EF5-uptake in the bile and small intestine. In contrast to the supradiaphragmatic tumors, this phenomenon imposes limitations on assessing tumors presenting weak [¹⁸F]EF5-uptake. Especially when located near to the intestine, metastases may be easily be mistaken to physiological uptake and remain unnoted.

Non-invasive hypoxia imaging with [¹⁸F]EF5-PET/CT is possible, but its clinical use is restrained by the weak tumor uptake of the tracer compared to the non-specific uptake in excretory organs. The potential usefulness of with [¹⁸F]EF5-PET/CT in OC could be complementary to FDG-PET/CT with the intent to determine high-risk patients. The role of hypoxia in OC is intensively studied and [¹⁸F]EF5-PET/CT forms an attractive tool for patient stratification.