Subcutaneous Administration of Biotherapeutics: An Overview of Current Challenges and Opportunities

Potential differences in the safety, immunogenicity, and efficacy of subcutaneous versus intravenous administration are best visualized by comparing the pharmacokinetic profiles of a therapeutic protein that can be administered via either route. A biotherapeutic intravenous infusion or injection directly into the bloodstream usually results in immediate maximum serum concentrations (C_max), whereas the pharmacokinetic profile of subcutaneously injected therapeutic proteins is typically characterized by a slow absorption rate from the subcutaneous extracellular matrix with C_max levels below those achieved with intravenous dosing. This absorption pattern of macromolecules into the blood is a consequence of their limited permeability across the vascular endothelia; therefore, the lymphatics provide an alternative absorption pathway into the circulation system [70, 71]. However, absorption via the lymphatics has also been described as a barrier for complete uptake of subcutaneously injected molecules. Other factors that can contribute to incomplete bioavailability of subcutaneously injected molecules can include the composition, volume, pH, and viscosity of the dosing formulation; interactions with interstitial glycosaminoglycans and proteins; and enzymatic degradation [70, 72, 73].

Traditionally, mAb treatments in oncology were developed for intravenous infusion based on a bodyweight- or body surface area-adjusted dosing regimen that aimed to correct potential interpatient variability and drug distribution and elimination issues [23].

Generally, when developing a subcutaneous dosing alternative for a molecule that is already on the market or in late-stage development with an intravenous bodyweight- or body surface area-adjusted regimen, a change to a fixed dose or dose bands for predefined weight categories could be considered. A fixed dose would require less complex handling, avoid the need for dose calculations, and reduce the potential for dosing errors. A regimen that needs to consider a patient’s bodyweight or size can be challenging, especially when the subcutaneous formulation is considered to facilitate home- or patient self-administration and requires a ready-to-use injection device.

Today, based on emerging knowledge about the pharmacokinetics of mAbs, considerable pharmacokinetic/pharmacodynamic data from clinical studies, and extensive PopPK modeling, there is evidence that fixed dosing of mAbs could generally be justified without impairing the safety and efficacy of the treatment [20, 23]. Wang et al. [20] used data from mAb studies published in peer-reviewed journals to evaluate body size-based dosing and fixed dosing for 12 mAbs in terms of their population and individual performance in adult patients. They found that the different dosing approaches performed similarly across the mAbs investigated. Body size-adjusted administration resulted in lower pharmacokinetic variability for five mAbs, whereas the pharmacokinetic variability of the other seven mAbs was lower with a fixed dosing approach. The average variability in area under the plasma concentration–time curve (AUC) and C_max of all 12 mAbs was similar following the two dosing approaches (AUC 42.4 versus 44.2%, C_max 30.1 and 30.3% by fixed dosing and bodyweight/body surface area-based dosing, respectively). The authors recommended that fixed dosing should be the preferred approach in adult first-in-human studies because of the advantages in ease of dose preparation, reduced costs, and reduced chances of dosing errors [20].

Moreover, as described by Hendrikx et al. [23], mAbs typically distribute within the blood plasma and extracellular fluids only, which increase less than proportionally with an increase in bodyweight. Depending on the mAb, intracellular degradation after binding to the target can be the primary route of elimination and is not affected by body size. Therefore, administering mAbs with a fixed dose represents a valuable treatment alternative to weight- and body surface area-adjusted administration.

Generally, whether a dosing approach is acceptable depends on the width of the targeted range of exposure, and the therapeutic index must be large enough to accommodate the exposure differences introduced by flat dosing across the body weight ranges.

When developing the subcutaneous formulation for trastuzumab and rituximab, the change from intravenous to subcutaneous administration included a switch from weight- and body surface area-adjusted dosing to a fixed dose [21, 39, 74]. The selection of a subcutaneous fixed dose was based on a pharmacokinetic bridging approach assuming that a subcutaneous dose achieving serum trough concentrations (C_trough) non-inferior to those with intravenous dosing would achieve comparable saturation of target receptors and result in noninferior efficacy [32, 33]. Previous comparisons of different dosing regimens [75, 76] revealed that trastuzumab or rituximab C_max did not correlate with clinical efficacy. Therefore, despite the lower C_max after subcutaneous administration, the risk of underexposing patients with the novel subcutaneous dosing regimen was considered low.

