Role of Daratumumab in Combination With Standard Therapies in Patients With Relapsed and Refractory Multiple Myeloma

Multiple myeloma (MM) is a hematological malignancy characterized by renal insufficiency, bone lesions, anemia, and hypercalcemia. In this modern era of medicine, even with the development of drugs like immunomodulatory agents (IMiDs) and proteasome inhibitors (PI), the treatment of MM prevails as a challenge. However, even after the attainment of total remission, relapse of MM and disease progression is frequent. That is why there is an urgent requirement to develop novel monoclonal antibody drugs. The latest drugs for the treatment of relapsed and refractory MM (RRMM) approved by the Food and Drug Administration (FDA) are elotuzumab and daratumumab. In this article, we will discuss daratumumab with different combination therapies. The literature exploration was done using PubMed, Medline, PubMed Central, and Research Gate. Keywords used to search are monoclonal antibodies, daratumumab, RRMM, and novel agents. Our review article, which includes 21 relevant articles, demonstrated that daratumumab in different combinations showed significant progression-free survival (PFS) without severe safety concerns. However, while observing all the studies, neither of them studied the combination therapies of daratumumab in end-stage renal disease (ESRD) patients. Hence, more randomized controlled clinical trials should be done to understand and compare the effect of the combination of daratumumab with the standard of care therapies in ESRD patients.


Introduction And Background
Multiple myeloma (MM) is defined as the abnormal proliferation of plasma cells (PCs) with increased monoclonal antibody production. It is the second common hematological malignancy. MM is characterized by organ dysfunction, including renal insufficiency, anemia, bone lesions, and hypercalcemia. The SEER Data Registry, also known as US surveillance, epidemiology, and end results, estimated that new MM cases in 2021 are 34,920, and estimated deaths in 2021 are 12,410 [1]. Relapsed myeloma is defined as previously treated myeloma that progresses and needs salvage therapy initiation. It does not meet the criteria for either Relapsed and Refractory MM (RRMM) categories or primary refractory myeloma [2]. When the disease is nonresponsive while on salvage therapy or progresses within 60 days of the last cycle in patients who have achieved minimal response or better at some point is defined as RRMM [2]. When the disease is nonresponsive in patients who have never achieved a minimal response and neither got better with any therapy is defined as primary refractory myeloma [2]. Despite enormous advances with the advent of proteasome inhibitors (PI) and immunomodulatory agents (IMiDs), relapse of MM and disease progression is very common even after achieving complete remission. Hence, the treatment of MM remains a challenge [3]. Patients' survival rate with frequent relapses or patients who are refractory to treatment is very low [4]. The median overall survival (OS) in patients with three or more prior therapy lines including an IMiDs or PI or double refractory to a combination of both PI and IMiDs was only about eight months, detailed in a recent retrospective report of real-world survival outcomes [5]. Hence, new treatment combinations and therapies are required urgently in patients with RRMM [4].
In 2015, the Food and Drug Administration (FDA) approved two monoclonal antibodies to treat RRMM, elotuzumab, and daratumumab [6]. Despite the utilization of these novel agents, nearly all patients experience relapsed disease [6], but comparatively prolonged survival with the use of these novel agents. Daratumumab is a human IgGκ monoclonal antibody that targets CD38, expressed in abundance in hematopoietic cell types, especially myeloma cells [7,8]. Daratumumab induces the death of PCs by antibody-dependent phagocytosis, apoptosis, and cell-mediated cytotoxicity. In addition to that, daratumumab works as an immunomodulator by increasing the activity of cytotoxic T-lymphocytes by inducing and decreasing CD38+ immunosuppressive myeloid and lymphoid cells [9,10].
A pooled observation of two monotherapy studies produced a median OS of 20.1 months and an overall response rate (ORR) of 31%. It showed a strong response and clinical benefit in patients with stable disease responses or better [5]. Based on the above findings, daratumumab was approved in the United States for a monotherapy drug (16 mg/kg) for treatment of MM patients who have received three or more than three prior therapy lines, including a PI and an IMiDs, or who are double-refractory to a PI and an IMiDs [5]. Daratumumab is extensively studied in various combinations with the standard of care therapies for RRMM. Daratumumab-based combinations showed significant benefit in phase three and phase one/two clinical trials [11][12][13]. But there is limited data on the complete role of daratumumab in a combination of the standard of care therapies. This article discusses different combination therapies' efficacy and safety, and the use of these combination therapies in different settings.

