MAbs 2G12, 4E10 and 2F5 are broadly neutralizing human IgG1 antibodies which bind to the surface gp120 or transmembrane gp41 envelope glycoproteins of HIV-1. In a recent proof-of-concept study we have demonstrated that at high doses a cocktail of these three monoclonal antibodies exhibits in vivo inhibitory activity against virus replication in some patients with established HIV-1 infection (42
). Interestingly, the main antiviral effect seen was primarily attributable to the 2G12 antibody, as evidenced by unambiguous phenotypic and genotypic escape studies. In contrast, no in vivo escape against 4E10 and 2F5 was detected. Here we present a formal pharmacokinetic analysis of a clinical trial which was used to explore whether the differential pharmacokinetic properties between the two anti-gp41 antibodies and anti-gp120 antibody 2G12 might partially be responsible for the unexpected differences in the activities observed in vivo. The objective of the present study was to evaluate the pharmacokinetics of the three antibodies and to explore whether they induce the development of an endogenous antibody response. Despite the use of 30% higher doses of 2F5, the peak and trough plasma concentrations of 2G12 achieved were substantially higher than those of 2F5 and 4E10 achieved. The higher 2F5 dosage was chosen based on the results of the preceding phase I studies, where a shorter half-life of 2F5 compared to those of 2G12 and 4E10 was found (1
The distribution and elimination pharmacokinetics of the three antibodies presented in this report indicate that they have kinetics which are similar to those of other antibodies, with small volumes of distribution and relatively long elimination half-lives (15
). The central volumes of distribution observed in the present study were 0.035 to 0.047 liter/kg, which is equivalent to the assumed standard plasma volume (3 to 5% of total body volume). This suggests that the disposition of the antibodies is initially limited to the vascular compartment. However, the steady-state distribution volumes attained during the postdistributive phase were approximately three times greater, which indicates that a significant quantity of the antibodies is delivered to the tissue compartment. It is important that by using a two-compartment model, the apparent distribution volumes are strongly underestimated in cases where antibody binds with a high affinity to extravascular sites because the assumption of a rapid equilibrium between both compartments is not fulfilled. In addition, this approach assumes that elimination occurs exclusively from the central compartment, which is probably also not correct. Elimination of antibodies is very likely taking place in the central and peripheral compartments simultaneously. However, our experimental data did not allow the differentiation of this. In analogy to other therapeutic antibodies with kinetics comparable to those of the antibodies used in this study, it may be expected that at least 50 to 80% of the given dose was distributed to tissues. Nevertheless, determination of tissue concentrations would be needed to confirm this. In previous studies tissue concentrations ranging from 10 to 50% of the simultaneous plasma levels were found for most antibodies, and in some studies even much greater tissue concentration-to-blood concentration ratios of 2 to 13 were reported (28
). Although the clearance values and the half-lives were highly variable, they were in the same range as those reported for other humanized IgG antibodies (28
). As in the earlier studies, 2G12 was found to have a significantly longer elimination half-life than 4E10 and 2F5 (1
). All three antibodies are of the same IgG1 subtype and differ only in their variable regions. However, minor changes in structures or glycosylation patterns may affect the pharmacokinetics of antibodies and may be possible reasons for these alterations.
The repeated antibody infusions at a high dose level of 3.3 g weekly were well tolerated by the patients. No severe side effects or toxicities were reported during the study. We specifically checked whether a humoral immune response against the passively administered human antibodies developed, since this could have safety implications for the patients and as such responses could be associated with an allergic reaction or could result in the more rapid clearance of other humanized antibodies possibly needed for therapy in the future. Importantly, no significant induction of an immune response against the monoclonal antibodies was found. The slight increase in the anti-4E10 IgM response, which was detected in one sample at week 12, is probably meaningless, as an endogenous antibody response would typically be observed within 7 to 10 days following administration (28
) and no transition to an anti-4E10 IgG response was found. Of note, an endogenous immune response directed against the monoclonal antibodies would most likely elicit both κ- and λ-specific IgG simultaneously as well as significant levels of IgM. Thus, there is a minor chance that some potential κ-specific response was missed.
Recently, it has been suggested that 4E10 and 2F5 have polyspecific autoantibody reactivities, and therefore, these antibodies might potentially predispose individuals to autoimmunity if they were infused into patients (21
). In our trial, despite the high and sustained levels of these two antibodies, we have not found a clinical correlate for the in vitro findings reported by Haynes et al. (21
). In particular, no clinical symptoms or signs compatible with any autoimmune disease appeared during or after the study (poststudy follow-up, 13 to 18 months). Thus, we conclude that despite these in vitro findings by Haynes et al. (21
), further clinical investigation with these antibodies is possible. Yet, if such a clinical investigation is undertaken, systematic investigations for laboratory parameters suggestive of autoimmune disease should be undertaken.
The multiple-dose phase II study described here established the long-term kinetics of MAbs 2G12, 4E10, and 2F5 following the administration of high doses. The three antibodies have plasma kinetics that correspond to a two-compartment model. Comparison with the kinetics of other therapeutic antibodies indicate that 2G12, 4E10, and 2F5 have distribution and elimination kinetics that are similar to those of the other compounds of this rapidly growing class of therapeutic agents. At present, approximately 20 monoclonal antibody preparations (antibodies, antibody fragments, or antibody fusion proteins) are on the market. These drugs are in use for a wide range of therapeutic applications in oncology as well as because of their activities against arthritis and other inflammatory disorders. Owing to their large molecular sizes, they are often associated with more complex pharmacokinetic and pharmacodynamic properties than small-molecule drugs. A saturable interaction with target antigen may influence antibody disposition and may potentially lead to nonlinear distribution and elimination. Moreover, independent of antigen, saturable binding to the intracellular receptor FcRn protects IgG from catabolism. Consequently, IgG antibodies often exhibit concentration-dependent elimination. A detailed overview of the pharmacokinetic and pharmacodynamic characteristics of antibodies was recently published by Lobo et al. (28
Numerous trials have evaluated the pharmacokinetics of antibodies used clinically, but the results of relatively few long-term studies have been published. In most instances only a small number of dosing intervals was observed and sample collection was completed within a few weeks. Some exceptions of studies with antibody preparations that were administered by the intravenous route (10
) or the extravascular route (6
) with sample collection periods ranging from 5 to 27 weeks can be found. In the same way, only a few reports addressing the clinical pharmacokinetics of antibodies directed against HIV have been published. The first trial assessed a chimeric monoclonal antibody to the V3 loop of the HIV-1 envelope gp120 over 21 weeks (18
). Subsequent studies evaluated the kinetics of monoclonal antibody F105 directed to the CD4-binding site of gp120 (7
), a humanized antibody binding to the V3 epitope GPGRAF (11
), and a humanized anti-CD4 antibody (25
). Overall, a single study with HIV-infected patients (18
) and only few studies in other disease settings (6
) have so far established the pharmacokinetics of antibodies after long-term multiple-dose administration in humans.
The comprehensive data presented here indicate that the anti-HIV-1 envelope monoclonal antibodies examined show distribution and elimination kinetics similar to those seen for other human-like antibodies and are thus expected to be largely distributed to the tissue compartment. Notably, although MAbs 2F5 and 4E10 have the shortest half-lives, we could not identify a distribution phenomenon that would explain the apparently lower in vivo activities of the two anti-gp41 antibodies compared to that of the gp120 antibody 2G12. While our clinical trial suggested that 2G12 had a dominant effect on the study outcome, the in-depth analysis of the pharmacokinetic data and evaluation of the distribution parameters revealed no differences between these antibodies that could reflect on their in vivo activities.