The intravenous and subcutaneous dosing regimens for trastuzumab and rituximab have been described previously [21, 39, 74]. Subcutaneous trastuzumab is administered with a fixed subcutaneous dose from cycle 1 onwards, omitting the need for a loading dose. Subcutaneous rituximab still requires a first body surface area-adjusted intravenous dose. For both mAbs, the dosing frequency is the same as with the respective intravenous dosing schedules. The pivotal phase III studies used a noninferiority design to compare the pharmacokinetics, efficacy, and safety of the fixed subcutaneous versus intravenous regimens. The trastuzumab HannaH study was a randomized, open-label study in the (neo)adjuvant setting in patients with HER2-positive, operable, locally advanced, or inflammatory breast cancer (see Sect. 3 for more details on trial design). The coprimary endpoints were C_trough at pre-dose cycle 8 before surgery and pathologic complete response (pCR) [39]. The rituximab Sabrina study was a randomized, open-label study in patients with previously untreated grade 1–3a, CD20-positive follicular lymphoma. The primary endpoint for stage I was C_trough at pre-dose cycle 8, and the primary endpoint for stage II was overall response at the end of induction [21, 74]. Patients were randomly assigned (205:205) to intravenous rituximab 375 mg/m² or subcutaneous rituximab 1400 mg plus chemotherapy.

In both the HannaH and the Sabrina studies, subcutaneous administration of trastuzumab and rituximab resulted in noninferior C_trough and efficacy to intravenous dosing and a similar safety profile [21, 39, 74]. In HannaH, the geometric mean ratio of C_trough subcutaneous to C_trough intravenous was 1.33 (90% CI 1.24–1.44), and 40.7% (95% CI 34.7–46.9) of the intravenous group and 45.4% (95% CI 39.2–51.7) of the subcutaneous group achieved a pCR. The difference between groups in pCR was 4.7% (95% CI − 4.0 to 13.4). In Sabrina, the geometric mean ratio of C_trough subcutaneous to C_trough intravenous was 1.5 (90% CI 1.3–1.7). Overall response at the end of induction was 84.9% (95% CI 79.2–89.5) in the intravenous group and 84.4% (78.7–89.1) in the subcutaneous group. The fixed-dose regimens for trastuzumab and rituximab were further supported by exposure–response analyses of response and grade ≥ 3 adverse events, which did not identify a statistically meaningful impact of bodyweight or body surface area, respectively, nor of exposure or route of administration of the respective mAbs [21, 74].

It is of note that while the pharmacokinetic-non-inferiority clinical bridging approach was successful for rituximab and trastuzumab, the situation can be different for other mAbs in other indications, especially when C_max is expected to contribute to efficacy.

While the abovementioned studies demonstrated that C_max did not contribute to the efficacy of trastuzumab and rituximab, the question remains as to whether the lower C_max with subcutaneous administration results in fewer hypersensitivity and infusion-related reactions (IRRs) than intravenous administration, where higher maximum serum levels are generally achieved.

The underlying mechanism of immune reactions to mAb infusions remains unclear. These reaction symptoms can be characterized by flushing, rash, fever, rigors, chills, dyspnea, mild, and/or severe hypotension with(out) bronchospasms, cardiac dysfunction, and/or anaphylaxis. For mAbs, such reactions are typically mild to moderate and occur predominantly during the first infusion [77]. In clinical practice, one way to reduce the extent and rate of hypersensitivity and IRRs is to continue dosing at a slower intravenous infusion rate [77, 78]. From a pharmacokinetic perspective, this slowing of the intravenous infusion rate results in a slower increase of mAb serum levels, similar to what is achieved with subcutaneous dosing. Importantly, although serum drug levels will still be lower with subcutaneous administration than those achieved with slower intravenous infusion, the principle of lowering mAb serum concentrations in both dosing strategies is the same. It has, therefore, been proposed that when C_max-related adverse reactions are observed during intravenous infusion, a change to subcutaneous dosing may be advantageous for the patient [79].

Alemtuzumab (Campath-H1) is an example of an mAb with marked IRRs during the first intravenous infusion. The biotherapeutic is directed against CD52 cells and is approved for the intravenous treatment of chronic lymphocytic leukemia. Small pilot studies using subcutaneous infusion reported that immediate flu-like toxicities associated with intravenous treatment were less severe when using the subcutaneous route of administration [80]. Furthermore, acute subcutaneous IRRs, such as rigor, rash or urticaria, nausea, hypotension, or bronchospasm, were rare or absent in contrast to the observations previously made for intravenous infusions [81].

