New generation of compstatin family attributed with sub-nanomolar affinity and enhanced PK properties were reported (Qu et al 2013, Primikyri et al 2017, Berger et al 2018). Peptide drug development has traditionally been obstructed by factors such as restricted administration routes, poor cell penetration, low metabolic stability and rapid plasma elimination (Craik et al 2013).
While cell penetration is of inadequate importance in the compstatin, because the target molecules are primarily found in the circulatory system, the other features have been examined and optimized during preclinical development of compstatin analogs.
In previous study, It was confirmed that the cyclic nature of the peptide a crucial contributor to its stability in the plasma. The acetyl group of the initial analogues further contributing to protecting the N-terminus from proteolytic enzymes (the C-terminus has always been capped by amidation) (Sahu et al 2000). The N-terminal alteration of compstatin Cp20 with non-proteinogenic amino acids yields two potent active analogs termed as CP30 and CP40. In newly developed analogs, the non-proteinogenic amino acids such as sarcosine (used in the compstatin analogue Cp30) or D-tyrosine (used in the compstatin analogue Cp40) have substituted the acetyl group (Qu et al 2013). Cp40 is so far the strongest C3 ligand with higher binding affinity in sub-nanomolar concentration. The fast elimination of the first generation compstatin analogs in plasma of human and NHP have created a major problem in therapeutic development of compstatin analogues for systemic drug administration. The clinical candidate POT-4 (Potentia) has been evaluated for primery treatment of AMD via intravitreal injection, thus largely bypassing these limitations. In recent PK study have been conducted in cynomolgus monkeys following single intravenous injection of second generation compstatin analogs (Cp20, Cp30, Cp40). Significantly, PK evaluation have revealed in NHP apparent terminal half-life value of up to ~12 h (Qu et al 2013). These analogs have highly valuable plasma half-life value (CP20, half life = 9.3 h; CP30, half life = 10.1 h; CP40, half life = 11.8 h). Moreover, there is a strong hint that these compstatin analogs actually follow a targetdriven model since C3 is present plentiful in plasma protein. In fact, the half life values determined for the three tested analogs were established to be directly connected to their binding affinities for C3, with Cp40 showing the longest plasma residence.
Even though the positive PK profile of the second generation compstatin analogs make them directly cooperative for systemic use, an further extension of the time that the drugs are present in plasma may confer a beneficial sign for long term administration. Distinct strategies have been assessed for maintaining the concentration of compstatin analogues in circulation that surpass the C3 concentration. The modification by attaching groups to either N- or C-terminus of the compstatin analogs with mildly affecting their inhibitory activity, has significantly assisted these approaches. Following this approach, the poly ethylene glycol (PEG) moieties were conjugated to the compstatin analogs (Jevsevar et al 2010). The size of branched PEG group (40 kDa) was anticipated to decrease the renal elimination and increase the half-life of compstatin analogs in a targeted dependent manner. The PEGylation remarkably increased the half life of Cp40, when PEGylated Cp40 was injected into cynomolgus monkey (Risitano et al 2014). The half life is around 5.5 days (increased by 11-fold compared with non-PEGylated Cp40). This finding indicated a possible option for clinical administration. One the potential primary risk to be measured with therapeutic PEGylated compounds is the development of immunogenicity in vivo (Zhang et al 2014). Observations suggested that anti-PEG antibody is developed and are correlated with the loss of therapeutic efficacy (Verhoef et al 2014, Ivens et al 2015, Zhang et al 2016). Moreover, the use of large size PEG (>20 kDa) has been related with side effects such as cellular vacuolation or increase toxicity due to deposition within the liver or decrease renal clearance (Zhang et al 2014, McDonell et al 2014).
