Pixantrone

Pixantrone is a cytotoxic aza-anthracenedione. Unlike approved anthracyclines (doxorubicin and others) and anthracenediones (mitoxantrone), pixantrone is only a weak inhibitor of topoisomerase II. Moreover, unlike anthracyclines or anthracenediones, pixantrone directly alkylates DNA forming stable DNA adducts and cross-strand breaks. Furthermore, because it incorporates a nitrogen heteroatom into the ring structure and does not have ketone groups, pixantrone has less potential for generating reactive oxygen species, binding iron, and forming alcohol metabolites that are felt to cause the cardiac toxicity of anthracyclines. Due to this unique structure, pixantrone produced minimal cardiotoxicity in animal models compared with doxorubicin or mitoxantrone.

A comprehensive retrospective population PK/PD analysis of Phase 1 trials and combination regimens (Phase ½) demonstrated that progression-free survival and Grade 2-3 neutropenia were related to pixantrone exposure.

Absorption

Following intravenous administration, plasma concentrations of pixantrone reached the maximal concentration at the end of infusion and then declined poly-exponentially. The pharmacokinetics of pixantrone was dose-independent in the 3 mg/m² to105 mg/m² dose range and no substantial differences were observed when the medicinal product was given as a single agent or in combination studies. Average exposures as single agent accounted for:

From an analysis of population PK data, for a target recorded dose of 50 mg/m² of pixantrone the median 28-day cycle exposure was 6320 ng.hr/ml (90% CI, 5990-6800 ng.hr/ml), for 3 doses/4 week cycle.

Distribution

Pixantrone has a large volume of distribution of 25.8 l and is approximately 50% bound to plasma proteins.

Biotransformation

Acetylated metabolites are the major biotransformation products of pixantrone. However, in vitro, conversion of pixantrone into the acetylated metabolites by either NAT1 or NAT2 was very limited. In human urine, the compound was mainly excreted unchanged, and very small amounts of phase I and phase II acetylated metabolites were found. Therefore, metabolism does not appear to be an important elimination pathway for pixantrone. Acetylated metabolites were pharmacologically inactive and metabolically stable.

Elimination

Pixantrone has a moderate to high total plasma clearance of 72.7 l/hr and a low renal excretion accounting for less than 10% of the administered dose in 0-24 hours. The terminal half-life ranged from 14.5 to 44.8 hr with a mean of 23.3 ± 8.0 (n=14, CV=34%) and a median of 21.2 hr. Due to the limited contribution of renal clearance, plasma clearance is mainly non-renal. Pixantrone may be metabolised in the liver and/or excreted in the bile. As metabolism appears to be limited, biliary excretion of unchanged pixantrone may be the major elimination pathway. Hepatic clearance approximates the hepatic plasma flow, suggesting a high hepatic extraction ratio and, therefore, efficient parent active substance elimination. Hepatic uptake of pixantrone is possibly mediated by OCT1 active transporters and biliary excretion by P-gp and BCRP.

Pixantrone had only a weak or no capability to inhibit P-gp, BCRP, and BSEP transport mechanism in vitro.

Pixantrone did inhibit OCT1-mediated metformin transport in vitro, but is not expected to inhibit OTC1 in vivo at clinically relevant concentrations.

Pixantrone was a poor inhibitor of OATP1B1 and OATP1B3 uptake transporters in vitro.

Linearity/non-linearity

Pharmacokinetics of pixantrone proved to be linear in a broad range of doses, from 3 mg/m² to 105 mg/m².

Pharmacokinetic/pharmacodynamic relationship(s)

A relationship between plasma exposure to pixantrone and neutrophil count has been observed.

After a single intravenous administration of pixantrone at 29 mg/kg and 38 mg/kg, immediate deaths were seen in mice (114 mg/m², LD10). Decreases in white and red blood cells and alterations in bone marrow, spleen, kidney, and testes were observed. Similar findings were reported in rats, and in dogs at 116 mg/m². In dogs, tachycardia and electrocardiography (ECG) changes occurred immediately after treatment.

In repeated-dose studies in mice, rats, and dogs, the main findings were myelotoxicity, nephrotoxicity (except dogs), and testicular damage.

In dogs, pixantrone given at 0.5 to 0.9 mg/kg for six cycles did not cause mortality or severe clinical signs, including ECG or body weight changes. Males were more sensitive to treatment, with respect to reduction in white blood cells and platelet count (reversible) and lymphoid depletion (spleen and thymus), as well as the marked toxicity to reproductive organs, as expected from a cytotoxic agent. Except for a transient increase in exposure in females after the third cycle, there were no marked differences in pharmacokinetic parameters. Males showed, however, slightly higher exposure than females.

In dogs, the heart was not affected by treatment, as no ECG changes were seen at different treatment times, nor heart changes were detected at gross- and histopathology. Kidney function and histology were similarly not affected both in 4- and 26-week studies.

The cardiotoxic potential of pixantrone compared with equiactive doses of doxorubicin and mitoxantrone in treatment-naïve and doxorubicin-pre-treated mice was evaluated. Pixantrone dimaleate up to 27 mg/kg given twice a week for 4 weeks did not induce any cardiotoxic effects, while mitoxantrone, as expected, was cardiotoxic at all tested doses (0.6, 1.6, and 1.5 mg/kg). Slight nephropathy was induced by pixantrone. Minimal cardiotoxicity of pixantrone was also demonstrated with repeat treatment cycles at the same doses.

Genotoxicity studies confirmed the potential for clastogenic effects in mammalian cells in vitro and in vivo. Pixantrone was mutagenic in the Ames test, increased the number of chromosomal aberrations in human lymphocytes, and increased the frequency of micronuclei in vivo.

Pixantrone caused maternal and foetal toxicity in rats and rabbits, even at a dose as low as 1.8 mg/kg given on days 9-11 of pregnancy, higher doses resulting in abortions and total embryo resorption. Embryotoxicity was characterised by reduced mean foetal weight, foetal malformations and incomplete or delayed foetal ossification. No long-term animal studies have been performed to establish the carcinogenic potential of pixantrone. No local tolerance study was conducted.

Pixantrone has been shown to cause phototoxic effects on 3T3 cells in vitro.

In a colony-forming units study in mice, the myelotoxicity of pixantrone and mitoxantrone administered at their LD10 (pixantrone dimaleate 38 mg/kg and mitoxantrone 6.1 mg/kg) was similar.