Recently, published data on the tolerability of first-dose subcutaneous administration for daratumumab have become available. Daratumumab is an mAb approved for the intravenous treatment of multiple myeloma, and in clinical trials (monotherapy and combination treatments; n = 820) intravenous dosing was associated with IRRs in 46% of patients with the first infusion, 2% with the second infusion, and 3% with subsequent infusions. With the first intravenous infusion, 4–9% of patients had grade 3 events, and with second or subsequent infusions < 1% of patients had grade 3 events. Median time to onset of a reaction was 1.4 h [65]. An open-label, multicentre, dose-escalation phase Ib study assessing the safety, pharmacokinetics, and efficacy of subcutaneous daratumumab 1200 mg (n = 8) and 1800 mg (n = 33) found lower rates of IRRs with daratumumab. IRRs were reported in 9 of 41 patients (22%) and were mostly grade 1/2 in severity; these included chills, fever, rigors, vomiting, itching, edema of the tongue, noncardiac chest pain, and wheezing. All IRRs developed during or within 6 h of the first subcutaneous infusion. No IRRs were reported with subsequent infusions [82].

Overall, while there is a trend towards a reduction in rate and severity of IRRs with the subcutaneous route for the above examples, data are currently still limited, and head-to-head clinical studies with different mAbs are required to further investigate the topic.

Following subcutaneous injection, molecules reach the systemic circulation via either the blood capillaries or the lymphatic system. Unlike small molecules, biotherapeutics with molecular weights of > 20 kDa exhibit limited transport into the blood capillaries and cross into the circulation system predominantly via the lymphatics [10, 71]. This increased exposure to the lymphatic system has led to the suggestion that subcutaneous administration of biotherapeutics could be more immunogenic than intravenous dosing, but this hypothesis may not be universally valid [83].

In this context, not only the formation of antidrug antibodies (ADAs) with the different administration routes per se but also their impact on the exposure, efficacy, and safety of the biotherapeutic needs to be considered. With the pivotal noninferiority trials for trastuzumab and rituximab, the HannaH and Sabrina studies, respectively, two larger head-to-head studies comparing the immunogenicity of intravenous versus subcutaneous administration with the same dosing regimen have become available [21, 74]. In the HannaH study, 8.1% of patients (30/296) treated with intravenous trastuzumab and 15.9% of patients (47/295) receiving subcutaneous trastuzumab developed antibodies against trastuzumab [65]. Trastuzumab pharmacokinetics and pCR rates within each treatment arm were similar regardless of trastuzumab–ADA status or treatment cycle. Likewise, the incidence of administration-related reactions was not statistically different independent of the ADA status [84]. In the Sabrina study, which was performed in a CD20-positive follicular lymphoma treatment setting, the incidence of treatment-induced/enhanced anti-rituximab ADAs in the subcutaneous group was low and similar to that observed in the intravenous group (2% (4/205) versus 1% (2/205), respectively) [65]. In patients testing positive for rituximab ADAs, no impact on B-cell depletion, efficacy, pharmacokinetics, or safety was found [74]. A lack of impact of ADAs on efficacy, safety, or pharmacokinetics was also described for abatacept or tocilizumab in RA [85, 86]. However, these data cannot be directly compared with the HannaH and Sabrina studies, as the intravenous and subcutaneous treatments were given at different dosing frequencies.

A recent review looking into the subcutaneous administration route and its impact on immunogenicity for therapeutic proteins highlighted the importance of further clinical research in the field, with the aim to understand key factors that are likely to contribute to the immunogenicity in addition to the route of administration. Namely, the composition of the drug delivery formulation, the indication and disease state of the patient population, the mode of action of the molecule, differences in the dose and dosing frequency, or concomitant medications that could modulate the immune system should all be considered [11].

In addition, as many of the biologic therapeutics given subcutaneously today are immunosuppressive drugs, such as rituximab, tocilizumab, abatacept, or golimumab, a key question that needs to be answered in future trials is whether therapeutic proteins intended to stimulate the immune response can be administered subcutaneously without triggering significant ADA formation, and whether these ADAs would, in turn, impact the pharmacokinetics, efficacy, or safety compared with intravenous administration.

A drawback of subcutaneous administration of biotherapeutics is the incomplete bioavailability of the injected molecule, which can range widely from 50 to 80% for mAbs and even more for other biotherapeutics. The underlying presystemic catabolism at the subcutaneous administration site or in the lymphatic system is still poorly understood with respect to both the involved enzymes and their translation across species. For mAbs, subcutaneous bioavailability appears to be inversely correlated with clearance after intravenous dosing, so that mAbs with a lower intravenous clearance exhibit higher subcutaneous bioavailability [87]. This correlation may be due to the involvement of hematopoietic cells (e.g. macrophages or dendritic cells) in both subcutaneous first-pass clearance and systemic clearance after intravenous dosing [88]. Richter and Jacobsen [10] have only recently reviewed the current understanding and knowledge of this area, so we do not discuss it further.