A different approach for increasing the residence of drugs in the circulation relies on the exploiting the transporter/deport function of abundant protein present in the circulation such as albumin. It has previously been shown that conjugation of peptide drugs to albumin-binding peptide (ABP) increase their half life in circulation (Dennis et al 2002). This approach was effectively approved with first-generation compstatin analogs by adding an ABP to terminus through a mini-PEG linker. Finding suggested that ABP-fusion peptides displayed a prolonged plasma residence after injection in mice along with retaining inhibitory potential, in contrast to compstatin, the ABP moiety attaches to albumin of mouse as well as human (Qu et al 2009). A same approach involving a low molecular weight albumin-binding molecule (ABM) to increase the residence time in circulation is used in clinic (Lauffer et al 1998). This approach was applied to improve the PK properties of compstatin analogue Cp20 by N-terminal conjugation of different ABM moieties to Cp20. This study resulted in the most potent C3b ligand with subnanomolar binding affinity (KD =150 pM) of resulting derivative ABM2-Cp20 (Huang et al 2013). ABM2-Cp20 displayed a 20-fold higher binding affinity for C3 over the parental peptide, making it the most potent compstatin analogue. This interaction of ABM2-Cp20 with albumin from different species (mouse, rabbit, bovine, baboon and human) were evaluated. The competition experiments showed that ABM2-Cp20 binds initially to site II on human serum albumin (HAS). Albumin binding molecule is proficient to get better the plasma protein binding of ABM2-Cp20 which suggests its favorable PK profiles.
Recently, conjugation of relatively short polymers of PEG with CP40, that is, mPEG (1k), mPEG(2k), and mPEG(3k), significantly increased the solubility of the parent peptide. The length of the PEG chain with the 1 kDa mPEG being the shortest chain that was able to improved the solubility of Cp40 (Berger et al 2018). Other then PEGylation, conjugation with Lys residue to the C- terminus of Cp40 also improve the peptides solubility. In in-vivo sudy, the analogs mPEG(3k)-Cp40, Cp40-KK, and Cp40-KKK confirmed highest solubility at pH ~7.5 (>245 mg/mL). Interestingly, these analogs were injection subcutaneously by 2 mg/kg in cynomolgus monkeys in a single injection resulted in approximately doubling the Cmax value when compared to that obtained for unaltered Cp40, with Cmax for Cp40 < mPEG(3k)-Cp40 < Cp40-KK < Cp40-KKK. In addition to improvements in Cmax, the terminal half-life of the compound was also increased from 44.5 to 59 h, with t1/2 for Cp40 ~Cp40-KKK < mPEG(3k)-Cp40 < Cp40-KK (Table). Better PK profile of the new generated analogs were also revealed in their increased AUC0-120h (Cp40 < Cp40-KK < mPEG(3k)-Cp40 < Cp40-KKK) and slower CL/F (Cp40 > Cp40-KK > mPEG(3k)-Cp40 ~ Cp40-KKK). In addition, the time of C3 saturation was extended (Cp40 ~ Cp40-KK < mPEG(3k)-Cp40 < Cp40-KKK). These PK parameters points out a similar profile between Cp40-KKK and mPEG(3k)-Cp40 with a tmax of 2-6 h and 48 h of target saturation followed by slow clearance. Slower release of these compounds into the systemic circulation from subcutaneous compartment, potentially added to the longer saturation of the circulating C3. Even though the association rate and binding affinity of mPEG(3k)-Cp40 was less and the dissociation rate of all the derivative analogs was very comparable, higher solubility of mPEG(3k-Cp40 and the higher solubility of mPEG(3k)-Cp40 outcome in a higher C max , establishing the overall PK profile. Cp40 and Cp40, Cp40-KK was easily absorbed into the circulation in comparison of mPEG(3k)-Cp40. So the Cmax was also higher than that of unaltered compstatin analogs Cp40, due to differentiation of solubility. Cp40-KK clearance started quickly but slowed over time resulting in an extended t1/2 when compared to that of the parental peptide after reaching the Cmax. Cp40-KKK absorption was slower unlike the Cp40-KK into the systemic circulation and its PK profile was more favorable. Interestingly, Lys cleavage products were identified in the circulation after injection of Cp40-KK and Cp40-KKK into the animals, demonstrating the presence of enzymatic restriction in vivo as well as in vitro in monkey and human. In contrast, Cp40-KKK was mainly cleaved into Cp40-KK in vivo while in vitro studies using NHP or human plasma showed preferential cleavage of Cp40-KKK to Cp40-K. These finding indicates that Lys-Lys bonds are digested both in the subcutaneous compartment and in the circulation. Further investigation is therefore required to identify the Lys-cleaving enzyme and assess the biological relevance of this phenomenon.
Undoubtedly, promising PK profiles will prepare the direction for further clinical development of lead compstatin analogs. Along the way, open questions concerning disposition, route of excretion, immunogenicity and the potential generation of metabolites still need to be answered.