Effects of erlotinib therapy on tumor tracer uptake
The present study demonstrated that tumor [11C]erlotinib V
T decreases significantly during erlotinib therapy.
To our best knowledge, this is the first clinical study that investigated the change of radiolabeled EGFR TKI uptake in patients off and on treatment using the same EGFR TKI. In the presence of therapeutic concentrations of erlotinib, tumor [
11C]erlotinib uptake decreased. This was presumably caused by occupancy of EGF receptors by abundantly present non-labeled erlotinib, i.e., due to a decrease in available binding sites. Blocking studies in xenograft models provide support for this mechanism. Using [
11C]erlotinib, Petrulli et al. showed that NSCLC xenografts with activating EGFRmut (HCC827) in mice had lower tracer uptake when cold erlotinib was given along with the tracer [
16]. In addition, Abourbeh et al. showed in mice-bearing HCC827 xenografts more than 50 % reduction in tumor [
11C]erlotinib uptake after administration of excess non-labeled erlotinib [
17]. Similar results were obtained with other radiolabeled TKI, such as [
18F]afatinib [
18] and [
11C]PD153035 [
19]. The fact that there was consistent decrease in [
11C]erlotinib uptake in the present study supports the notion that uptake of [
11C]erlotinib is, at least in part, due to specific binding. Furthermore, in the presence of therapeutic concentrations of erlotinib, obtained by taking a fixed oral dose of 150 mg erlotinib daily, there was still residual tumor tracer uptake. Interestingly, from a pharmacokinetic perspective, this may indicate that there may be room for increasing the therapeutic concentration of erlotinib, as at maximal concentration, the specific binding would be absent.
Erlotinib therapy is known to induce metabolizing enzymes, such as CYP1A, CYP3A4, and CYP3A5 [
20]. Also, in vitro data suggest that erlotinib stimulates the metabolism of midazolam in human microsomes, suggesting that erlotinib could induce its own metabolism and thus also increase the clearance of [
11C]erlotinib [
21]. However, this was not observed in the present study. On the contrary, parent fractions at 60 min post injection were significantly higher during erlotinib therapy. Possibly, the presence of abundant non-labeled erlotinib also saturated the metabolizing enzymes, thereby slowing down the metabolism of [
11C]erlotinib. Moreover, patients on therapy had higher blood activity concentrations, normalized to injected dose and patient weight. This may also be caused by higher concentrations of circulating parent tracer due to the blocking of receptors and enzymes by high concentrations of non-labeled erlotinib.
Among the nine evaluable patients, two were scanned first under erlotinib therapy and stopped therapy immediately thereafter. In these two patients, the abovementioned findings were also true, i.e., V
T(E+) was lower than V
T(E−) and metabolites(E+) were lower than metabolites(E−). This supports the notion that the presence of non-labeled erlotinib determined these pharmacokinetic changes by the abovementioned mechanism.
High tumor sensitivity to erlotinib could potentially cause a large decrease in
V
T. Namely, in the absence of cold erlotinib, EGFR-TKI-sensitive tumors are expected to have high [
11C]erlotinib
V
T values as compared to resistant tumors [
1]. Once cold erlotinib is added, EGF receptors become blocked causing
V
T to drop. The results of this study confirmed that the patients with the largest decrease in
V
T did have responsive tumors; however, there was no clear association between decrease in
V
T and tumor response. To illustrate, three patients (patients 8, 2, and 6) had a large (i.e., approximately 50 %) decrease in
V
T. Patient 8 was treated with erlotinib therapy for a few weeks only. Erlotinib was stopped, as he refused to continue therapy due to a pneumonia that he ascribed to erlotinib. He did have some tumor regression with erlotinib during these few weeks, however, not enough to be declared a partial response. Patient 2 had a complete response to erlotinib therapy after 3 months. Patient 6 had a slow disease progression prior to erlotinib scanning; she stopped erlotinib therapy after her first scan but developed a severe flare of her disease within 1 week. Her second scan showed increased tumor volume and increased
V
T; this illustrates that her tumor still had significant amount of sensitive clones. These cases demonstrate that high decrease in
V
T can occur in sensitive tumors; however, there was no clear association, that is, responders did not exclusively show high decrease, as there were two other cases with partial tumor response to erlotinib therapy who showed moderate decrease in
V
T of 29 and 17 %. On the other hand, as a result of erlotinib therapy, changes can occur in the size of the tumor, its concentration of vital tumor cells and possibly its EGFR density. These changes can occur in a period as short as 7 to 14 days after initiation or discontinuation of therapy and may also influence
V
T. Therefore, any tumor response to erlotinib therapy may also influence the decrease of
V
T during therapy. However, the limited number of patients scanned does not allow for extensive elaboration. Future studies including more patients should investigate the correlation between response and change in uptake.
The decrease in V
T varied between 17 and 58 %. This high level of variability disqualifies V
T(E+) as substitute for V
T(E−). Any quantitative comparison between patients or between different time points in a single patient should be performed using V
T(E−). However, for intrapatient interlesional comparison at a single time point, V
T(E+) may still be considered. Whether tumor TKI sensitivity can be predicted by [11C]erlotinib V
T(E+) remains to be investigated.
Simplified uptake parameters
For simplification of future scanning protocols, SUV was not found to be a suitable uptake parameter, as it did not correlate with V
T, both on and off therapy. SUV normalizes on the basis of injected dose and patient weight, which is less accurate as compared to TBR that normalizes on the basis of the blood pool activity itself. For example, during erlotinib therapy, the blood tracer concentrations were higher. Due to this increased tracer availability, the absolute amount of tumor tracer binding may have changed in varying extent. Tumor SUV does not take this variable into account, whereas TBR does. Contrary to SUV, arterial and venous TBR showed an excellent correlation with arterial V
T, supporting the use of both arterial and venous TBR in the time interval of 40 to 60 min post injection in future whole body static scanning protocols.
Interestingly, venous TBR values were lower than the arterial TBR values, especially in patients off therapy. This was due to higher plasma activity in venous samples than in arterial samples. As no difference in metabolism was observed between venous and arterial samples, the higher venous plasma activity values were only caused by a higher venous concentration of parent molecules. The reason for this finding is unclear. Possibly, the interstitial compartment together with EGFR molecules that are highly expressed at the epidermal tissue compartment act as a capacitator, by reversibly binding [11C]erlotinib molecules. So, venous plasma collects not only the unbound [11C]erlotinib molecules coming from the arterio-capillary route but also the [11C]erlotinib molecules being released from the interstitial and peripheral tissue compartments. This can also explain why patients on therapy, who have more EGFR saturation, have less veno-arterial activity difference. Another cause that may be considered is the fact that a single venous cannula was used for tracer injection and blood withdrawal, which implies that venous activity may increase due to the presence of tracer molecules that remained sticking to the cannula wall during injection. However, this mechanism is unlikely as it cannot explain why the veno-arterial difference was higher in off therapy than on therapy. Nevertheless, venous sampling was found to be suitable for interlesional quantitative comparison using TBR, as long as venous values are not interchanged together with arterial values.