1. Species-Dependent Pharmacokinetics of CMX001, An Orally-Available Prodrug of Cidofovir Being Developed for Treatment of Poxvirus Infection
BM Lampert, MR Almond, and LC Trost
Chimerix, Inc., Durham, North Carolina USA
47th SOT Annual Meeting
March 16-20, 2008
Seattle, Washington
ABSTRACT #
CMX001 is an orally available lipid conjugate prodrug of cidofovir, a nucleotide analog approved for treatment of CMV but active against many double-stranded DNA viruses. The clinical utility of cidofovir is limited by poor bioavailability, requiring intravenous infusion complicated by nephrotoxicity. CMX001 is under development for treatment of smallpox, a potential bioterrorist threat. CMX001 provided complete protection from mortality in mouse and rabbit models of poxvirus infection but was less efficacious in a cynomolgus monkey model. The pharmacokinetics of CMX001, cidofovir, and CMX064, the major (inactive) metabolite, were evaluated in single and multiple oral dose studies in mice, rats, rabbits and cynomolgus monkeys. Following oral administration, CMX001 was well absorbed in mice, rats and rabbits with Tmax occurring in 1 to 6 hours. In these species, Cmax and AUC increased about 1-3 fold greater than in proportion to dose. Slight accumulation was observed between the first dose and steady state. By contrast, values for Cmax and AUC in monkeys were about 5 to 10-fold lower and Tmax occurred earlier, at 0.5 to 2 hours, both likely a reflection of the rapid metabolism of CMX001. The terminal phase half-life ranged from 3 to 9 hours in rodents and rabbits but was only 1 hour in monkeys. Values for cidofovir Tmax ranged from 8 to 12 hours in mice, rats and rabbits but only 4 hours in monkeys. Cmax and AUC increased approximately in proportion to dose in all species, however, systemic exposure to cidofovir was 2 to 4 fold lower in mice and monkeys compared with rats and rabbits. The half-life ranged from 10 to 30 hours resulting in modest accumulation after multiple administrations. Systemic exposure to CMX064 was highest in monkeys, followed by mice and rabbits, and lastly rats. Recent experiments employing intramuscular dosing produced much higher systemic exposure to CMX001 in monkeys providing rationale for conducting additional efficacy experiments in this species. Human pharmacokinetic studies are ongoing.
METHODS
Studies in mice, rats, rabbits and cynomolgus monkeys were conducted to provide toxicokinetic (TK) and pharmacokinetic (PK) data for the nonclinical development of CMX001. The TK/PK component of the studies included blood sampling (n=3/timepoint) following single oral dose administration at 3 dose levels in mice, sampling after the first daily dose and 12th dose at 3 dose levels in pregnant female rats and rabbits, and sampling on Days 1 and 7 during 7 days of dosing at 2 dose levels in male cynomolgus monkeys. Following a washout period, a second phase was conducted in the monkey study during which the same dose was administered for 7 days by the intramuscular route. With the exception of mice, the animals were not fasted prior to dose administration.
Noncompartmental PK analyses were conducted using WinNonlin version 5.1 (Pharsight Corporation, Mountain View CA) to generate estimates of maximum plasma concentration (Cmax), time of maximum plasma concentration (Tmax), half-life of elimination (t1/2elim), and area under the plasma concentration versus time curve (AUC). The primary determinants of the observed concentration-time profiles and the derived PK parameters for CMX001, cidofovir (CMX021), and CMX064 include the bioavailability of CMX001, the relative rates of conversion to cidofovir or CMX064, and the volume of distribution and clearance of the three entities. For the purpose of comparing across species, the doses administered are expressed in the tables in mg/m2, however the dose was administered on a mg/kg basis.
Table 2. Relative Steady State Pharmacokinetic Parameters (Dose Normalized)
Figure 2. Mean Single Dose and Steady State Plasma Concentration of CMX001, Cidofovir and CMX064 in Cynomolgus Monkeys Given 4 mg/kg CMX001 by the Oral or Intramuscular Route (n=3)
Species
Dose (mg/m2)
CMX001
Cidofovir
CMX064
Cmax/D
AUC0-24/D
Rat 3
0.21
0.50
2.16
5.60
BQL ND 9
0.80
8.07
1.97
34.26 27
1.76
11.66
1.75
29.71
0.33
7.11
Rabbit 6
0.97
7.28
12.47
6.42
6.96 18
1.13
11.50
0.74
15.78
7.33
51.83 54
1.99
8.31
0.83
16.09
8.80
52.78
Monkey 12
0.34
0.54
0.45
6.08
12.75
85.58 48
0.11
0.24
0.55
10.06
8.85
74.27
CMX001
Cidofovir
CMX064
Abbreviations: BQL, below the quantifiable limit; ND, not determined (due to insufficient data)
Figure 1. Dose Normalized Mean Plasma Concentration (based on mg/m2) of CMX001, Cidofovir and CMX064 in Mice, Rats, Rabbits and Monkeys Following Oral Administration of CMX001
CMX001
Cidofovir
CMX064
RESULTS
CMX001
CMX001 was well absorbed in mice, rats, and rabbits with Tmax ranging from 0.5 to 6 hours. Tmax occurred earlier in monkeys than rodents, possibly due to rapid metabolism of CMX001 in monkeys.
CMX001 terminal-phase half-lives were longest in rats at approximately 9 hours. Half-lives were intermediate in mice and rabbits at 3 to 5 hours, and much shorter in monkeys, approximately 1 hour. Due to the short half-life in monkeys, no accumulation occurred.