Review
We searched the databases PubMed, Medline, PubMed Central, and Research Gate using a combination of keywords. Keywords used to search are daratumumab, RRMM, monoclonal antibodies, and novel agents. The articles included are those from the last five years. Grey literature is not included in this review article. Only articles with free full-text available are included in the study -no location specifications. Only articles in the English language are included.

Mechanism of action of daratumumab
CD38 is a type II transmembrane glycoprotein involved in numerous intracellular and extracellular functions. CD38 expression is at low levels in the normal lymphoid cell lineage and normal myeloid cell lineage. However, CD38 is expressed at high levels on malignant myeloma cells [8]. Krejick et al., in their flow cytometric analysis on 148 patients, confirmed the increased expression of CD38 on myeloma cells [14]. Daratumumab is an IgG1 human monoclonal antibody against CD38. Daratumumab kills myeloma cells via different pathways include antibody-dependent cellular phagocytosis (ADCP), antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), direct apoptosis, and immunomodulatory action.

Antibody-Dependent Cellular Phagocytosis
ADCP is one of the Fc component-medicated mechanisms that contribute to the anti-tumor activity of daratumumab. Daratumumab (IgG1 monoclonal antibody) binds to CD38; the Fc component of daratumumab binds to FcγRs on the macrophages and then phagocytosis of opsonized tumor cells [15,16]. This mechanism contributing to the anti-tumor activity of daratumumab is well demonstrated by Overdijik et al. in their in vivo study [10]. It is one of the important mechanisms because of macrophages' ability to engulf multiple opsonized tumor cells and the ability to engulf quickly. The efficacy and quick action of macrophage-induced phagocytosis is demonstrated by time-lapse imaging microscopy [10].

Antibody-Dependent Cellular Cytotoxicity
This is one of the Fc component-mediated mechanisms of action. Fc component of daratumumab binds to FcγRs on effector cells. Effector cells mainly release toxic components and thereby results in lysis of the tumor cells [15,16]. Casneuf et al. demonstrated that even though there is a reduction in the number of natural killer (NK) cells as these cells express CD38, there is no demonstrable change in the safety and efficacy of daratumumab. This no change in safety and efficacy is because the remaining NK cells can contribute to ADCC, other granulocytes contribute to ADCC, and other mechanisms of actions also play a role in the efficacy of daratumumab [17].

Complement-Dependent Cytotoxicity
Fc binding to the complement activates complement and results in the production of membrane attack complex, leading to increased permeability of membrane and damage to the cell [16].

Direct Apoptosis
Programed cell death is triggered by cross-linking CD38 by inhibitory FcγRIIb and activating FcγRs [16].

Immunomodulatory Action
Increased expression of CD38 is noted on B-regulatory cells and myeloma-derived suppressor cells. As daratumumab is a monoclonal antibody to CD38, these cells are sensitive to daratumumab and hence promote anti-tumor activity. Krejick et al. found that a shift towards positive versus negative immune cells might contribute to anti-tumor activity. But further studies were required to prove whether this T-cell shift results in actual anti-tumor activity or not [14]. The mechanism of action of daratumumab is illustrated in Figure 1.

Modes of resistance
Resistance pattern to daratumumab may be primary or secondary resistance.

Mechanism of Primary Resistance to Daratumumab
The level of expression of CD38 itself affects the efficacy of daratumumab. MM cells with high levels of CD38 expression are more sensitive to daratumumab than cells with a low level of CD38 expression [15].

Mechanism of Secondary Resistance to Daratumumab
Decreased expression of CD38 following daratumumab infusion causes decreased efficacy. Soluble CD38 and antidrug antibodies might neutralize CD38 theoretically, but no clinical evidence till now [15]. ADCP resistance is mainly caused by Fc receptor polymorphism; low monocyte to myeloma ratio decreases the activity of daratumumab. CD47 expression inhibits phagocytosis of myeloma cells. ADCC resistance is usually caused by increased NK cell destruction, an increased bone marrow stromal cells, and polymorphism of the Fc receptor. CDC resistance is due to CD46, CD55, and CD59 are membrane-associated complement inhibitors that protect normal cells from complement medicated damage. These inhibitors also protect myeloma cells from complement-mediated damage. At the time of progression with daratumumab therapy, increased expression of CD55 and CD59 is shown. Resistance to direct effects is due to a decrease in CD38 expression. Resistance to immunomodulatory action is due to a decrease in activated T cells [16]. Clemens et al., in their analysis of pharmacokinetics in 223 patients in Gen 501 and SIRUS study, showed that daratumumab exhibits non-linear pharmacokinetic characteristics after intravenous administration. Maximum concentration (C max) increase is in proportion to an increase in dose after initial infusion. After subsequent infusions, the C max increase is greater than the dose-proportional increase. The reason for this increase in C max is postulated either due to target-mediated saturation or due to a decrease in CD38 containing cells by decreasing tumor burden [18]. This study also studied the dose-effect and confirmed that 16 mg/kg is a better target saturation and ORR than 8 mg/kg. Based on the consideration of pharmacokinetic characteristics, the suggested daratumumab dose is 16 mg/kg weekly for eight weeks, every two weeks for 16 weeks, and four weeks after that [18].

