Accounting for disagreements on average cone loss rates in retinitis pigmentosa with a new kinetic model: Its relevance for clinical trials.

Since 1985, at least nine studies of the average rate of cone loss in retinitis pigmentosa (RP) populations have yielded conflicting average rate constant values (-k), differing by 90-160%. This is surprising, since, except for the first two investigations, the Harvard or Johns Hopkins' protocols used in these studies were identical with respect to: use of the same exponential decline model, calculation of average -k from individual patient k values, monitoring patients over similarly large time frames, and excluding data exhibiting floor and ceiling effects. A detailed analysis of Harvard's and Hopkins' protocols and data revealed two subtle differences: (i) Hopkins' use of half-life t0.5 (or t(1/e)) for expressing patient cone-loss rates rather than k as used by Harvard; (ii) Harvard obtaining substantially more +k from improving fields due to dormant-cone recovery effects and "small -k" values than Hopkins' ("small -k" is defined as less than -0.040 year(-1)), e.g., 16% +k, 31% small -k, vs. Hopkins' 3% and 6% respectively. Since t0.5=0.693/k, it follows that when k=0, or is very small, t0.5 (or t(1/e)) is respectively infinity or a very large number. This unfortunate mathematical property (which also prevents t0.5 (t(1/e)) histogram construction corresponding to -k to +k) caused Hopkins' to delete all "small -k" and all +k due to "strong leverage". Naturally this contributed to Hopkins' larger average -k. Difference (ii) led us to re-evaluate the Harvard/Hopkins' exponential unchanging -k model. In its place we propose a model of increasing biochemical stresses from dying rods on cones during RP progression: increasing oxidative stresses and trophic factor deficiencies (e.g., RdCVF), and RPE malfunction. Our kinetic analysis showed rod loss to follow exponential kinetics with unchanging -k due to constant genetic stresses, thereby providing a theoretical basis for Clarke et al.'s empirical observation of such kinetics with eleven animal models of RP. In contrast to this, we show that cone loss occurs in patients with increasing -k values during RP progression. And as the Hopkins' protocol selects more advanced RP cases than Harvard's to assure avoidance of ceiling effects (Harvard does this by kinetic monitoring), we show increasing -k kinetics to be the reason Harvard obtains more +k and small -k values. Thus the combined effects of (i) and (ii) produce Harvard's smaller average -k value. The relevance of the increasing biochemical stress model for optimizing clinical trials is discussed.

Rationale for an experimental treatment of retinitis pigmentosa: 140-month test of hypothesis with one patient.

BACKGROUND: Numerous mutations in over 100 rod genes are the well-established cause of apoptotic death of these cells and development of night blindness in retinitis pigmentosa (RP). Cone death is either concomitant or follows rod death with resultant loss of critical peripheral and central day vision. As cones are generally not encumbered by genetic mutations, the causes of their death and its prevention are the central problems of RP research. Currently no FDA-approved medications are available for retarding RP progression. HYPOTHESIS: It is proposed that cones, which are outnumbered 20:1 by rods, undergo apoptosis as a consequence of neurotrophic factor deficiencies and oxidative stresses accompanying massive rod death: increased retinal oxygen tension; leakage of lipid-peroxidation catalysts from disrupted membranes; reactive oxygen species from active/hyperactive microglia ingesting rod-apoptotic bodies. Accordingly we developed and tested a treatment regimen with a range of antioxidants in combination with the off-label use of deprenyl (1 mg/day), a safe antiapoptotic agent, which also upregulates eight neurotrophic factors. Since deprenyl inhibits only one of four mitochondrial apoptotic pathways, we added the antibiotic minocycline (100 mg/day) to our protocol at month 76. Minocycline complements deprenyl's therapeutic properties: it inhibits all four apoptotic pathways; inhibits apoptosis-initiating proteins; as phenol exerts powerful antioxidant properties; upregulates three antioxidant enzymes; downregulates oxidative/inflammatory microglia activities. Its safe long-term use for acne and rheumatoid arthritis received FDA approval; it passes the blood/brain and blood/retinal barriers readily; and because of its rapid and complete absorption causes no intestinal disturbances. The National Eye Institute has initiated in 2010 and 2011 clinical trials with minocycline (200 mg/day) for diabetic macular edema and retinal branch vein occlusion. TESTING OF HYPOTHESIS: The hypothesis was tested for 140 months with one RP patient monitored by Humphrey Perimetry, which was quantitated by two parameters: (a) sum of decibel units, (b) number of detected light sources (visual field). Although no decline was observed in these parameters during the first 50 months of treatment, they declined by 10-28% during months 50-65. These declines reversed upon introduction of minocycline: over the total 140-month treatment, the right eye visual field showed 0% decline and left eye 13.3% decline. Rate constants for logarithmic decline of visual field measured prior to treatment indicate that visual fields would have decreased by 64% and 70%, respectively by month 140 in the absence of treatment.

Detection of phencyclidine in hair.

Phencyclidine (PCP) can be detected in human hair wih commercially available radioimmunoassay regents. hair samples of all subjects admitting PCP use were positive, while thin-layer chromatographic urine analyses were positive in only one of seven cases. Presumably the drug is incorporated into the hair during periods of drug use and then retained in that particular section of the hair for its lifetime. Earlier results in this laboratory in a more detailed study of opiate retention in hair indicated not only that nanogram levels of the drug could be measured in a single strand of hair, but also that sectional analysis of the strand could indicate the time of drug use. The PCP results again suggest that the hair sample could serve as a valuable tool in the determination of drug abuse histories. The sample accessibility and stability and the long-term retention of the drugs in hair exemplify the potential advantages of the hair sample over the body fluid sample.

Radioimmunoassay of hair for determining opiate-abuse histories.

Heroin and morphine metabolites can be detected in hair with the use of commercially available radioimmunoassay reagents and with minor sample preparation. Hair samples obtained from morphine-treated mice and heroin users contained nanogram levels of the drug per milligram of hair (single human hair). The results of the hair analyses for all subjects admitting the use of heroin were positive, whereas the results of only 30% of thin-layer chromatographic urinanalyses of these same subjects were positive. In addition, differences in drug concentration for sections of hair near the scalp and near the distal end correlated with the length of time the drug had been used. These results exemplify the potential advantages of the use of hair analysis over urine and serum analyses in terms of accessibility, sample stability, and long-term retention of information.