Preclinical studies have revealed that both CDC and ADCC are more
effective when the expression of CD38 is high, but immediately following
the first infusion with DARA, there is a pronounced reduction of the
expression of CD38 on the remaining MM cells [9, 23, 27]. In vitro
studies showed that all-trans retinoic acid (ATRA) improves the
DARA-mediated CDC and ADCC by upregulation of CD38 on MM cells including
MM cells, which were resistant to DARA monotherapy [27]. This point
towards a potential clinical advantage of a high expression of CD38 on
myeloma cells. Indeed, a high expression of CD38 before initiation of
DARA is a predictor of obtaining a partial response or better [23].
Based on these observations, a phase 1/2 study combining ATRA with DARA
in DARA-refractory patients was initiated [28]. Despite of the
promising in vitro experiences, the results of the clinical study were
disappointing. The overall response rate was only 5% and the mean PFS
2.8 months. Panobinostat can also upregulate the expression of CD38 by
myeloma cells and enhance the anti-myeloma effect of DARA in vitro but
given the disappointing clinical results with ATRA it is unlikely that
this avenue will be pursued further [29].
The fact that CD38 expression by MM cells is downregulated immediately
after initiation of treatment with DARA but a response may be maintained
over many months or even years suggests that other modes of action of
DARA than CDC, ADCC and ADCP are important to obtain a long term
response [23]. The long-term effects of DARA may be more dependent
on its immunomodulatory effects (reduced production of immunosuppressive
adenosine, elimination of CD38 positive regulatory cells) or the
inhibition of formation of nanotubes and transfer of mitochondria from
stromal cells to the MM cells. Thus, low CD38 expression by myeloma
cells may in fact be beneficial to maintain control of the disease and
one may indeed envision CD38 as a growth and survival factor for MM
cells. A clinical aspect of this discussion is whether the treatment
with DARA should continue with a change of partner drug despite
progression on a DARA containing regimen, or whether treatment with DARA
should be paused, so the MM cells regain the CD38 expression, which
takes about 6 months [23]. Recently, Agoston and colleagues did a
retrospective, nationwide evaluation of all MM patients in Denmark
receiving DARA before 2019. The course of therapy for 474 patients were
evaluated, and the OS from the time point where each patient progressed
on DARA for the first time was determined. They found that the median OS
for patients continuing to receive DARA but combined with another
anti-myeloma therapy than before the relapse was 23.6 months, and the OS
for patients not receiving DARA after progressing on DARA was 11.3
months. The difference was clinically significant in a multivariate
analysis counting for age, cytogenetic risk, time since diagnoses among
others [30]. This points towards a benefit of continuing treatment
with DARA without interruption and suggests that the low CD38 expression
by MM cells imposed by DARA is compatible with and may in fact support a
continued clinical response.
In the clinical setting complement activation caused by DARA has only
been measured during the first eight weeks of treatment and only by
measuring native complement factors, among these C2 [23]. The
results show that there is a decrease of the native complement factors
after initiating DARA therapy, but the signal disappears after a few
weeks of treatment, indicating that the complement system is mainly
active during these first weeks or that an increased production of
complement factors compensate for the consumption. In an attempt to make
CDC even more potent, a new CD38 mAb is under development [31]. This
CD38 mAb called GEN3014 has a hexamerization-enhancing mutation, which
increases the binding of C1q and thereby enhance the activation of the
complement cascade [32]. The data from the first in human trials of
DARA and GEN3014 confirms this assumption. There was a much more
profound decrease in C2 in patients treated with GEN3014 than in
patients treated with DARA [23, 31]. To explore the impact of
complement activation during long-term treatment with DARA, the
complement split product C3d was measured in PB of patients who had
received DARA for a median of minimum 300 days. The level of C3d in
patients receiving DARA that had obtained a partial response or better
was compared to patients progressing on DARA. There were no significant
difference between the two groups. This supports the hypothesis that CDC
is of less importance for the long-term response to DARA.
T cells
The MM bone marrow microenvironment seems to be in an immunosuppressive
state and both B- and T-cells shows significant impairment of their
functions, possibly due to an increased concentration of adenosine in
the bone marrow [33]. Such impairment is reflected by higher
expression of markers of T-cell exhaustion such programmed cell death-1
(PD-1) and cytotoxic T-lymphocyte associated protein-4 (CTLA-4)
[34]. Several checkpoint inhibitors that block PD-1, PD-L1 or CTLA-4
have been approved for solid cancers but, so far, checkpoint inhibitors
have not shown convincing efficacy in MM [35]. T-cell immunoglobulin
and ITIM domains (TIGIT) is an inhibitory checkpoint molecule highly
expressed by CD8+ T-cell from MM patients and may play a more important
role in the inhibition of the T-cell response against MM [36].
When initiating treatment with DARA, the number of
CD4+CD25+CD127dimTregs decline immediately. These Tregs are also CD38+and therefore sensitive to DARA. This decline is followed by an
expansion of CD8+ T cells and an increase in the
ratios of CD8+:CD4+ and
CD8+: Tregs [11]. These immunomodulatory changes
are more pronounced in patients with a good, clinical response to DARA.
Whether exhaustion of T-cells plays a role in the development of
resistance to DARA is not fully clarified, but adding durvalumab, a
PD-L1 inhibitor, to DARA in DARA-refractory patients did not induce a
clinical response [37]. Iversen et al. examined the expression of
checkpoint molecules on CD8+ T-cells isolated from the bone marrow of
newly diagnosed MM patients (NDMM) and patients relapsing on DARA
[38]. They found no difference in the expression of PD-1 on
CD8+ T-cells in DARA-refractory patients compared to
NDMM patients. Furthermore, PD-1 was only expressed on around 20% of
CD8+ T-cells from MM patients, whereas TIGIT was
present on around 80% (Figure 3). Paiva et al. confirmed the
observation on the percentage of PD-1 expressing CD8+T-cells [39]. Two studies confirm that TIGIT is more frequently
expressed by CD8+ T-cells in myeloma patients than
PD-1 [36, 40]. Altogether, it seems that PD-1 is not a key immune
regulatory checkpoint in MM. Iversen et al. did not find a difference in
the expression of TIGIT on CD8+ T-cells when comparing
NDMM to DARA-refractory patients, which was supported by the findings by
Guillerey [36]. Neri et al . found a higher expression of
TIGIT on T-cells from DARA-resistant patients compared to responders
[41]. Therefore, it seems that there is a difference in the
expression of TIGIT on T-cells when comparing DARA-refractory patients
to patients currently in a clinical response, but not when comparing
DARA-refratory patients to NDMM. Anti-TIGIT mAbs are currently being
tested in clinical trials of patients with relapsed MM.