Pharmacokinetics of daratumumab
Although daratumumab was proven as an effective drug, it is also associated with infusion-related reactions (IRR) in 50% of patients. Most of them are mild and related to early infusions. Usmani et al., in their study, mainly focused on the safety and pharmacokinetics of subcutaneous delivery of daratumumab with hyaluronidase. Two doses were tested in this study, the first dose is 1,200 mg, which is equivalent to the daratumumab IV dose of 16 mg/kg, and the second dose is 1,800 mg, which is equivalent to the daratumumab IV dose of 24 mg/kg. The second dose is tested considering the bioavailability of less than 100% for subcutaneous delivery. Serum daratumumab concentrations in the 1,800 mg subcutaneous group are almost similar to daratumumab 16 mg/kg infusion. Treatment site reactions include pain, erythema, and induration. Most of them were resolved without intervention. IRR is less common than IV infusion. As there are considerably fewer side effects with maintaining efficacy, subcutaneous daratumumab can be considered an alternative [19]. Mechanism of action, modes of resistance and pharmacokinetics are mentioned in Table  1.

Combination therapy
Daratumumab monotherapy at a dose of 16mg/kg was studied in Gen 501 and SIRUS study and showed significant ORR in one-third of the population. In addition, treatment efficacy and side effect profile of daratumumab combined with various standards of care regimens in RRMM were studied in different phase clinical trials. Those results from different clinical studies are reviewed here in this article.