Cidofovir
The rate of appearance of cidofovir in the systemic circulation is dependent upon the absorption rate of the prodrug, CMX001, as well as the rate of conversion of CMX001 to cidofovir.
Tmax values for cidofovir in the mouse and rabbit occurred approximately 8 hours after administration of CMX001. Tmax occurred later in rats at about 12 hours after dosing and earlier in monkeys, about 4 hours after dosing.
The half-life of cidofovir ranged from 17 to 30 hours in the 4 species.
CMX064
The pharmacokinetic profile of CMX064 is complicated by the successive steps of oxidation required for its formation; however, it is the major metabolite identified to date.
Across all 4 species, the Tmax values for CMX064 occurred at 2 to 5 hours and the half-life ranged from 2.5 to 5 hours.
INTRODUCTION
CMX001 is a lipid (1-0-hexadecyl-oxypropyl, HDP) conjugate of the acyclic nucleotide analogue cidofovir. Cidofovir (CDV), which is FDA approved for the treatment of cytomegalovirus induced retinitis in AIDS patients, is active against all five families of double stranded DNA (dsDNA) viruses that cause human morbidity and mortality. The clinical utility of the drug, however, is limited by the requirement for administration by intravenous infusion and the risk of acute nephrotoxicity. Formation of the HDP lipid conjugate increases in vitro antiviral activity by up to 1000-fold, promotes high oral bioavailability and eliminates nephrotoxicity1. CMX001 is currently in Phase I/II clinical development for the prophylaxis and treatment of smallpox, and is entering development for use in prophylactic and pre-emptive therapy for dsDNA viral infections that cause graft loss in transplant patients.
Chimerix’s lipid conjugate technology employs covalent coupling of a synthetic lipid to drug via a phosphonate moiety2. The goal of lipid conjugation is to increase oral bioavailability, increase potency and target delivery to specific tissues. The molecules are designed to resemble natural lipids that are readily absorbed from the small intestine and distributed to tissues via plasma and/or lymph. Hydrophobic lipid conjugates are proposed to be incorporated into chylomicrons released into the lymphatic system and ultimately venous blood while hydrophilic conjugates are proposed to be absorbed and pass directly into portal blood. Once in the cell the drug is cleaved from the lipid carrier by hydrolytic action of phospholipases on the phosphorous-O-alkyl bond3.
CONCLUSIONS
CMX001 Cmax and AUC after oral administration were highest in mice and rabbits followed by rats. Values in monkeys were approximately 5- to 10-fold lower, possibly due to extensive metabolism in this species. At steady state, systemic exposure increased slightly in rats and rabbits but decreased in monkeys.
Cidofovir Cmax and AUC were highest in rats followed by rabbits and mice. Cmax values in monkeys were again lowest of all species. Systemic exposure to cidofovir increased after multiple doses relating to it’s long half-life of elimination.
CMX064 Cmax and AUC were highest in monkeys followed by rabbits indicating oxidation pathways are the major determinant for the prodrug and cidofivir in these species. CMX064 Cmax and AUC were lower in mice and negligible in rats. Compared with single dose exposure, systemic exposure to CMX064 after multiple doses of CMX001 increased in rabbits, decreased in rats and remained about the same in monkeys.
Compared with oral administration, intramuscular administration of CMX001 in monkeys produced higher Cmax and greater systemic exposure to CMX001 and cidofovir after a single dose; however, exposure to cidofovir at steady state was approximately equivalent regardless of dose route.
Table 1. Relative Single Dose Pharmacokinetic Parameters (Dose Normalized)
Proprietary Technology to Increase Oral Bioavailability: Formation of a Lipid Conjugate of Cidofovir
Species
Dose (mg/m2)
CMX001
Cidofovir
CMX064
Cmax/D
AUC0-inf/D
Mouse
1.2
0.82 ND
BQL 6
1.31
8.52
2.82
21.13 30
2.31
13.45
0.36
35.60
3.22
27.84
Rat 3
0.09
0.44 9
6.03
0.91 27
0.89
7.76
0.69
0.24
Rabbit
1.05
6.80 18
1.39
8.25
0.29
15.34
5.24
41.38 54
8.53
0.28
5.65
47.21
Monkey 12
0.21
0.39
0.18
14.33
96.61 48
0.27
0.66
5.17
11.88
97.19
Table 3. Mean Single Dose and Steady State Pharmacokinetic Parameters in Cynomolgus Monkeys Administered 4 mg/kg CMX001 by the Oral or Intramuscular Route
REFERENCES
G.R. Painter, L.C. Trost, R. Lanier, B.M. Lampert, M.R. Almond, A. Robertson and K.Y. Hostetler (2008) CMX001: Anti-VARV, anti-HCMV viral polymerase inhibitor. Drugs of the Future. (submitted for publication).
G.R. Painter and K.Y. Hostetler (2004) Design and development of oral drugs for the prophylaxis and treatment of smallpox infection. Trends in Biotechnology 22:
N. Gummad and A.K. Menon (2002) Transbilayer movement of dipalmitoylphosphatidylcholine in proteoliposomes reconstituted from detergent extracts of endoplasmic reticulum. J. Biol. Chem. 277:
Route
Phase
CMX001
Cidofovir
CMX064
Cmax
AUC
Oral
Single Dose
12.92
31.71
13.47
248.25
569.67
Steady State
5.29 ND
26.53
880.11
425.33 IM
8.74
347.00
23.13
269.00
Abbreviations: AUC = hr*ng/mL (0-inf Single Dose, 0-24 Steady State); Cmax = ng/mL; IM, Intramuscular; ND, not determined (due to insufficient data)
Abbreviations: BQL, below the quantifiable limit; ND, not determined (due to insufficient data)