IL2 and its closely related IL15 are cytokines capable of stimulating immune cells via interactions with shared signaling receptor components, IL2Rβ and IL2RγC. Specifically, IL15, a potent stimulant of CD8
+ T cells and natural killer (NK) cells [
18,
19], is a promising cancer immunotherapeutic agent being explored by several companies. ALT-803, the lead IL15 analogue in the field, is a complex of an IL15 superagonist N72D mutant and a dimeric IL15 receptor α Su/IgG1 Fc fusion protein that was found to exhibit enhanced biologic activity and potent anti-tumor efficacy in vivo with a substantially longer serum half-life than recombinant IL15 [
20‐
22]. These preliminary findings indicate that this IL15 superagonist complex could serve as an ideal immunostimulatory cancer therapeutic.
In fact, ALT-803 has entered the clinic in a series of early phase clinical trials. Wrangle et al. reported ALT-803 in combination with nivolumab to be safe to administer in an outpatient setting in patients with advance non-small cell lung cancer. The recommended phase 2 dose of ALT-803 was noted to be 20 μg/kg given once per week subcutaneously (SQ) in combination with 240 mg intravenous nivolumab every 2 weeks [
9]. Similarly, Romee et al. reported ALT-803 alone was well tolerated in patients who relapsed > 60 days after allogeneic hematopoietic cell transplantation. The recommended phase 2 dose of ALT-803 in this setting was 10 μg/kg intravenously or subcutaneously weekly for 4 consecutive weeks [
11]. In our, phase Ib study of ALT-803 and BCG administered intravesically in BCG-naïve patients with NMIBC, we demonstrated that the combination therapy was well tolerated and the phase 2 recommended dose of ALT-803 was 400 μg/instillation [
8].
Previously, we published the efficacy of ALT-803 intravesical therapy in a carcinogen induced rat model [
6]. In order to further evaluate the possible advantage of SQ injection of ALT-803, as noted in two phase I clinical trials, we set out to compare the biological activity of SQ ALT-803 and intravesical ALT-803 with or without intravesical BCG. We assessed the anti-tumor efficacy and immune stimulation of each regimen. All treatments demonstrated similar anti-tumor efficacy and SQ ALT-803 alone and intravesical BCG plus SQ ALT-803 showed non-inferiority to standard of care, intravesical BCG alone. All treatment regimens were well tolerated. Notably, SQ ALT-803, when administered alone, activated CD8
+ T, NK and NKT cells in the PBMC while activating NKT cells in the spleen. ALT-803 has been shown to stimulate the activation, proliferation, and expansion of CD8
+ T cells and NK cells in rodent models [
22], in cynomolgus monkeys [
23] and in human phase 1 studies [
9‐
11]. Unlike our previous study in which a rat carcinogen induced model was used [
6], we were unable to demonstrate a statistically significant difference in the number of CD8
+ T and/or NK cells infiltrating the tumor and bladder, though there was a trend of increased CD8
+ T cells in the intravesical BCG with SQ ALT-803 treatment group. The difference between our results in rats and mice may be multifactorial. First, although the two models are rodents, they have been known to exhibit biological and molecular differences [
24]. Second, we assessed tumor infiltrating cells 2 weeks after completion of therapy in the current mouse model and 1 week after the last treatment in the rat model which may not be the appropriate time point. Third, a larger number of animals in each treatment group may have been required to show a significant statistical difference (i.e., the study was powered for changes in tumor size not infiltration of immune cells). Interestingly, serum cytokines were significantly elevated with intravesical BCG treatment alone (i.e., TNFα and IL13) as well in SQ ALT-803 with intravesical BCG treatment group (TNFα, IL5, IL6 and IL13). Urine cytokines were significantly elevated with intravesical BCG treatment alone (IFNγ) as well as with SQ ALT-803 with intravesical BCG group (IL13 and IFNγ). Therefore, the data suggest that systemic administration of ALT-803 resulted in systemic changes, notably increased numbers of CD8
+ T, NK and NKT cells within the peripheral blood and spleen as well as induction of key cytokines,
e.g., IL5, IL6 in the sera and IL13 and IFNγ in the urine compared to only intravesical BCG administration. Thus, it is possible that systemic ALT-803 therapy may result in a broader activation of CD8
+ T and NK cells as well as NKT cells compared to intravesical ALT-803, which could translate into a greater potential to effectively eradicate extravesical tumor (i.e., metastatic disease to lymph node or distant tissues). This concept of eliciting a systemic immune response is being tested in the current clinical trial entitled “Different strains of BCG with or without vaccine in high grade NMIBC” (NCT 03091660) in which intravesical BCG is administered with or without SQ BCG vaccine. It is speculated that SQ BCG vaccine will elicit a broad systemic immune response, which would supplement the local immune response. As we and others have noted, BCG elicits a broad, non-specific immune response [
25]. We offer the idea that SQ ALT-803 is ideal in this setting to elicit a long-term CD8
+ T cell, NK cell and NKT cell responses both systemically and locally.