Therapeutic Efficacy of Daratumumab in Different Combinations
Daratumumab with bortezomib and dexamethasone (D-Vd): The CASTOR study is multicenter, randomized controlled trial (RCT). Patients included in the study were the patients with RRMM, those who had at least some positive response to previous therapy, and those who got at least one dose previously. The patients excluded from the CASTOR study had significant side effects to bortezomib or patients who were refractory to bortezomib therapy. They were randomly distributed into two groups. One group got bortezomib and dexamethasone (Vd), and another group got D-Vd. Progression-free survival (PFS) was the primary endpoint that was being studied. After a median follow-up of about eight months, pre-specified interim analysis was done by Palumbo [22].
Daratumumab with lenalidomide and dexamethasone (D-Rd): POLLUX study is a multicenter, RCT, phase three clinical trial. Patients with RRMM, according to the International myeloma workgroup, those who received at least one prior therapy were included in this study. Those who were refractory to lenalidomide or had unacceptable adverse events (AEs) with lenalidomide were excluded from the study. The patients were randomly assigned into two groups. One group received D-Rd, and another group received lenalidomide and dexamethasone (Rd). Pre-specified Interim analysis of this study was done by Dimpolous et al. after a median follow-up of 13.5 months, which showed significant PFS. The time to event analysis of PFS after 12 months was 83.2% in the D-Rd versus Rd is 60.1%. Significant PFS is maintained in all subgroups stratified based on the number of the previous therapy line, whether the patient is exposed to lenalidomide or not, a timeline of previous therapy, and age. ORR in the D-Rd was 92.9% versus 76.4% in the control group. A significantly higher ORR, deeper responses, and longer response duration were shown in this study. The daratumumab group showed a 63% lower risk of death or disease progression than the control group [12].
Dimpolous et al. conducted their updated analysis of the POLLUX study with additional 12 months of follow-up (total of 25.5 months), which showed a significantly reduced risk of progression or death than the control group. A 24-month PFS rate was 68.0% in the treatment group versus 40.9% in the control group. ORR is 92.9% in the D-Rd group versus 76.4% in the Rd group. This study is focused on minimal residual disease and clinically relevant subgroups. Also analyzed parameters in the subgroup analyses were conducted based on the number of prior lines of therapy, prior treatment exposure, and a treatment-free period after the last dose, and cytogenetic risk. The efficacy is maintained in all subgroups, including those with cytogenetically high risk [23]. Bahlis  limiting toxicities of the daratumumab and pom-dex. This study also focuses on daratumumab and pom-dex safety and tolerability. Those who received at least two lines of therapy were included in this study. The patients were eligible who have progressive disease on lenalidomide or bortezomib or a combination of lenalidomide and bortezomib. The ORR was 60%. ORR was maintained in all subgroups stratified based on various parameters. ORR in subgroups were 64%, 65%, and 55% in patients who received two, three, or more than three lines of prior therapy. Median survival was about nine months. This study showed that pom-dex could be safely combined with daratumumab. This study might complement the POLLUX study. In the POLLUX study, the addition of daratumumab for lenalidomide, which is immunomodulatory, showed significant ORR. The addition of pomalidomide also showed significant ORR, but this is a non-randomized study and needs a very extensive study to prove the combination of daratumumab with pom-dex to be effective. APOLLO study is an on-going study to determine the actual benefit from the RCT. This regimen is approved based on the phase one/two clinical trials without the phase three trial. This regimen is really helpful in people who are refractory to lenalidomide [26].
Hussain et al. did a retrospective cohort study to see the combination of daratumumab and pomalidomide helps overcome the resistance of either of them, considering their mechanism of action. This study showed that a combination of daratumumab and pomalidomide helps overcome resistance in some cases. But this is a retrospective cohort study with a limited population, needs a randomized controlled trial to confirm the association that has been shown in this study [27]. Siegel et al. in their clinical trial mainly focused on patients who are refractory or relapsed with lenalidomide. One-year PFS in those who relapsed after lenalidomide was 83.2%. One-year PFS for those who are refractory to lenalidomide treatment was 72.4%. PFS in patients with one or two prior lines of therapy was 78.8 % versus 69.0%. ORR was achieved irrespective of several prior therapy lines, refractoriness to lenalidomide, or previous bortezomib exposure. These findings demonstrate that pomalidomide, low-dose dexamethasone, and daratumumab combination is a safe and effective treatment for patients with RRMM immediately following disease progression on or after lenalidomide [28].
Daratumumab with carfilzomib and dexamethasone (D-Kd): Safety, pharmacokinetics, and preliminary efficacy from the D-Kd arm of the EQUULEUS were studied by Chari et al. The patients included in this study are RRMM, lenalidomide refractory patients, who had received one to three prior lines of therapy. The 12month PFS in patients who received D-Kd was 74%. In lenalidomide refractory patients, 12-month PFS was 65%, and bortezomib refractory patients, 12-month PFS was 60%. Median PFS was not reached in all patients. In the treatment group, the 12-month OS rate was 82%. ORR was 84% in lenalidomide refractory patients, and in bortezomib refractory patients ORR was 84%. Median OS was not achieved. The 12-month OS rate for all treated populations was 82%, lenalidomide refractory patients were 75%, and bortezomib refractory patients was 76% as it is a phase one study. Phase three trials are necessary to find out the efficacy of this combination and check the side effect profile. This regimen would be a helpful regimen for those with refractoriness to lenalidomide. There is one study going for D-Kd in lenalidomide-exposed patients [13].
Daratumumab with bortezomib, cyclophosphamide, and dexamethasone (D-VCd): D-VCd was studied in a multicenter, open-label, single-arm study which includes 14 RRMM patients along with 87 patients of NDMM. This study is limited as they involve only 14 patients and the main population in NDMM. The ORR after the completion of four cycles was 71.4 %. Median PFS was 13.3%, and 12-month PFs were 66.2%. 12month OS rate was 54.5%. As this study is a phase two study and no set of controls and a limited number of the population included in this study, it needs an extensive clinical trial to prove the efficacy [29].

Influence of Disease Characteristics, Patient Characteristics, and Other Monoclonal Antibodies in Combination Therapy
Except for the type of myeloma, there is no influence of disease and patient characteristics on the concentration of daratumumab. Daratumumab concentration is less in IgG type compared to non-IgG. Regarding efficacy, there is no change in the efficacy of IgG MM and non-IgG MM. No patient and disease factors analyzed by Yan et al. showed a statistically significant difference [30].
Hoylman et al. did a retrospective cohort study to find out the optimal sequence of daratumumab and elotuzumab. The effect of one monoclonal antibody on another monoclonal antibody was never studied before; ORR to initial monoclonal antibody was unchanged. ORR is higher in the cohort where daratumumab received a second dose compared to elotuzumab as a second dose. PFS for daratumumab when daratumumab first received was about three months. PFS for elotuzumab when elotuzumab first received was about six and half months. Median PFS for daratumumab when received second dose was about ten months. The median PFS for elotuzumab when elotuzumab received the second dose was about two months. Use of daratumumab before elotuzumab was associated with decreased cumulative PFS. Daratumumab retains activity whether it was used as first or second after elotuzumab. But elotuzumab did not retain activity when used after daratumumab [6].