As incomplete bioavailability typically results in the need for a higher dose for subcutaneous infusions than for intravenous infusions, costs of goods can be higher for subcutaneous formulations. In an attempt to increase the subcutaneous bioavailability of a biotherapeutic, subcutaneous infusions can be coadministered (or produced as coformulations) with the dispersion-enhancer hyaluronidase (discussed in detail in Sect. 4), an enzyme that facilitates spreading of an injected fluid in the subcutaneous tissue [18]. This increased dispersion in the interstitial tissue can result in a higher bioavailability of a co-injected molecule. Initial evidence supporting this hypothesis was found in a rat study on the absolute intradermal bioavailability for peg-interferon alfa-2b and the therapeutic antibody infliximab. Bioavailability was calculated to increase from 61 to 108% and from 59 to 94%, with and without co-injecting with hyaluronidase, respectively. In addition, the time to reach C_max (T_max) was lower for both molecules in the presence of hyaluronidase, indicating a faster absorption into the circulation system in the presence of the hyaluronidase (T_max of infliximab decreased from 48 to 18.7 h) [63]. The impact of hyaluronidase on the fraction absorbed is likely to depend on the molecule coadministered. In a study with trastuzumab with and without hyaluronidase using the mini-pig model, the fraction absorbed from compartmental pharmacokinetic analysis was similar across formulations and estimated to be 85%. Comparable to what was observed for intradermal peg-interferon alfa-2b and infliximab in the rat, subcutaneous trastuzumab absorption was more rapid from the hyaluronidase-containing formulation than from the control without hyaluronidase (average first-order absorption rate constants were 0.828 and 0.166/day, respectively) [32].

Limited clinical data on the impact of hyaluronidase on bioavailability are available for SCIG in the treatment of PI. Hyaluronidase-facilitated subcutaneous administration did improve the bioavailability by approximately 20% as compared with subcutaneous administration in the absence of the enzyme [89]. The authors reported that, when investigating the AUC, normalized by dose per kilogram, for IGHy compared with IGSC alone from a prior study, hyaluronidase improved the bioavailability of the immunoglobulin by approximately 20%. The ratio of AUC per dose per kilogram for IGHy/IGSC was 120.4% (90% CI 115.5–125.5). A low subcutaneous bioavailability in PI can increase the treatment burden for patients who require lifelong replacement therapy, as the resulting higher dose and larger dosing volume may necessitate administration at multiple injection sites, more frequent dosing, and dose adjustment to achieve immunoglobulin serum concentrations comparable to those with intravenous administration.

In a study of tocilizumab in healthy volunteers, coadministration of hyaluronidase resulted in a slightly increased tocilizumab exposure, a trend towards lower pharmacokinetic variability, and earlier T_max compared with administration without hyaluronidase [90]. The earlier T_max and increased absorption rate following subcutaneous coadministration with hyaluronidase has been shown to increase the absorption rate of insulin, resulting in superior glycemic control compared with insulin alone, meaning lower insulin requirements and reduced hypoglycemic excursions [91].

As understanding of the impact of the various anatomic injection sites on subcutaneous absorption remains limited [10], the injection sites for the subcutaneous formulations of trastuzumab and rituximab are restricted to those studied in clinical trials (thigh for trastuzumab and abdomen for rituximab) [64, 65]. Xu et al. [92] investigated the absolute bioavailability of golimumab, an anti-TNFα human IgG1κ, among three different injection sites. Healthy male volunteers (n = 78) were randomly assigned to either a single dose of golimumab 100 mg via a 30-min intravenous infusion (n = 23) or subcutaneous administration at the upper arm (n = 18), the abdomen (n = 18), or the thigh (n = 19). The absorption of golimumab was found to be similar independent of the injection site. The overall mean bioavailability of subcutaneous golimumab was 51% (upper arm, mean 52.0%, coefficient of variation (CV) 23; abdomen, mean 47.0%, CV 30; thigh, mean 54.1%, CV 34; p = 0.364). Similar findings were reported by Ortega et al. [93]. In a randomized, open-label, parallel-design, phase I study, 60 healthy volunteers were randomly assigned (1:1:1:1 ratio) to receive a single dose of either mepolizumab 250 mg by intravenous injection, subcutaneous injection (upper arm, abdomen, or thigh), or intramuscular injection. The mean bioavailability for each injection site was 64% (90% CI 55–73), 75% (90% CI 66–86), 71% (90% CI 62–82), and 81% (90% CI 71–94) for subcutaneous abdomen, arm, thigh, or intramuscular, respectively. While the above studies show comparable bioavailability independent of the anatomic site of injection, results may differ depending on the individual biotherapeutic. Therefore, additional clinical evidence is required to allow a more general conclusion on whether different injection sites can be used for a biotherapeutic without impairing the established efficacy or safety profile.