AEs Associated With Daratumumab in Different Combinations
Although AEs associated with a combination of medications are high, most of them are manageable and almost similar to the AEs associated with individual medication involved in combination therapy. In the interim analysis of the CASTOR study, although there were more hematological and non-hematological AEs reported in the D-Vd group, the treatment discontinuation because of side effects is almost similar. No cases of immunogenicity or no cases of hemolysis were reported in the treatment group [20]. Increased incidence of IRR in the daratumumab group and most of them were associated with the first dose of daratumumab infusion. Most of them are grade one or two [20]. Even with an extended follow-up of another 12 months, the safety profile remains unchanged. No new cases were reported [21]. AEs with D-Rd are similar to knowing Rd complications demonstrated in POLLUX study [12]. AEs related to the D-Rd group with extended follow-up are higher than the control group, but the discontinuation rate is low [23]. A higher incidence of infusion reaction is evident in the daratumumab group over 75 years in a study by Mateos et al. [22]. Side effects profiles were similar in both the East Asian and Japanese groups, consistent with the overall population in the study by Suzuki et al. [25]. Though higher neutropenia rates were observed in the daratumumab plus pom-dex group, febrile neutropenia and grade three/four infections are comparable with pom-dex alone [26]. Yimer et al., in their study, show that AEs are more common in RRMM than NDMM in patients receiving D-VCd [30]. Daratumumab therapeutic efficacy and safety as combination therapies are mentioned in Table 2.

TABLE 2: Therapeutic Efficacy and Safety of Daratumumab in Combination Therapies
RCT -Randomized controlled trial, D-Vd -daratumumab, bortezomib, and dexamethasone, Vd -bortezomib and dexamethasone PFSprogression-free survival, OS -overall survival, ORR -overall response rate, , D-Rd -daratumumab, lenalidomide, and dexamethasone, Rdlenalidomide and dexamethasone, Pom-dex -pomalidomide and dexamethasone, D-Kd -daratumumab, carfilzomib, and dexamethasone, D-VCd -Daratumumab, bortezomib, cyclophosphamide and dexamethasone, TEAEs -treatment effective adverse events, MRD -minimum residual disease, CR -complete response, VGPR -very good partial response Our study has some limitations; only articles in the English language and the free text available articles are included. Most of the studies are performed on Caucasians and African Americans but not on Asians except the study by Suzuki et al. on the East Asian population. Patients in these studies are followed for a limited time and need more follow-up to assess OS rate. There are no RCTs found on daratumumab use as combination therapies in end-stage renal disease (ESRD) patients, which is an important association with MM. Only one study by Cejalvo et al. did a retrospective analysis that showed a single agent is safe in ESRD patients. However, this study has limitations; this is a small observational retrospective study [31]. RCTs are necessary to assess the safety and efficacy of single-agent and combination therapies.

Conclusions
The main objective of this research article is to study the safety and efficacy of daratumumab with the current standard regimes for MM. D-Vd compared to Vd exhibited prolonged PFS in interim and long-term follow-up data. While observing the extended follow-up, all subgroups maintained the prolonged PFS including a subgroup based on cytogenetic risk. Prolonged PFS was observed in the interim and extended follow-up in the D-Rd group. While considering all subgroups (including high cytogenetic risk), notable prolonged PFS was maintained on the D-Rd group compared to the Rd group. Therefore, D-Vd and D-Rd were approved for the treatment of RRMM. During phase ½, clinical trial efficacy and safety were observed with daratumumab plus pom-dex, especially useful in patient's refractory to lenalidomide. During phase one clinical trials, D-Kd and D-VCd also showed promising efficacy. More RCTs are necessary to prove the efficacy and safety of D-Kd and D-VCd. While observing all the studies, none of them mentioned the combination therapies of daratumumab for ESRD patients. Hence, more RCTs are necessary to understand and compare the effect of combinations of daratumumab with the standard of care therapies in ESRD patients.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.